THE IMPACT OF PACKING LINE PROCESSES ON GLOSS DEVELOPMENT, AND THE
USE OF GLOSS AS AN INDICATOR OF COATING INTEGRITY AND QUALITY OF ‘RED
DELICIOUS’ APPLES (Malus domestica)
By
EKUWA ENYONAM QUIST

A DISSERTATION

Submitted to
Michigan State University
in partial fulfillment for the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Packaging

2011

ABSTRACT
THE IMPACT OF PACKING LINE PROCESSES ON GLOSS DEVELOPMENT, AND THE
USE OF GLOSS AS AN INDICATOR OF COATING INTEGRITY AND QUALITY OF ‘RED
DELICIOUS’ APPLES (Malus domestica)
By
Ekuwa Enyonam Quist

Apples are coated with food grade waxes to maintain and improve quality, such as color
and firmness, and to enhance gloss. In this study, a combination of simulated packing line
cleaning and wax application processes were used to evaluate their efficiency in cleaning and
developing gloss on ‘Red Delicious’ apples.
Commercially available alkaline and neutral detergents were applied in a pilot packing
line while varying the temperature and dwell time in the dump tank (DT) together with the rinsewater pressure and dwell time on the washing brushes (WB) to simulate the cleaning process. A
solvent extraction system quantifying surface residue after cleaning was developed to evaluate
cleaning efficiency. The alkaline detergent was more effective in acquiring a cleaner surface than
the neutral detergent. Increasing the temperature of the DT also resulted in a significantly cleaner
surface.
Cleaned fruit were subsequently waxed with commercially available shellac fruit coatings
at different viscosities; they were applied and “dried” in different environmental conditions of
temperature and relative humidity (RH) to investigate the conditions conducive for gloss
development and the maintenance of the quality attributes of weight and firmness. A nondestructive device for measuring gloss on curved surfaces was developed and correlated to the
human perception of gloss. The wax viscosity with RH in the drying zone, and the duration of
drying significantly affected the gloss. The inability to remove moisture from the apple surface

in the drying zone under high RH conditions in the environment decreased the ensuing gloss as a
result of dilution and therefore a decrease in the deposition of the coating thickness. Wax with
higher viscosity “dried” under lower RH conditions produced significantly higher gloss fruit
compared to a lower viscous formulation. Furthermore, the decay of gloss during storage was
accompanied by decreasing attributes of weight and firmness as the uniformity and integrity of
the wax coating deteriorated.
Respiration as influenced by the availability of O2 was impacted by the barrier properties
of the peel and storage temperature. Hand-coated shellac ‘Red Delicious’ apples were “dried” at
o

o

50 C under two RH conditions of 25 and 60%, and stored at 4, 10 and 20 C for a month. In this
controlled system, it was determined that the drying treatments resulted in varied coating
thickness, surface roughness and gloss. Surface gloss was determined to be directly related to
coating thickness, and inversely to surface roughness through the use of micrographs. The
transmission of respiratory gases through shellac was studied by bar-coating polyethylene film
with different shellac wet thicknesses, 4 and 10 µm. The transmission rate of O2 was higher at
higher temperatures with a corresponding increase in respiration rate, production of CO2 per g of
fruit. The increase in respiration rate was accompanied by an increase in the loss of fruit weight.
All of these increases were less pronounced in higher gloss fruit. In contrast the internal CO2
concentration was higher for the higher gloss fruit. Coated fruit, compared to the uncoated,
recorded lower respiration rates with significantly reduced gloss and weight loss.

KEYWORDS: ‘Red Delicious’ Apples, Packing Line, Fruit Detergent, Shellac Coating, Gloss,
Microstructure, Quality, Respiration Rate, O2 and CO2 Transmission Rate.

© 2011
Ekuwa Enyonam Quist
All rights reserved

“To make a great dream come true, the first requirement is a great capacity to dream; the
second is persistence – a faith in the dream” – Hans Selye, MD

This is dedicated to my husband, Charles Mawuse Ofori, for his encouragement and incredible
support through all of my dreams, desires and goals.
It is also dedicated to my parents, Esi Walton, MD and Patrick K. Quist, MD for giving me a
sense of security in my dreams and abilities, instilling in me the discipline and legacy of
education, and supporting me in every step of life’s journey.

v
 

ACKNOWLEDGEMENT

I am thankful to my major advisor, Dr. Maria Rubino, for her guidance in this work. My
sincere gratitude also goes to my guidance committee - Dr. Rafael Auras for his insightful
contribution to the aspect of gloss in the study; Dr. Randy Beaudry for sharing his expertise in
Horticulture and allowing the use of facilities; Dr. Dan Guyer for his immense contribution to the
design and set up of the pilot packing line; Dr. Bruce Harte for his mentoring throughout the
course of this endeavor; and Dr. Janice Harte for her instructional leadership in the sensory
aspects of this work. Thank you all for your guidance and constructive comments.
I am highly indebted to the numerous researchers who made their labs and resources
available to me. Dr. Renfu Lu, Dr. Amos Mizrach and Mr. Ben Bailey of USDA/ARS/SBRU
deserve inexpressible recognition for designing the glossmeter used in my research and for the
countless hours they dedicated to making their labs available for my use. My heartfelt gratitude
goes to Dr. James Steffe for his directions and use of his rheology labs; and to Carol Fleger and
Dr. Hazel-Ann Hosein for assisting in using the SEM and AFM in their respective labs.
This research would not have been a success without the invaluable suggestions and input
of Dr. Rob Tempelman, Dr. Nora Bello, Dr. Sandra Herman and Dr. Deepa Aggarwal in the
experimental designs and statistical analyses. I am thankful to Mr. Dick Wolthuis who spent
hours helping with the setup of the pilot line. I am so grateful to the faculty and staff, both past
and present, at the School of Packaging for their immense contribution to this phase of my life.
My thanks go especially to Dr. Twede, Linda Estill, Colleen Wagner, Bob Hurwitz and April
Meersdom for their help in diverse ways.

vi
 

I would like to acknowledge the Michigan Apple Committee and Project GREEEN for
funding this research. My gratitude also goes to the numerous apple packaging houses in
Michigan who not only opened their doors to us but also supplied apples for the study.
My deepest gratitude goes to my husband, parents, siblings and in-laws for their constant
love, encouragement and support. To Kaye, Pokua, Minnie, the Pepprahs and Sefa-Dedehs,
thanks for bearing with my constant talk of “apples” and “waxes”. Thanks to my lifelong friends
from MSU - Ploy, Turk, Isinay, Norbismi, Hayati, Javier, Sanjeev, Cristina, Dave, and Carolina
just to mention a few. My thanks go to Martureo, the McCalls, Yeboahs and Hollons for being a
family and support system, and to my professional mentor, Dr. Aaron Brody, for the constant
‘parenting’.
You have in diverse ways contributed to achieving this goal and the best I can give in
return is to say to you all “God richly bless you beyond what you can ask or imagine.”

vii
 

TABLE OF CONTENTS

List of Tables .................................................................................................................................x
List of Figures ............................................................................................................................... xi
Chapter 1

Introduction ............................................................................................................1

Chapter 2

Literature Review ..................................................................................................17

Chapter 3

The Effects of Packing Line Washing Treatments and Waxing Conditions on the
Surface Cleanliness and the Gloss Development of ‘Red Delicious’ Apples
(Malus domestica) .................................................................................................66

Chapter 4

The Effect of Wax Application and Storage on the Gloss and Quality of ‘Red
Delicious’ Apples (Malus domestica) .................................................................111

Chapter 5

The Effect of Edible Coating Application Process and Storage Temperature on the
Respiration and Quality of ‘Red Delicious’ Apples (Malus domestica), and the
Transmission of Gases through the Coating .......................................................139

Chapter 6

Summary, Conclusions & Recommendations for Future Studies ......................165

Appendices ...................................................................................................................................169
Appendix A

Copy of the Final Report for the Michigan Apple Committee ...........................170

Appendix B

The Remainder of the Interactions of Packing Line Factors on the Level of
Cleanliness ..........................................................................................................193

Appendix C

Consumer Panel Consent Form ..........................................................................195

Appendix D

Questionnaire for the Sensory Evaluation of the Gloss on Apples - Consumer
Panel Rating ....................................................................................................... 198

Appendix E

Selected Panel Consent Form .............................................................................201

Appendix F

Questionnaire for the Sensory Evaluation of the Gloss on Apples - Selected Panel
Ranking .............................................................................................................. 207

Appendix G

Questionnaire for the Sensory Evaluation of the Gloss on Apples - Consumer
Panel Ranking .................................................................................................... 209

Appendix H

Rheological Profile of Shellac Coatings Used in this Research .........................211

viii
 

Appendix I

Digital Microscopic Images of Uncoated and Coated Apple Surface .................215

Appendix J

CO2 Transmission Rate through Shellac of Varied Thicknesses .........................218

ix
 

LIST OF TABLES

Table 2.1. Methods for quantifying quality attributes .................................................................37
Table 2.2. Possible Uses of Edible Films and Coatings ...............................................................53
Table 3.1. Factors (detergent time, temperature dwell time, temperature and spray rinse pressure)
on different sections of the washing line affecting the efficacy of the packing line in
achieving optimal surface cleanliness on apples ........................................................76
Table 3.2. Factors of the wax application treatments evaluated for their effects on gloss
Development ...............................................................................................................80
Table 3.3. Frequency table of panel ranking by glossmeter measurement ...................................95
Table 3.4. Frequency table of panel ranking by glossmeter measurement after merging the
moderately and highly glossy fruit into one category .................................................96
Table 4.1. Comparison of gloss index to coating thickness and surface roughness of unwaxed,
and low, medium and high gloss apples ...................................................................119
Table 5.1. Respiration rate of apples with varied coating thicknesses stored at 4, 10 and 20oC for
4 weeks.......................................................................................................................156
Table A.1. Temperature, Relative Humidity and Dew Point Chart ............................................185
Table H.1. Apparent behavior of fruit shellac coatings (original formulation and that of 7%
weight vaporized) at 20 and 40oC .............................................................................212
 

 

x
 

LIST OF FIGURES 

Figure 2.1. A schematic layout of a typical commercial apple packing line ................................27
Figure 2.2. Electron micrograph of the surface of apple fruit showing platelet wax crystals ......49
Figure 3.1. Flow diagram for the evaluation of cleanliness and gloss development on ‘Red
Delicious’ apples ......................................................................................................72
Figure 3.2. Customized glossmeter for non-destructive apple gloss measurement ......................81
o

Figure 3.3. Effect of 18 and 40 C dump tank temperature on apple surface cleanliness ............83
Figure 3.4. Effects of no detergent vs. using alkaline vs. neutral detergent on surface cleanliness
of apples .....................................................................................................................85
o

Figure 3.5. Two way interactions of 18 and 40 C dump tank temperature with (A) alkaline and
neutral detergents and (B) 60 and 120 seconds dwell time on the washing brushes on
the surface cleanliness of apples ...............................................................................87
o

o

Figure 3.6. Surface moisture (g) and temperature ( C) of cleaned apples “dried” at 50 C for 90
and 300 seconds in RH conditions of 25 and 60% ....................................................90
o

Figure 3.7. Apparent viscosity and shear stress of shellac fruit coating at 20 C with varied shear
rates .............................................................................................................................92
Figure 3.8. Interactions of varied coating conditions of viscosity, temperature, RH, and drying
time on gloss development .....................................................................................101
Figure 4.1. Flow diagram of the work plan ................................................................................114
Figure 4.2. AFM images of the apple surface with different amounts of coating ......................120
Figure 4.3. Scanning Electron Microscope micrographs of the apple surface with different
amounts of coating ..................................................................................................122
Figure 4.4. Relationship between coating thickness and gloss at low, medium and high gloss
levels ........................................................................................................................124
o

Figure 4.5. Interactions of coating conditions on gloss with storage at 4 C in 95% RH ..........126
Figure 4.6. Relationship between gloss and (A) weight, and (B) firmness of fruit (mean ± SE) in
o
6 weeks of storage at 4 C in 95% RH ..................................................................128

xi
 

o

Figure 4.7. Weight loss in apples during storage at 4 C in 95% RH for 8 weeks .....................130
o

Figure 4.8. Loss in fruit firmness with storage at 4 C in 95% RH for 8 weeks ........................132 
Figure 5.1. Flow diagram of the work plan ................................................................................144
Figure 5.2. Changes in gloss of unwaxed, low and high glossed apples at 1 and 4 weeks of
storage ......................................................................................................................151
Figure 5.3. Digital optical images of the apple surface with different amounts of coating ........152
o

Figure 5.4. Weight loss in unwaxed, low and high gloss apples stored at 4, 10 and 20 C ........153
Figure 5.5. Thickness of shellac coating on low and high gloss apples .....................................153
o

Figure 5.6. Internal CO2 of coated apples stored at 4, 10 and 20 C ..........................................157
Figure 5.7. O2 transmission rate through shellac coating of 4 and 10µm wet thicknesses at 10, 15
o
and 20 C .................................................................................................................159
Figure A.1. Effect of dump tank temperature on surface cleanliness (per 3 apples) ..................175
Figure A.2. Effect of humidity in the drying oven on gloss level of coated fruits ......................177
Figure A.3. Effect of coating formulation on gloss level of fruits dried in 60% RH for 90sec
during the pilot trial ..................................................................................................177
Figure A.4. Surface temperature of apples transitioning through the packing line ....................179
o

Figure A.5. Surface moisture on apples, treated in either a warm or cold dump tank under 15 C
and 70% RH, just prior to entering the wax applicator ..........................................180
Figure A.6. Comparing gloss on apples treated in either a warm or cold dump tank, and waxed
with the 'old' formulation .........................................................................................181
Figure A.7. Comparing gloss on apples treated in a warm dump tank and waxed with either the
'old' or 'new' formulation .........................................................................................182
o

Figure A.8. Weight loss in apples during storage at 4 C, 95% RH ............................................187
o

Figure A.9. Loss in fruit firmness with storage at 4 C, 95% RH ...............................................188
Figure A.10. Relationship between gloss and (A) weight, and (B) firmness of fruits during
o
storage at 4 C, 95% RH .........................................................................................189
xii
 

Figure A.11. Relationship between coating thickness and gloss ................................................190
Figure B.1. Two way interactions of (A) 60 and 120 seconds dwell time on the washing brushes
with alkaline and neutral detergents and (B) pressure of rinse spray with 30 and 60
seconds dwell time in the dump tank on the surface cleanliness of apples .............194
Figure H.1. Time-independent behavior of fruit shellac coating at 20, 50 and 10 RPM ............213
Figure H.2. Apparent viscosity of fruit shellac coating (original formulation and that of 7%
o
weight vaporized) at 20 and 40 C with varied shear rates ......................................214
Figure I.1. Digital microscopic images of the apple surface with different amounts of coatings at
200x magnification ................................................................................................216
Figure J.1. CO2 transmission rate through shellac coating of 4 and 10µm wet thicknesses at 10,
o
15 and 20 C ...........................................................................................................219

xiii
 

CHAPTER 1
INTRODUCTION

1

1.1. Introduction
With the upsurge of diet and health awareness in recent years, consumers are opting for
the healthy foods category under which fresh produce falls. However, appeal places a pivotal
role in determining the quality (texture, taste, etc.) of the produce and the consumers’ decision to
purchase, let alone consume them. The fundamental appeal is derived from attributes like
appearance, flavor, aroma and texture; these attributes have been used in many studies to
measure the overall quality.
Even though the state of Michigan is ranked third in apple production in the United
States, and apples are Michigan's largest fruit crop by volume [1], its ‘Delicious’ apples have
experienced a loss in market share with a decline in sales in the last few years [2]. This trend has
been attributed to the lack of appeal of the fruit to the consumer. Though the gloss of Michigan
apples are deemed to be lower than that of Washington’s and hence the relatively lower sales, it
has been documented by horticulturists that ‘Red Delicious’ producers in the United States are
faced with the pressure to provide optimum quality fruit after storage [3]. The problem is
therefore not confined to Michigan only.
Prior to being delivered to the consumer after harvesting, apples are subjected to a
number of postharvest processes including storage, washing, sorting, waxing, drying and
packaging. The apple fruit is then consumed as fresh or processed for use in pies, sauce, juice
and cider; the determinants of its uses are uniformity of color, size, and the absence/presence of
defects amongst many others. Thus the perceived quality is the dictation factor in its end use.
Apples, pears, oranges and other produce are coated with food grade waxes; the
applications of natural waxes such as shellac and carnauba on apples have been documented to
improve the fruit appearance by imparting gloss in addition to retarding weight and firmness

2

loss, and delaying ripening during storage [4]. A reduction in weight loss is the result of
decreased transpiration, by means of the presence of a gaseous barrier derived from the applied
coating, which translates to firmness retention. Thus coating fruit not only enhances appeal but
also serves as a barrier to moisture loss resulting in weight loss control and firmness
preservation, both of which are measures of quality and lead to enhanced preservation and
extension of shelf life.
Shellac and carnauba coatings are most popular in the produce industry. Though
carnauba wax provides a lesser gloss compared to shellac, the finish does not face the problems
of whitening [5]. In view of this a lower gloss may compensate for the risk of higher gloss with
the tendency of whitening in humid environments. However, it has been found by researchers
that shellac coatings are excellent for dark ‘Red Delicious’ apples as it “impacts gloss, hides
bruises and forms a modified atmosphere condition that tends to preserve firmness and prolong
shelf life for this variety” [6]. To optimize the aesthetics and preservation qualities of the fruit,
the surface of the fruit needs to be clean, warm and thoroughly dry prior to the application of the
coating for a good finish.

1.2. Research Driver
This study was motivated by the need to evaluate and improve the appeal and quality of
Michigan ‘Red Delicious’ by investigating the effect of the different factors on the packing line
on gloss development. The appearance of the fruit which is mainly the result of genetic and
environmental factors is enhanced by processes on the packing line, and the choice of ‘Red
Delicious’ was because it is the most produced and consumed variety in the US and Michigan.

3

Many studies aimed at enhancing the quality of ‘Delicious’ have looked at factors such as
firmness, soluble solids, internal ethylene concentration, and the development of coatings to
enhance quality. These studies have related fruit coatings to quality attributes such as weight loss
[6, 7], flesh firmness [4, 5], soluble sugar content [4, 7], coating permeability [6, 8, 9], volatiles
concentration [10-12], internal atmospheres [4, 6, 8] and fruit respiration [8, 10, 11] but only a
few [5, 6, 11] have directly correlated measurements of gloss to these attributes. Since applying
coatings to fruit introduces a barrier to moisture with modification in the atmosphere of the fruit,
changes in physiological activity occur. These changes, inclusive of respiration and transpiration,
lead to texture preservation by means of reduced moisture and weight loss. Thus, gloss derived
from applying coating has a direct impact on texture quality attributes, but the magnitude of the
correlation will be dependent on the uniformity and integrity of the dried coating which is
dictated to a large extent by waxing conditions, giving rise to the need to investigate the
relationship between these.
The gloss of the skin embodies the overall appearance which is what attracts the
consumer in the first place before intrinsic factors like flavor and firmness come into play; it is
therefore critical for Michigan apples to meet the high quality standards in appearance. Gloss
measurements to evaluate the quality of produce like apples is challenging because of the
curvature of their surface. Most commercially available glossmeters are predominantly designed
for flat surfaces and therefore require a device modification or destruction of the produce to
successfully measure the gloss of curved surfaces. Very few studies [13-17] have addressed the
challenge of measuring gloss of uneven curved surfaces, and the need to provide a solution to the
problem cannot be over emphasized.

4

1.3. Goal and Objectives
The goal of this research was to improve the appearance and value of Michigan ‘Red
Delicious’ by enhancing the gloss through optimized packing line processes. This was achieved
by evaluating and altering packing line protocols, as well as correlating gloss to quality attributes
of the fruit. A good wax finish for enhanced appeal requires that several factors on the packing
line such as the washing, drying and application of the coating be defined and monitored. Thus
the objective was to evaluate and improve the packing line to achieve the desired fruit finish and
enhanced quality by:1. Validating a new method to measure gloss
2. Identifying the parameters on the washing section of the line that will ensure efficient
cleaning of the fruit surface in preparation for further processing
3. Evaluating different wax application treatments on gloss development as a function of
environmental conditions of temperature and relative humidity
a. Studying gloss decay with storage
b. Studying the effect of gloss on the quality attributes of firmness and weight loss
during storage
4. Investigating the effect of coating on the peel barrier properties and analyzing the impact
of coating integrity and gloss on the respiration rate of the fruit, and hence their influence
on the loss or retention of quality
5. Correlating wax formulation to gloss and recommending formulations that work better in
the humid Michigan environment
6. Verifying the findings from the pilot study on a commercial production line

5

1.4. Hypotheses
To achieve good gloss and consumer appeal, cleaning and wax application variables as
well as environmental factors need to be optimized. Apples have a layer of natural wax together
with debris, chemicals deposited on the fruit from sprays and other extraneous variables that
have to be removed via efficient cleaning prior to packaging and consumption. Following
cleaning, the fruit are coated with food grade waxes. Temperature and humidity affect the
waxing, and according to packing house personnel, though no empirical data exist, the surface of
the fruit needs to be clean, dry and warm for a good wax finish.
Hypotheses were set in place to be tested and aid decision making on the effect of
packing line variables and environmental factors on the overall quality of ‘Red Delicious’.

Hypothesis I – The impact of the packing line on surface cleanliness
A combination of different cleaning parameters will result in varying degrees of surface
cleanliness. The use of fruit cleaner, increases in temperature of the dump tank, dwell time in
both the dump tank and on the washing brushes, and rinse pressure on the packing line will
improve surface cleanliness. These hypotheses were established to make statistical decisions to
prove or disprove the above:
Null Hypothesis (Ho): Fruit cleaned by different washing treatments will have the same
level of cleanliness.
Alternative Hypothesis (Ha): Different levels of cleanliness will be observed in fruit
cleaned by different washing treatments.

6

Rationale
Fruit cleaners are formulated to cleanse the fruit from contaminants and other deposits, thus the
use of appropriate agents will achieve the goal of a cleaned surface. Also most of the fungicides,
pesticides and chemical sprays used on the field are water soluble and therefore longer contact
with the cleaning system will increase dissolution. With efficient monitoring to avoid heat injury,
temperature is well able to remove the natural wax on the fruit, and pressure is able to dislodge
debris that may not easily be removed by the other washing factors considered.

Hypothesis II – The effects of coating formulation and environmental conditions on gloss
Cleaning fruit tends to remove the natural waxes from the surface of the fruit [8]. The use
of fruit coatings not only replaces the function of the natural wax removed during washing, but
imparts a sheen which enhances the aesthetic value of the fruit, thus improving visual appeal,
quality and marketability. However environmental conditions of temperature and humidity affect
the way the wax adheres to the fruit, markedly resulting in different levels of gloss. Gloss is
affected by the type of coating formulation and its flow properties. These hypotheses were
established to make statistical decisions to prove or disprove the above:
Null Hypothesis (Ho): The wax formulation, and the temperature and humidity conditions
in which it is applied and dried in have no effects on the sheen of the fruit.
Alternative Hypothesis (Ha): The wax formulation, and the temperature and humidity
conditions in which it is applied and dried in have effects on the sheen of the fruit.
Rationale
Fruit coatings containing volatiles, isopropanol and morpholine, in their formulation are
susceptible to changes in their rheological profiles and hence their flow behavior. A viscous

7

coating with reduced flow properties may inhibit uniform fruit surface coverage thus affecting
coating integrity which influences gloss. The environmental conditions on the packing line
influence the dew point. Warm conditions favor good curing of the wax, but if the surface
temperature is at or below the dew point, condensation will form on the apple surface and
interfere with wax application. High humidity conditions also pose the threat of washing off the
applied wax which will lower the level of expected gloss and affect quality.

Hypothesis III – The influence of coating on the diffusion of respiratory gases
The resistance of the fruit skin to respiratory gases and water vapor is altered with the
application of commercially formulated coatings, which form a barrier to the passage of gases
through the peel. These hypotheses were established to make statistical decisions to prove or
disprove the above:
Null Hypothesis (Ho): The respiration rate of fruit is not influenced by the application of
coatings.
Alternative Hypothesis (Ha): The respiration rate of fruit is influenced by the application
of coatings.
Rationale
Coated fruit in comparison to uncoated fruit have reduced respiration rates and water loss from
the fruit as a result of changes in physiological activity. The transfer of gases between the fruit
and the atmosphere may decrease as a result of the combined effect of reduced fruit respiration
and the coating barrier properties. Also resistance of the peel to respiratory gases can change in
response to environmental conditions of temperature and relative humidity. Peel resistance is
also highly influenced by the integrity of the coating layer and its characteristics. According to

8

Banks et al. [18], it is the manner in which the coating adheres to the peel, loosely or tightly, and
also the proportion of the pores in the peel that are covered and blocked by the coating that
determines the respiratory response of the fruit to the coating.

1.5. Research Plan
In order to achieve the goal of this research, the work was divided into 4 phases: I)
Surface Preparation; II) Pre-Waxing; III) Wax application; and IV) Packing Line
Recommendations. The work plan of the phases is detailed in Figure 1.1.
Phase I
In this phase apples were treated with combinations of the different factors listed in the
‘Phase I’ section in Figure 1.1.
1. A full factorial design was used to assess the effect of each factor and their 2-way
interactions on surface cleanliness prior to wax application. A method to extract the
residue remaining on the fruit after the application of each specific washing treatment
was developed as a means to quantify and evaluate the degree of cleanliness that resulted
from subjecting the fruit to different combinations of washing treatments.
2. Surface dryness is critical in achieving a good coating finish. Coated fruit were dried for
o

90 or 300 s at 50 C in either 25 or 60% RH; the purpose of varying the drying time and
humidity was to simulate different lengths of drying tunnels and climatic conditions
respectively. A method for quantifying surface dryness, by means of quantifying the
moisture left on the apple surface after drying, to enable a correlation with resultant gloss
was developed. The surface temperature of the apples emerging from the different drying
conditions was also measured.

9

Figure 1.1. Work plan of the study
For the interpretation of the references to color in this and all other figures, the reader is
referred to the electronic version of this dissertation.

10

Phase II
This phase together with Phase I was required to lay the foundation for Phase III; this
included developing a method to measure surface dryness, studying the rheological profile of the
shellac coatings to be used, and developing a device for measuring the gloss on curved surfaces.
1. Washed and dried fruit from Phase I were waxed with shellac coatings in Phase III. Gloss,
from the application of wax, needed to be measured to enable a correlation with waxing
application conditions. Since most of the commercial glossmeters are designed for measuring
gloss on flat surfaces, a customized gloss device with the capability of non-destructively
measuring the gloss on the curved surface of apples was designed and built. It was important
that the gloss measured by the new device could be perceived by the human senses, and
therefore required the use of a consumer panel to establish a correlation between the
instrument’s measurements and the human perception of gloss.
2. Shellac fruit coatings have a volatile solvent base, composed of isopropanol and morpholine.
Evaporation of these solvents concentrates the solid content which increases viscosity. An
increase in viscosity which impedes flow may affect coating spreadability on the fruit
surface. Rheological profiling of the waxes was conducted to enable a correlation between
the wax flow properties and the resulting gloss.

Phase III
Fruit emerging from the best identified washing treatment in Phase I were used in this phase.
o

1. The shellac waxes were applied in 15 or 35 C, and 60 or 85% RH. Following waxing, the
o

fruit were dried for either 90 or 300 s at 50 C in 25 or 65% RH. The coated fruit were stored

11

o

at 4 C in 95% RH regular atmosphere for 8 weeks and monitored for gloss, weight and flesh
firmness changes fortnightly.
o

2. Both coated and uncoated fruit were stored at 4, 10 and 20 C for respiration studies;
investigations included the effect of the presence of a coating layer, coating application
conditions by means of gloss levels, and storage temperature on the rate of respiration.
3. Coatings create a barrier to respiratory gases. The O2 and CO2 barrier properties of the
coatings were studied by measuring their transmission rates through coatings of different
o

thicknesses at 10, 15 and 20 C using polyethylene film as the carrier.
4. In an effort to better understand the effect of coating on gloss and the barrier properties of the
peel, with their corresponding effects on changes in weight, firmness and respiration rates,
microstructural studies were conducted. The roughness and thickness characteristic of the
coated peel that evolved from using varied amounts of coating were studied using the Atomic
Force Microscope (AFM) and Scanning Electron Microscope (SEM) respectively.

Phase IV
A scale up was carried out on a commercial packing line. The findings of the pilot trials
in the laboratory were adapted to a packing line at one of the packing houses in Michigan. At the
completion of the scale up a ‘guideline for good practices for achieving good coating finish’
document was made available to packers for their reference.

12

1.6. Overview
Chapter 2 of this document is the literature review. Chapter 3 elaborates on the effect of
washing fruit, using different cleaning treatments, on the surface cleanliness of the fruit, as well
as the design of a customized non-destructive gloss device. The effect of waxing conditions on
gloss, weight and firmness; and the changes in respiration rates with coating and varied storage
temperatures are discussed in chapters 4 and 5 respectively. Chapter 6 summarizes and concludes
the study.

13

REFERENCES

14

1.

USDA/NASS (2008). "Michigan Agricultural Statistics 2008 - 2009: Fruits". USDA
National
Agricultural
Statistics
Service.
http://www.nass.usda.gov/Statistics_by_State/Michigan/Publications/Annual_Statistical_
Bulletin/stats09/fruit.pdf, (July 13, 2009).

2.

ProjectGREEEN (2008). The science of shine proves appearance does matter. IssueAction-Impact 2(2): 1 - 8.

3.

Fellman, J. K.; Rudell, D. R.; Mattinson, D. S.; Mattheis, J. P. (2003). Relationship of
harvest maturity to flavor regeneration after CA storage of 'Delicious' apples. Postharvest
Biology and Technology 27(1): 39-51.

4.

Bai, J. H.; Baldwin, E. A.; Hagenmaier, R. H. (2002). Alternatives to shellac coatings
provide comparable gloss, internal gas modification, and quality for 'Delicious' apple
fruit. Hortscience 37(3): 559-563.

5.

Bai, J.; Alleyne, V.; Hagenmaier, R. D.; Mattheis, J. P.; Baldwin, E. A. (2003).
Formulation of zein coatings for apples (Malus domestica Borkh). Postharvest Biology
and Technology 28(2): 259-268.

6.

Bai, J.; Hagenmaier, R. D.; Baldwin, E. A. (2003). Coating selection for 'Delicious' and
other apples. Postharvest Biology and Technology 28(3): 381-390.

7.

Pandit, A. H.; Mir, M. A.; Wani, R. A. (2007). Effect of wax coating and growth
regulators with PP, HDPE and LDPE wrapping on the quality and storage life of apple
cv. red delicious. Plant Arch. 7(2): 647-650.

8.

Hagenmaier, R. D. (2005). A comparison of ethane, ethylene and CO2 peel permeance for
fruit with different coatings. Postharvest Biology and Technology 37(1): 56-64.

9.

Park, H. J., Bunn, J. M., Vergano, P. J. , and Testin, R. F (1994). Gas permeation and
thickness of the sucrose polyesters, semperfreshTM coatings on apples. Journal of Food
Processing and Preservation 18(5): 349-358.

10.

AvenaBustillos, R. D.; Krochta, J. M.; Saltveit, M. E. (1997). Water vapor resistance of
red delicious apples and celery sticks coated with edible caseinate-acetylated
monoglyceride films. Journal of Food Science 62(2): 351-354.

11.

Alleyne, V.; Hagenmaier, R. D. (2000). Candelilla-shellac: An alternative formulation for
coating apples. Hortscience 35(4): 691-693.

12.

Bai, J. H.; Hagenmaier, R. D.; Baldwin, E. A. (2002). Volatile response of four apple
varieties with different coatings during marketing at room temperature. J. Agric. Food
Chem. 50(26): 7660-7668.

13.

Nussinovitch, A.; Ward, G.; Mey-Tal, E. (1996). Gloss of Fruits and Vegetables.
Lebensmittel-Wissenschaft und-Technologie 29(1-2): 184-186.

15

14.

Ward, G.; Nussinovitch, A. (1996). Peel gloss as a potential indicator of banana ripeness.
Lebensmittel-Wissenschaft und-Technologie 29(3): 289-294.

15.

Mizrach, A.; Lu, R.; Rubino, M. (2009). Gloss Evaluation of Curved-surface Fruits and
Vegetables. Food and Bioprocess Technology 2(3): 300-307.

16.

Jha, S. N.; Matsuoka, T. (2004). Changes in Electrical Resistance of Eggplant with Gloss,
Weight and Storage Period. Biosystems Engineering 87(1): 119-123.

17.

Jha, S. N.; Matsuoka, T.; Miyauchi, K. (2002). PH - Postharvest Technology: Surface
Gloss and Weight of Eggplant during Storage. Biosystems Engineering 81(4): 407-412.

18.

Banks, N. H.; Dadzie, B. K.; Cleland, D. J. (1993). Reducing gas exchange of fruits with
surface coatings. Postharvest Biology and Technology 3(3): 269-284.

16

CHAPTER 2
LITERATURE REVIEW

17

2.1. The Apple Fruit
The US is one of the four key apple producers in the world, coming second to China [1]
but third to the EU-27 in 2009 [2]. Michigan is the third largest apple producer in the US after
Washington and New York; it produced 12% of US apples in 2009 compared to 54 and 14%
from Washington and New York respectively. Its production was down 4% from 2007 and 3%
from 2006 as a result of frost and hail damage. However, its production was up 6% from 2008
achieving record highs above historic levels [3]. Apples are Michigan's most valuable fruit crop
and the largest by volume, representing over half of the state’s total fruit production [4]. Out of
the hundreds of commercial apple varieties, the most prevalent dominating the US and Michigan
orchards is the ‘Red Delicious’, followed closely by the ‘Golden Delicious’. However, the ‘Gala’
and ‘Fuji’ apples are rapidly gaining market share [1, 5].
With the development of new products and technologies, in the last decade, that result in
reduction in product losses and open avenues for new markets, the apple industry worldwide is
faced with intense competition. These factors resulted in the saturation of the supply chain, seen
in the scenario where fruits from the previous year have to be dumped because of the onset of the
harvest of new fruits. There is however capacity to increase utilization of fruits especially in the
geographical areas deemed not to meet the minimum requirements of fruit consumption [6].
Regional and national discussions on the need to increase fruit and vegetable consumption to
curb future public health issues resulting from low produce consumption have been ongoing [6,
7]. Efforts have been made to identify the underlying factors contributing to the low fruit
consumption among populations.
A recent Swiss surveyed 5062 visitors to a local food exhibition and showed that apple
consumption trends are influenced by both gender and age. Out of the 14.3% of the consumers

18

that ate more than 6 apples per week, 10% were under the age of 40, 20% between 50 – 70years
and over 40% above the age of 70 [8]. Péneau et al. [8] attributed this trend to the possibility of
the older generation doing so out of tradition when there were fewer options. Other studies have
also shown the general significant low consumption rate of fresh produce by the younger
generation [9]. The percentage of men consuming less than an apple a day was double that of
10% by women [8]; a similar trend was reported by Thompson et al. [9]. Consumer education on
health benefits from consumption trends will not only promote health and reduce waste, but will
also open avenues for unexploited capacity in the potential growth of the fruit industry.
Nutritional quality has been categorized as one of the attributes used to judge freshness and
quality but only a minority make a purchase decision on the basis of nutritional profile [10]. A
2006 study on consumers’ perception of apple freshness by the same Swiss team [8] revealed
that not much importance was attributed to nutritional value. However in the cases where it did,
age and consumption rate as well as gender influenced the consideration of nutritional quality.
More importance was given to derived nutrient with an increase in the consumers’ age and in
consumers eating at least 5 apples per week. Females also attributed more importance to
nutritional value than males.
More emphasis on changes in consumer attitudes to consumption than modifications to
supply [6] may sound like the best solution to increasing fruit utilization. Though a glossy
appearance like in many fresh produce may not necessarily have a parallel to quality, it is used to
judge apple quality [11, 12]. Modern consumers are reported to demand impeccable appearance
which comprises the intensity and uniformity of skin color and blemish-free fruits [13].
Michigan ‘Red Delicious’ apples have suffered a loss of market share and reduced sales as a
result of perceived lower quality in comparison to other cultivars of the crop and the same

19

cultivar from different regions [14]. Therefore how does the industry change the consumer
consumption trend if the fruit is not appealing enough to influence their purchasing intent, let
alone consumption of it?
Over 60% of all Michigan apples are processed; in 2008 about 73% of the apples were
processed, up 7 and 9% from 2007 and 2006 respectively but down 16% in 2009 as a result of a
better growing season and larger sized fruits [3]. The increase in the amount used for valueadded processed products from previous years was the result of weather damaged and undersized fruits. While Michigan apples are the main source for apple sauce, fresh and processed
fresh-cut slices and apple cider [5], fresh apples have a higher economic value than those sold for
processing into value-added products. The fresh market generated $0.355/lb in comparison to
$0.135/lb for that sold for processing in 2008, whereas in 2009 it generated $0.215/lb and
$0.07/lb respectively [3]; the lower pricing in 2009 was the result of the abundance of fruits.
Therefore the economic need is to increase the proportion of the sale of fresh Michigan apples to
increase its economic value and the revenue generated from it.
‘Red Delicious’ has received great attention from producing and exporting areas of the
world. In addition to a high percentage of red coloration for red apples, most guidelines dictate
optimum pre- and post-storage appearance and intrinsic qualities for highest grade fruits [15].
According to Fellman et al. [15] there is the need to improve the industry’s capacity to produce,
store and market optimal quality fruits all year round in order to fortify the consumer’s
confidence in the ‘Red Delicious’ apple. Though the initial purchase may have been made on the
basis of appearance, good edible quality is what assures the consumers’ satisfaction, confidence
and repeated purchases [16]. Therefore the need for apples to meet the high quality standards in
flavor and texture besides appearance cannot be overemphasized.

20

Just as research in orchard management, postharvest handling and storage have allowed
the production and availability of quality fruits all year round, it is believed that the processing
used in Michigan can be optimized through research to produce glossy apples that will compete
with those from other regions. This study aimed at improving the visual quality of Michigan
‘Red Delicious’ to the same level as those harvested and packaged in Washington, and in so
doing gain market shares. Improving visual appeal mainly achieved by enhancing gloss with fruit
coatings comes with added benefits of quality preservation. This is discussed further in this
document. This study also addresses the need to evaluate and quantify gloss to facilitate its use
as an indicator of fruit quality.

2.2. What is Gloss?
The appearance of both inanimate and animate objects is crucial to humans, and specular
gloss (light distribution by the commodity) is widely used to measure the quality of a surface
[17]. Gloss is used to describe the appearance of a material and is basically the interaction of
light with the surface of interest. It therefore describes a surface’s ability to reflect light in the
specular direction. The topography of a surface, the angle of the incident light and the refractive
index of the surface affect gloss; it is the superposition of these factors that influence surface
appearance [17, 18].
Gloss is a geometric attribute that arises from the spatial distribution of light from an
object. It is the attribute of surfaces that is responsible for the perception of a shiny or lustrous
appearance, and its perception is highly dependent on how light is distributed by an object.
According to Hunter et al. [18], gloss, as associated with specular reflection, varies from one
surface to the other by:

21

(i)

the amount of light, relative to the beam, reflected in the specular direction

(ii)

the pattern and degree to which the reflected spreads away from the specular direction

(iii)

how varying the specular angle changes the specular reflection.

The perception of gloss, according to the same team [18], is a combination of six sensations, not
necessarily in equal proportions:
(i)

Specular gloss occurs in medium-gloss surfaces like those of book paper, paint and
plastics. It is the shininess and brilliance of highlights, usually measured at 45o to the
normal.

(ii)

Sheen is the shininess, viewed at almost grazing angles of 85o, of low-gloss (matte)
surfaces like that of paper and paint.

(iii)

Contrast gloss or luster is the contrast between specularly reflecting areas and other
areas. Low gloss surfaces of textile fiber, yarn and cloth, newsprint, bond paper,
diffuse-finish metals, fur and hair are examples of surfaces that exhibit this visual
criterion.

(iv)

Absence-of-bloom gloss is the absence of a haze or milky appearance adjacent to
specularly reflected beams. This occurs in high- and semigloss surfaces in which
specularly reflected highlights may be seen.

(v)

Distinctness-of-image gloss, which is the distinctness and sharpness of mirror
images, occurs in all types of high-gloss surfaces in which mirror images may be
seen.

(vi)

Surface-uniformity gloss deals with the freedom from visual surface nonuniformities such as texture which diminishes the intensity of gloss. It can occur in all

22

types of medium-to-high gloss surfaces with the absence of texture or markings from
which the position of a surface may be identified.

2.2.1. How Surface Characteristics Dictate Gloss
When evaluating visual properties, the gloss of the surface is one of the most important
properties considered; gloss measurements are a routine in assessing quality. Specular gloss is
an important factor in the estimation of the quality of smooth and rough surfaces [17]. The
amount of light reflected, when light encounters an object, is largely dependent on the nature of
the surface, whether it is smooth or rough. Reflected light, perceived as specular reflection, is
responsible for gloss of an optically smooth surface. The lack of a glossy reflection is indication
of a rough surface; light is scattered in many directions when it encounters a rough or textured
surface. The intensity of the specular reflection is therefore weaker, and the irradiance
distribution is shorter and wider as the surface roughness increases. Also, all of the specularly
reflected light may therefore not be collected by the photodetector leading to lower readings [17,
18]. In summary, light distributions determine the perception of our judgment of an object [18].

2.2.2. The Measurement of Gloss
Appearance evaluation, which used to be an art, has in recent years become a science.
Appearance measurements are made to obtain numbers that represents the way objects look. This
crosses a broad range of industries, from food to automobile to clothing and furniture, where
consumers will judge appearance and link it to what they deem as quality. Appearance
measurements span from the developmental stages (research and development) to production
(quality control of the raw products, during processing and the finished product) to the end use

23

(performance evaluation) [18]. The first step in designing instruments that can measure what the
human eye perceives is to recognize the relationship between appearance and the specific optical
phenomena. The four main optical phenomena are recognized as diffuse reflection, specular
reflection, diffuse transmission and regular transmission [18].
Visual sense is complicated and experience plays a major role in the evaluation of what is
observed. Nerve impulses, on encountering light patterns, are complexly sorted for the
identification of objects, movements, etc. “Patterns of light entering the eye are the stimuli on
which appearance judgments are based” [18]. At a tender age, using other senses for
confirmation, one tends to develop the abilities for visual discrimination using the eyes and
brain. There is a certainty for branding an object as dirty or clean, glossy or matte, fresh or stale,
without necessarily being aware of the optical processes that led to the judgment; these
evaluations are done with very little conscious analyses of the specific optical attributes that lead
to the decision [18].
Whereas the eye can distinguish between a low gloss reading due to a poor image
reflecting quality of the object and that as a result of surface curvature deflecting the specular
reflection, an instrument cannot. Though the eye is more versatile in assessing numerous
geometric factors simultaneously, it is subjective with observer variability. An instrument is
better able to provide repeatable evaluations than the unaided eye. Also, an instrument possesses
the ability to output geometric measurements with numbers whereas the eyes lack that ability to
numerically express attributes though it is a sensitive comparator. Most gloss devices are based
on the detection of specular gloss which is the ability of a surface to reflect light specularly [17],
but the design of instruments to make similar inferences as humans requires the knowledge of
the behavior of light and its interactions with objects. An investigation of the human response to

24

light and the decisions on the attributes of an object, defines the proper scale to use for
appearance-measuring instruments, once an established correlation with visual perception is
made [18]. Of the varying current universal methods for measuring gloss, each method for a
specific class of surfaces was designed with correlations between visual appraisals and
instrumental values of gloss. Each optically unique product requires its own unique method for
measuring gloss which is the reason for the varied American Society for Testing and Materials
o

(ASTM) angular (20, 45, 60, 75 and 85 ) methods employed in gloss instrumentation to meet
unique application requirements. Low angle methods are known to better distinguish high-gloss
specimen while the high angle methods are best known for use on low-gloss materials [18].
Two categories of instruments are employed in appearance measurements. Physical
analysis instruments like the spectrophotometer measure the physical properties of the light
distributed by the object, while psychophysical analysis instruments such as glossmeters and
colorimetric spectrophotometers are designed with information about how the observer perceives
the distributed light. The latter’s measurements are correlated to the human perception of the
attribute of interest. Both spectral and geometric considerations are made in the design of such
instruments. Spectral properties are most important in designing color-measuring devices but for
geometric devices (gloss, haze measurements) it is the angular and directional dimensions of the
instruments that are important. However, both properties must be efficiently controlled for the
accuracy and reproducibility of measurements [18]. Appearance-measuring instruments like the
glossmeters are designed with a light source, placement for the object to be measured, a light
receiver, and a signal measurement device. For glossmeters, the photodetector (light receiver) for
the specular reflection is placed at the same but opposite angle to that of the incident beam. Light
sources used in appearance-measuring devices include the incandescent, fluorescent, xenon arc
25

and laser. The incandescent source is the most common because of its low cost, ability to control
output with a power control, ability to produce a steady intensity output, among other compelling
factors [18].
The judgment of food qualities is greatly influenced by the perception of factors such as
gloss, oiliness, stickiness and softness. Using conventional glossmeters for most foods require a
destructive preparation of the surface for measurements, because the need for an extended flat
surface makes it impossible to measure gloss of the invariably curved and uneven surfaces of
food efficiently [19]. Measurement of reflectance with respect to the angular properties of the
incident and reflected can be made with a goniophotometer. A goniophotometer measures the
quantity of light reflected or transmitted as a function of the angle of incidence, providing
information about the spatial distribution and therefore geometric attributes [18, 19].
Goniophotometric properties – changes in the intensity of the specularly reflected as a function
of the incident and viewing angles – give rise to the perception of gloss of curved surfaces [19].

2.3. Postharvest Handling and Storage
Apples are harvested commercially with a range of different maturities depending on it
immediate end use. Once the fruits leave the orchard they are either packed immediately for the
consumer, or stored in refrigerated or controlled atmosphere (CA) for future processing.
Improper handling and storage aggravate defects that may already be present or initiate
deterioration, both of which lead to the loss of quality.
Produce like apples that are not field-packed upon harvesting, are picked into field
containers and transported to packing houses for storage and further processing [20]. Figure 2.1
depicts the typical unit operations used in an apple packing house for delivery to the consumer.

26

Apples

Refrigerated
or controlled
atmosphere
storage

Dump
tank

Wash
& rinse
Wax

De-water

Dry

Pack

Regular
atmosphere
storage

Grocery

Figure 2.1. A schematic layout of a typical commercial apple packing line

Apples are the predominant horticultural crop stored under CA with about 60% of its
production stored in CA for prolonged storage [13, 21]. In CA storage the atmospheric
composition is usually altered by reducing oxygen and/or raising CO2. This is a way to prolong
the storage life of fresh produce by retarding ripening through reduced ethylene production or
27

action [13]. The recommended standard CA storage for ‘Delicious’ is 0.7 - 2% O2 and 2% CO2
in 0 - 1oC. A relatively high humidity of 90 - 95% is recommended to prevent water loss and
shriveling [13, 21]. An optimized CA storage will therefore retard respiration of the produce and
allow longer storage of seasonal crops.
Packing houses in Washington State clean and pre-size fruits prior to CA storage as
observed in a recent visit. Cleaning prior to storage ensures the removal of chemical residues,
field debris and microbes (to an extent), and therefore reduces the potential decay by spoilage
agents during storage. This also provides an added benefit of minimizing the contamination of
the facility. However since the natural occurring waxy layer is partially removed during
cleaning, the fruits are more prone to rapid water loss and are therefore stored for shorter periods
in CA than fruits that are not cleaned prior to CA storage. Pre-sizing enables the packers to sort
the fruits according to their end use, and also provides a means of controlling size and weight
uniformity when the fruits are ready for further processing for the consumer. The practice of presizing reduces down-time during processing since a bin of uniformly sized fruits minimizes
differences in product quality and will invariably be directed to the same packaging and
distribution line.
Coming out of refrigerated or CA storage, fruits are introduced onto the packing line
where they are cleaned, washed and sorted prior to packaging. First the fruits are delivered from
the field bins to the packing line which is accomplished by either dry bin dumps or water dumps
into a dump tank [20]. In dry bin dumping, padding is secured over the lid and the fruits
delivered through a controlled flow opening, whereas as the name suggests in water dumping the
fruits are delivered directly into water. Submerging the bins allows the fruits to float readily and
freely in the water which serves to protect the fruits from impact damage [20]. Since dump tank

28

water accumulates fungal spores and bacteria which can infect postharvest produce wounds,
sanitation of the dump tank is important. Chlorine and water ozonization are frequently used to
control decay-causing organisms. Detergent brush washes followed by clean water rinses are
used to remove soil and other contaminants from the surface of the produce [20].
After washing, apples are dewatered and dried either by drying tunnels or heavy duty
industrial propeller fans to prepare the surface for waxing. Thorough cleaning and sufficient
dewatering are important to obtain a good wax finish. With no interference from dirt and other
extraneous materials on the surface, a good wax adhesion/setting can be achieved. However, a
good wax finish is the interaction of a clean, dry, and warm fruit. In my communication with
numerous apple fruit specialists, it was conveyed that the use of warm water in the dump tank
raises the temperature of the skin in preparation for waxing. Warm fruit minimize moisture
condensation on the surface of the fruit as they are introduced into a warm waxer. The excess
water on the fruit surface dilutes the applied coating, resulting in low quality sheen. Using a
heated dump tank not only warms the fruit in preparation for waxing but also aids in the cleaning
of the fruit surface. Therefore, the importance of having clean-warm-dry fruits entering the
waxing section of the packing line is paramount.
Currently no standard detection systems for the degree of cleanliness exist though there
are several complex methods for the determination of the presence of residual crop fungicides
and their extraction [22]. Most of these methods are, however, destructive, comprising fruit
homogenization, and pre-concentration and extraction of the residues using organic solvents.
Ethyl acetate [23, 24], acetone [25], acetone with methanol [26], and methylene chloride [27] are
some of the solvent systems used in the extraction. In the research by Ong et al. [28] using
chlorinated and ozone washes is the only non-destructive procedure reported, to the author’s

29

knowledge, that evaluates residual chemicals on whole fresh apples after they have been cleaned
by various treatments. Based on the information garnered from the above mentioned studies, we
developed a method to evaluate the level of cleanliness after subjecting the fruits to different
combinations of washing treatments. The method which utilizes a solvent extraction system by
rinsing the fruit surface with isopropyl alcohol is described in detail in Chapter 3.
Waxes derived from natural sources are applied to the fruits, after cleaning, to help
reinstate some of the properties of the removed natural wax. Applied waxes need to be dried –
usually by use of convection dryers – to achieve the proper wax properties [20]. In the industry
either cold or hot air is used to achieve drying [29], with hot air commonly used for water-based
wax emulsions [20]. Drake et al. [29] investigated the effect of the temperature used in drying
o

wax on apple quality and found that cold dried (32 F) ‘Gold Delicious’ apples tended to be
o

firmer than those that were hot dried (140 F), and also tended to loss less weight over the 90
days of refrigerated storage. In the case of ‘Red Delicious’, firmness was not affected by the
temperature of the wax dryer, and though the waxed fruits dried with the hot method tended to
lose weight the weight loss was not economically significant. Following drying, waxed fruits are
either manually or mechanically sized and sorted, after which they are either hand or
mechanically packaged. Pallets of packaged fruits are temporarily cold-stored [20] while
awaiting pickup and delivery to a consumer point of purchase.
On delivery to the consumer, a decision on the end use of the produce is made on the
basis of quality judgment from the appearance without necessarily having tasted it. Thus the
applied coating serving as an edible package not only has to serve the ultimate function of
preservation but also clearly communicate ‘quality’ by virtue of an attractive visage.

30

2.4. What is Fruit Quality?
Quality is the degree of excellence, and in the case of produce, the absence of defects [16,
30]. The role of texture, appearance and flavor in the consumers’ perception of freshness and
quality has been documented by several studies [8]. Ninety six percent of consumers surveyed by
Zind [31] indicated intrinsic attributes of taste, freshness and ripeness as the most important
selection criterion, in comparison to 94% who indicated that appearance and the condition of the
product was more important in their selection criterion [32]. Thus in grading produce, intrinsic
quality goes hand-in-hand with visual quality.
The goal of production, handling, storage and distribution is to provide consumer
satisfaction and is critical to product quality [6, 30]. For red colored apples, quality is based on
the intensity and characteristics of the red skin - their appearance [33]. However, the complete
reliance on appearance to judge quality sacrifices critical attributes like firmness, color and
aroma. After successfully jumping the hurdle of appearance, the consumers’ acceptability and
preference of apples are highly influenced by price [6] and prior experience with firmness [34]
which is said to be the consumers’ primary edible quality factor contributing to their choice of
fruit. An European study by Péneau et al. [8] reported that the age of consumers and the
frequency of consumption of apples had an influence on the perception of freshness based on
appearance; consumers eating less than 2 apples per week gave more importance to appearance,
and the importance attached to appearance decreased with increasing age. The understanding of
the consumers’ perception and preferences of apples and produce in general requires the
integration of economics, marketing, psychology, postharvest and sensory science [6].
Quality is used frequently in the post-harvest world but according to Shewfelt [30] it is
rarely defined and there are as many concepts as there are many perspectives on postharvest

31

handling. The quality of produce changes from harvest time to consumption, and the relative
importance and meaning of quality changes at each stage of production [30, 35]. Both subjective
and objective factors define quality depending on the party of interest. Culture, economics,
psychological, ethical and religious views influence the subjective while the organoleptic and
physicochemical characteristics and food safety influence the objective. All these together create
a wide and varying concept of quality [36]. The producers’ view of quality is good appearance
with minimal defects in a high yielding, disease resistant, easily harvested commodity with good
shipping qualities. To market distributors, appearance is paramount to quality while attaching
importance to firmness and long storability. Appearance, firmness, good flavor and nutritive
value are the attributes the consumer uses to judge quality [16]. The limiting factor in delivering
quality produce to the consumer is therefore a lack of appreciation of the different perspectives
of quality which stems from orientation. Product-oriented quality which is the focus of
postharvest researchers, producers and handlers considers the intrinsic attributes of the produce
such as taste and firmness whilst consumer-orientation, which is the focus of consumers,
marketers and economists, considers the needs and wants of the consumer [30]. It is the merger
of both product- and consumer-orientation of quality that will produce optimum produce quality.
This study aimed at achieving both.
To summarize, the overall quality of the fruits is greatly influenced by the preharvest
conditions, the stage of maturity at harvesting, the harvesting methods and postharvest handling
[16, 37, 38]. Quality results from a complex interaction of preharvest factors spanning from
cultivar and rootstock genotype, mineral nutrition during growth, irrigation, and canopy
manipulations to crop rotations [38]. According to Crisosto et al. [38] the selection of the correct
genotype for specific growing conditions will ensure maximum postharvest quality. A mature

32

fruit is one that has completed natural growth and development, and is the “stage at which a
commodity has reached a sufficient stage of development that after harvesting and postharvest
handling, its quality will be at least the minimum acceptable to the ultimate consumer” [39]. It is
therefore recommended to pick fruits at optimum maturity for a greater chance at ensuring
maximized fruit quality from harvest to consumption, while keeping in mind that poor
temperature management reduces the quality and maximum potential shelf life of produce [16,
40]. Fruits harvested too early or too late in the season are more susceptible to physiological
disorders and have shorter storage life than those picked at optimum maturity –e.g. fruits picked
immature may not fully ripen and those picked over-mature tend to be softer, easily damaged and
more susceptible to senescent breakdown [13].
The quality of fresh produce is also limited by the internal gas concentrations; the ease
with which gases diffuse through the peel of the produce plays a vital role in the preservation of
quality [41]. When the supply of O2 required for respiration and the escape of CO2, a bi-product
of respiration, are blocked, the interior O2 and CO2 are reduced and raised respectively. This
alteration in interior gases results in variation in product quality [41], and therefore makes it
important to maintain internal gas concentrations within limits, irrespective of whatever
processes are applied to the whole fruit in preparation for marketing and to improve consumer
appeal, that do not cause unacceptable changes in quality.
Since quality in the produce industry is complex and its concept varies with different
markets and stages in the supply chain, certain controls and measures are set in place to ensure
the consistency in delivery of quality commodities from farm to fork.

33

2.4.1. Quality Assurance and Control
Dealing with apples which lack uniformity of color in some varieties, size, shape, and
general organoleptic properties pose a challenge for quality control and assurance. Another
challenge is the different concepts of quality in different markets. Quality assurance is the
process designed for correct implementation of specific steps to ensure that quality control
measures are met, while quality control is the process designed to ensure the delivery of adequate
quality through product evaluation. Quality assurance and control protocols are set in place to
monitor, evaluate and ensure that quality standards are met and also to guarantee the continued
delivery of quality fruits to the consumer.
Quality control starts in the field and continues through the supply chain to the end user
with each step having the potential to either maintain or reduce quality [10, 16]. Very few
postharvest procedures can improve quality which makes it essential that a commodity is
harvested when it will provide maximum quality, and that harvesting is carefully carried out to
minimize injuries to ensure the maintenance of the quality at harvest [16]. Hazard analysis and
critical control points (HACCP) is fundamental and forms the basis of the majority of control
systems enforced [10]. It is used to identify and assess potential hazards and risks, and to
establish process steps (critical control points) to control, minimize and eliminate the occurrence
of such hazards and risks.
Both manual and automated systems are used on most packing lines for quality
management. Hand-sorting is usually used to separate products on the basis of color, size,
surface blemishes and grade. Small fruits and vegetables require more sorting decisions than
larger commodities; the use of systems that turn and rotate eases the sorting decision as they
provide surface visibility from all angles. In manual grading, however, it is important to

34

adequately train workers and minimize monotony and fatigue for consistency and accuracy [10,
20]. Automated systems include sorting of fruits by weight or dimension, and detection and
elimination of defective units using size sensors and electronic imaging and/or light reflectance
systems respectively. The latter is also used for color sorting. X-rays or light transmittance are
used for the detection of internal defects [16, 20], and non-destructive testing for internal quality
of texture and flavor are also monitored [10]. The units which do not meet the set standards are
diverted onto another lane where they are graded accordingly.
Final sorting prior to packaging, as a quality control measure that quality standards have
been met, is practiced in some facilities [20]. Packaging, an important quality management step,
is accomplished either manually or mechanically. It is critical that a commodity is correctly
labeled with variety, size and grade, and packed into the right containers. To meet legal grades
and quality standards, the product and packaging are inspected before final padding and closure
of the package [20]. Governmental agencies also play a role in the assurance of quality with their
primary role being the assurance of the delivery of pest- and disease-free products to both local
and international markets, thus ensuring food safety which is the primary non-commercial
product assurance [10].
Other quality control measures prior to packaging include frequent changing of the water
in the dump tank to minimize the buildup of microbial contaminants and ensure efficient
cleaning not only in preparation for waxing but also for the assurance of food safety. Using
warm water in the dump tank to help raise the surface temperature of the fruit is another control
measure to facilitate the efficient interaction of the wax with the fruit surface for a good coating.
Frequent cleaning of the waxing brushes is also practiced to prevent the drying of the wax in the
brushes which renders them hard and inflexible not only causing injury to the fruit but also

35

inhibits the ability to spread the coating evenly. Wear and tear on brushes is closely monitored to
facilitate timely replacements to help prevent commodity injury from worn out brushes.
With measures in place for quality assurance and control, quality attributes are evaluated
to verify that standards have been met.

2.5. Quality Attributes and their Evaluation
An attribute is a quality or characteristic of the commodity in question. These
characteristics usually exhibit changeable properties and several methods have been designed for
their measurement and evaluation. The ease of measurement and evaluation varies with the
accumulated knowledge base for its properties and also in the range in which it is measured.
Quality attributes are categorized as extrinsic and intrinsic, with the extrinsic factors playing a
critical role on the judgment of intrinsic factors. The extrinsic quality factors are mainly, but not
limited to, appearance (color, size, gloss, surface blemishes). Texture (firmness, juiciness) and
flavor (aroma and taste - sweetness, sourness) form the basis of intrinsic quality factors. Various
equipment or techniques exist for quantifying these attributes to enable an efficient analysis of
their effect on the perception of produce quality. Due to the high potential of individual units of
the same crop being different in quality attributes, several fruits are required for measurement to
minimize the variations that may arise. Table 2.1 lists some of the research and/or commercial
methods used for quantifying quality attributes.
Gloss, weight, and firmness preservation are the quality attributes of interest in this study.
Gloss, which is a measure of the shiny fruit surface, is derived from the application of wax. The
wax layer creates a barrier to gases and its characteristics determine the level of gloss. The gloss
level and barrier properties are highly dependent on the thickness and uniformity of the layer.

36

Table 2.1. Methods for quantifying quality attributes
Equipment/Method(s)

Attribute*
Appearance
Color
Gloss/Sheen
Defects/Disorders

Reference(s)

Chromameter
Reflectometer
Glossmeter
Visual inspection

[42], [33]
[43], [44]
[42], [44]

Texture
Firmness/Crispiness Penetrometer
Texture Analyzer

[15], [33]
[34]
[37], [45]

Gas chromatography
Gas chromatography
Gas chromatography
Gas chromatograph

[15], [41]
[43], [37]
[42], [43], [41]
[43], [44]

Acoustical Firmness Sensor

Aroma/Volatiles
Ethylene
O2 consumed
CO2 produced
Ethanol content
Taste
Total soluble solids

Refractometer
[42], [43]
Near Infra-Red
[34]
Titratable acidity
Titration with NaOH
[33], [45]
pH meter
*All of these attributes in some studies were also evaluated by a consumer panel

The barrier directly influences the uptake and release of respiratory gases and water vapor; a
thicker and uniform coating layer provides a higher barrier compared to a thinner and an uneven
coated surface. The rate of respiration and transpiration affect the maintenance of weight and
firmness; high respiration and transpiration rates result in a rapid loss of weight and firmness.
It is therefore important to evaluate peel permeability to better understand respiration
rates to facilitate a correlation to changes in gloss, which will also be correlated to weight and
firmness.

37

2.5.1. Fruit Respiration and Peel Permeability
Respiration is a metabolic process which involves the oxidative break down of organic
reserves to provide energy and carbon skeletons for biochemical processes. The oxidative break
down of the stored organic substances results in the formation of simpler molecules such as CO2
and H2O with the release of energy [10, 46]. Glucose, a common substrate for respiration, if
completely oxidized is represented as
C6H12O6 + 6O2

6CO2 + 6H2O + Energy

Respiration is influenced by both intrinsic and extrinsic factors. The type of commodity and the
stage of maturity are two of the intrinsic factors that affect respiration. Climacteric commodities
have high respiration rates at the onset of development which declines until the climacteric rise
occurs with ripening or senescence. On the other hand non-climacterics exhibit high respiration
rates early in development but the rate declines steadily during maturation. Temperature is the
most important extrinsic factor affecting respiration. Enzymatic denaturation may occur at high
temperatures resulting in reduced respiration, and at low temperatures physiological injury may
occur, leading to an increase in respiration [46]. Oxygen and CO2 concentration as well as
physical stress such as wounding are other extrinsic factors that affect respiration. A reduction in
O2 to less than 10% (by volume) is what is required to achieve a reduction in respiration.
However, this reduction is temperature dependent such that when temperature is lowered the
oxygen requirement is reduced [10, 46].
Respiration rate is an excellent indicator of ongoing metabolism and therefore provides a
useful tool to estimate the quality profile [10]. Three common methods are used in the
determination of respiration rates: the static or closed; flow-through or flushed; and the
permeable systems [46]. In the closed system, changes in O2 and CO2 concentrations in a gas
38

tight container, of known volume, are measured and used to estimate respiration rate. In the
flow-through system, a gas mixture flows at a constant rate through an impermeable container.
Generally it is the absolute difference in O2 and CO2 concentrations between the inlet and outlet
when the system is in steady state that is used to calculate respiration rates. In the permeable
system, steady state O2 and CO2 concentrations in a package of known permeability and
dimensions are measured. Respiration in this system is estimated based on mass balance and the
other measured variables [46].
Atmospheric O2 must pass through the peel of the fruits to the interior for normal
respiration and CO2 produced by respiration must also escape through the peel to be discharged
from the interior of the fruit to minimize variations in quality [41]. This phenomenon is highly
influenced by the peel permeability, which is altered early in processing through washing and
waxing and results in changes in the normal concentration of internal respiratory gases [41, 47]
The changes in the peel barrier properties occur in two ways [41]:

Diffusion through openings such as the lenticels, stomata, stem scars and injuries in the peel



Classic permeance – dissolution of a gas into a barrier on the high concentration side,
diffusing through the barrier and exiting on the low concentration side.

Permeation through a membrane or coating is the diffusion of a permeant through the
layer which is then sorbed/desorbed from/into the external/internal environment [48]. The
diffusion and solubility properties therefore determine the mass transfer behavior of a material
which is expressed as the permeability coefficient (P). The permeability coefficient is therefore a
combination of the effects of the diffusion (D) and solubility (S) coefficients, and determines the
barrier properties for the material in question [49]:
39

P=DxS

(1)

D describes how fast the molecules of the permeant move through a unit area of the membrane
while S is the quantity of molecules sorbed by the membrane. At steady state, the former is a
function of temperature and independent of concentration; this behavior is described as Fickian.
Henry’s law of solubility is what describes S, which is also temperature-dependent and holds in
situations where there is no pemeant-polymer interactions at low concentrations and low partial
pressure [48, 49].
The permeability coefficient is defined as the quantity of permeant dissolved and sorbed /
desorbed per unit area, per unit time at a specified pressure gradient and through a specified
thickness:

P

ql
Atp

(2)

Where P = Permeability Coefficient
q = Quantity of Permeant
l = Thickness of Medium
A = Surface Area
t = Temperature of the Environment
∆p = Pressure gradient

Because of the high tolerance of ‘Red Delicious’ (the dominant cultivar used in the
development of fruit coatings) to high gas barriers, apple coatings have focused on development
of visual gloss with little on the needs that arise from the effects of high barrier to gaseous
exchange [43]. It is well documented that coating applied to the fruit surface serving as a
protective layer may also serve as a barrier to gaseous exchanges with the atmosphere [41, 43,
40

47]; as a result respiration may become unaerobic with undesirable changes [20, 43]. On the
basis of this, investigations have been conducted on the changes in the internal atmosphere of
fruits with waxing and the development of new coatings with improved barrier properties [41,
43, 50, 51].
As expected, coatings with lower permeabilities result in low internal O2 and high
internal CO2 [43, 50, 52]. These reports confirm a 1953 study on the effect of skin coating on the
behavior of apples in storage by Trout et al. [53]. Trout and his team reported that “coating
increased the resistance of the skin to gaseous diffusion and thus greatly reduced the internal
oxygen concentration, increased the internal carbon dioxide concentration, reduced respiration
rate and retarded ripening changes by varying degrees”. Gaseous diffusion occurs through the
pores and the cuticle of the fruit peel. The application of surface coatings increase the resistance
to gaseous exchange by covering the cuticle and pores of the skin which culminates in decreases
in the transmission rates of gases between the internal and external atmospheres [54]. Thus
surface coatings, through these effects, have the tendency to modify the composition of the
internal atmosphere, suppress the respiration rate and reduce transpiration in fruits [54].
A recent study by Hagenmaier [41] showed that high barrier coatings not only caused
significant decreases and increases in internal O2 and CO2 respectively, but also showed much
higher variations in the internal gas concentration of different individual fruits with the same
coating in comparison to different individual non-coated fruits. This is a classic indication of
higher variation in the quality of fruits coated with high barrier waxes. The variability in the
individual fruit response to a coating treatment is due to the inherent variability in the fruit skin
resistance to gas diffusion and hence respiration rate [54]. The variation in the proportion of
blocked pores, by coating, as well at the extent to which the coating adheres to the fruit surface,

41

especially along the edges of the pores, account for differing fruit response to the same coating
treatment. A loosely adhering coating offers the opportunity for gaseous exchange in the space
between the fruit and the coating, whereas the opposite applies in the situation of a tightly
adhering coating. Essentially coating a pore (blockage) eliminates its contribution to gaseous
exchange [54]. The extent of internal atmospheric changes depend on the storage temperature,
thickness and type of wax used, the viscosity at which it is applied and also the variety, peel
anatomy and condition of the fruit [52, 53, 55]. The wax characteristics are important as these
factors influence the blocking of pores on the peel to gases [56]. Viscosity, a controllable factor
is particularly important since flow properties influence the distribution and coverage of coating
on the fruit surface. It therefore affects the thickness of the coating layer and hence the barrier
properties which influence quality.
Coatings can create a modified atmosphere similar to CA which may change in response
to environmental temperature and relative humidity, and a combined effect of fruit respiration
and coating permeability [50], thus having a direct impact on produce quality. The effects of
storage temperature and humidity have been shown over and over again to affect fruit quality.
High humidity will not prevent moisture loss if the temperature of the produce is not near that of
the air temperature [55], and it also increases the permeability of shellac-based coating to O2 and
CO2 [57]. Low temperature storage not only extends the shelf life of temperate fruits but also
protects non-appearance quality attributes such as texture, nutrition, aroma and flavor [55].
Studies by Hall et al. [58] report that the temperature and the maturity of the fruit are the most
important factors that influence the storability of coated fruits.
The first few days after coating are said to be the most critical period, where a rise in
internal carbon dioxide concentration was observed. Many other studies have also shown the

42

changes in storage temperature and humidity on the significant demand for respiratory O2
indicating how environmental conditions affect fruit respiration and coating permeability [50,
59]. Outside the range of proper temperature maintenance, the loss in apple firmness is
concomitant with an increase in skin color indicating a direct effect of temperature on the
physiological activities of ripening and changes in firmness [55]. Only 7% of apples are held at
the recommended and required temperature storage at the retail level; 97% of the fruits
experience temperature above the recommended range making them victims of the greatest
temperature abuse [55, 60]. However, the robust nature of the fruit reduces the rate of quality
loss unlike produce like strawberries which perish rapidly in abused temperature conditions. The
rate of mass loss depends to a great extent on the nature of the peel surface of the commodity
type and cultivar [55].

2.5.2. Firmness and Weight
Firmness is the primary edible quality factor for consumer preferences and acceptance of
fruits [34, 61]. Loss of firmness as a result of softening is characterized by fruit ripening [62].
During ripening, firmness declines partly as a result of cell wall disassembly and degradation of
the polysaccharides of which it is composed [63, 64] due to enzymatic activity of
polysaccharide-modifying enzymes secreted into the cell wall from the symplast [64]. It has been
extensively documented that these enzymes including cellulase and pectinase are mostly
hydrolases cleaving their primary substrate using water, their secondary substrate [62, 65]. The
cell wall provides rigidity and adhesion of intercellular structure, and the extent of its
modification has a direct bearing on the degree of textural changes. Overall, the declining
strength of the cell wall and intercellular connections will determine the firmness and textual

43

changes of a fruit [64]. The cell wall is composed of a network of polysaccharides, proteins and
some phenolics. The wall polysaccharides are categorized as pectins, hemicelluloses and
cellulose [65], and though the component varies among species the generally composition is of
equal amounts to about one-third of the dry weight, while structural proteins make up only about
1 to 10% of the dry weight [64]. Pectins however form about half of the polymeric content of the
cell wall [64]. Pectin is a major constituent of the middle lamella and contributes to cell adhesion
[61, 62]. The components of the cell wall are linked by numerous bonds including hydrogen
bonds, ionic calcium bridges and ester bonds [64, 65]. The degradation of cell wall polymers
collapse these adhesive bonds leading to loss in firmness [62].
The cell wall is highly hydrated with dissolved solutes, ions and soluble proteins
including enzymes [64, 65] and the decline in firmness with ripening is also attributed to
reduction in tugor pressure. This is the result of water loss by the fruit and accumulation of
osmotic fluids in the apoplast, which together reduces the expansionary pressure on the walls
cumulating in textural changes [64, 66]. Depolymerization of xyloglucans and the solubilization
and depolymerization of polyuronides [62, 64] also result in the loss of tissue firmness and
ripening of fruits. This is due to a loss in intercellular contact as the cell wall is more open and
hydrated causing softening of the fruit tissues [64].
Relative humidity is crucial in determining moisture loss, and it has been shown in the
case of apples that low RH more than temperature is the main cause of deterioration [67]. The
rate of water loss from a commodity is dependent on the water vapor pressure deficit which is
the difference between the actual vapor pressure and the saturated vapor pressure [55]. Reduced
water loss prevents shriveling and textural changes that lead to a loss in weight and firmness. In a
study on mangoes, a climacteric fruit exhibiting rapid ripening after harvest just like apples, the

44

firmness of coated fruits was found to be significantly different from that of the uncoated [51].
Coated mangoes required 7.0 – 5.3 N to compress 2 mm compared to the significantly different
2.8 N needed to compress the same distance for uncoated fruits. The coating, by virtue of
reinforcing the moisture barrier properties, impacted physiological changes in the cell wall which
translated to the retention of firmness. Meng et al. [63] in a study on peaches also found firmness
to decrease rapidly at higher storage temperature. Fruits stored at 10oC softened rapidly with a
significant decrease in firmness compared to 5oC storage with an effective maintenance of
firmness.

2.5.3. Gloss
The subject of gloss has been discussed in detail in section 2.2, but this sub-section on
gloss will focus more on the application of waxes to achieve a glossy surface and also relate the
discussion in section 2.2 to the quality attributes of weight, firmness and respiration.
Coating apples impacts gloss, the shine on the fruit surface that gives the fruit an
attractive appearance. The application of coatings to apples is an essential unit operation in the
packing of the fruit as it not only enhances visual appeal and quality perception, but it also
improves preservation as discussed in the previous section. Shellac derived from the secretions
of an insect, Tachardia lacca [68], and carnauba from the leaves of the Cerifera palm [69] are
the two commonly used waxes in the fruit industry. Since glossy surfaces are considered
attractive, fruit waxes are formulated to be glossy [19]. A shiny surface may not be valued for all
produce but is valued for apples [11, 12] and is beneficial for its sale [56]. Apples are assessed
for quality based on surface texture, color, translucence and gloss – as part of its visual structure
[19].

45

Surface gloss, a measure of specular reflectance is dependent on the light source and its
intensity, as well as the surface characteristics of the commodity [12, 70, 71]. It is one of the
main visual attributes that influence the evaluation and grading of fresh produce [71], though a
good shiny looking commodity may not necessarily have a direct correlation to freshness and
quality, and therefore doesn’t guarantee a tasty and crisp product. Aside from the intensity of the
light source and surface morphology of the commodity, gloss is also dependent on the uniformity
and thickness of the coating. Applying a viscous wax or voluminous amounts increases the layer
of coating which translates to higher gloss. However a viscous coating with reduced flow
properties may not provide an even distribution and coverage on the fruit and the inconsistencies
in the coating layer may translate into lower gloss.
To make objective evaluations of gloss levels to successfully correlate it to visual appeal
and quality, physical measurements are required. Gloss measurements, as discussed in section
2.2.2, are often made by projecting a light beam at a specified angle onto a surface and tracking
the amount of reflected light [70]. Numerous devices exist for measuring gloss but most are
designed for flat surfaces which would mean a modification to the device to efficiently and nondestructively measure the gloss of uneven and curved surfaces like those found in fresh produce
as was reported in recent studies by Bai and his team [43, 72].
Commercial glossmeters designed for curved and uneven surfaces have evolved in recent
years but there is limited data on their efficiency of measuring gloss of fruits and vegetables [70].
Research by Nussinovitch et al. [12] report the successful construction of a device capable of
measuring gloss on a variety of produce including oranges, bananas, onions, eggplants and
tomatoes. Mizrach and his team [70] also recently developed an automated device for measuring
the gloss of apples, with a repeatability error of 16% for 90% of their measurements. This

46

automated device was initially used to measure gloss of the apples used in this study, but because
of the complexity of the mechanical automation, a new but simpler device operating on similar
principles to that of Mizrach et al. [70] was constructed in the laboratory of Dr. Lu. Details on
the features and operation of the prototype gloss device are given in Chapters 3 and 4.
Although the coating barrier, by impacting physiological activity, is able to slow down
the deterioration process, changes in the degree of gloss are observed with prolonged storage and
associated with changes in other quality attributes. An example of this is the observation that
decreased gloss of banana peel indicated a loss of weight during storage [73]. This observation is
attributed to the fact that gloss is related to surface composition and morphology, and thus a
change in morphology results in changes in surface gloss [71]. Studies over the years like that of
Hagenmaier et al. [47] report the decay of gloss from shellac with storage. The same authors
investigated the ethanol content (associated with off flavors) with waxing and found that shellac
coated fruits had higher ethanol content and this is a sacrifice of flavor for appearance. Bai et al.
[56] also report the decay of gloss from six different waxes over a 4-week testing period.
Investigations by Fellman et al. [74] on the effect of refrigerated storage on the changes in
appearance of waxed ‘Delicious’ fruits showed that cold drying of wax caused greater whiting
than hot drying for both shellac and carnauba. The incidence of shellac whiting on cold dried
apples was observed to decrease with a month of refrigerated storage but increased after 2
months. A similar pattern was observed in the hot dried fruit with a relatively smaller amount of
wax coating. The authors suggest that “a naturally occurring cuticular component of apples
influences the whiting reaction of shellac waxes”. A ripening period (ambient temperature
storage) of 8 days, however, totally eliminated the incidence of whiting. This they explained was
the replacement of a plasticizer in the shellac formula by natural substances, and a decrease in

47

natural wax synthesis of waxed fruits. They recommend holding waxed apples in refrigerated
storage for less than one month after packaging to minimize whiting.

2.6. Waxes and Packaging
The practice of enrobing fruits and vegetables with edible coatings was reported in the
twelfth and thirteenth centuries in China where wax was used on oranges and lemons. Although
the mechanism of preservation was unknown to the Chinese back then, the process was sustained
due to the realized shelf life extension of the coated compared to the uncoated [75]. It wasn’t
until the 1930s that hot-melt paraffin became commercialized for use as edible coatings on
apples and pears [75]. Edible packages are mostly films composed of a polymer base and serve
to function in the same way as inedible packages. As the name suggests an edible film aside from
providing packaging functionalities is considered safe for human consumption. Edible films are
found in a variety of food categories including wax coatings for fruits and vegetables, casings for
sausages and chocolate coatings for confectionaries. They are also very common in the
pharmaceutical industry in enteric protective coatings and shells for capsules.

2.6.1. Natural Wax Coating
Natural waxes by virtue of their numerous useful properties have been used by mankind
for centuries. Waxes in nature function to protect plants from destructive environmental
influences such as microbes, drying and chemicals. They have been found to be highly resistant
to chemical and biological degradation, and hence its use in food protection [76]. Natural waxes
on fruit surfaces function to reduce the loss of moisture and provide water repelling properties to
the fruit, which are important to retain chemicals that are applied to the skin [77].

48

Natural waxes are small crystals on the surface of the fruit and the chemical composition
of the wax determines the shape of the crystals [77]. The crystals are highly impervious to water
vapor and gaseous exchanges with the barrier properties highly dependent on the intercrystalline
lattice; tightly packed crystals are less permeable to gases in comparison to a loosely packed
lattice [78]. Figure 2.2 below depicts the characteristic crystals on apples. Light reflection and
scattering by the waxy crystals on the fruit surface is responsible for the perceived surface
appearance [77]. This principle is the same as that on which the perception of gloss from applied
waxes work.
With growth and maturity the natural wax on fruit increases, but its quality and quantity
changes during postharvest [77, 79]. There are however a few varieties that have been
investigated to record an initial increase in surface wax during storage but decreases after about
the 3rd month in storage. These apple varieties include the ‘Calville Blanc’, ‘Cox’s Orange’ and
the ‘Dougherty’ [79].

Figure 2.2. Electron micrograph of the surface of apple fruit showing platelet wax crystals.
Source: Kolattukudy [77]

49

Natural waxes contain mostly straight-chain carboxylic acids and alcohols in the region of ca.
C12 – C36 [76], and apple surface waxes are reported to be composed of hydrocarbons, alcohols,
fatty acids, ursolic acid and α-farnesene [79]. Increases in the various components were
associated with a corresponding increase in surface wax content. The main hydrocarbons in a
study on ‘Granny Smith’, ‘Dougherty’ and ‘Sturmer’ cultivars are recorded to be n-nonacosane
and n-heptacosane. Alcohols from C14 to C28 were also reported with

n-tetracosanol, n-

octacosanol and n-heptacosanol being the most abundant [79].
Earlier studies by Morice et al. [79] report the additional findings of numerous earlier
studies indicating an increase in the fatty acid fraction of several apple cultivars during storage.
Of the three cultivars studied by Morice et al. [79] it was found that with the exception of the
‘Sturmer’, the linolenic acid content of the ‘Granny Smith’ and the ‘Dougherty’ increased with
storage. The linoleic acid in the Dougherty’ and ‘Sturmer’ showed no trend during storage but
increased in the ‘Granny Smith’. There was however a decline in the levels of oleic acid with
storage for all cultivars. The amount of stearic acid also increased for all cultivars, in contrast to
plamitic acid in the ‘Granny Smith’ and the ‘Dougherty’. The higher saturated acids components
showed little changes during storage and the ursolic acid component was reported to increase
with the total wax content.
One of the major differences between the natural and commercial wax is the absence of
ursolic acid which is the major cyclic component of apple fruit wax [77]. Commercial waxes are,
however, able to provide similar functions of the natural wax, which leads to a discussion on the
details of their composition and functionalities.

50

2.6.2. Coatings as Edible Packaging
It is estimated that 25 to 80% of fresh produce go to waste due to spoilage as reported by
various authorities [80], but with the understanding of the respiratory mechanism of produce,
several techniques have been successfully developed to extend their shelf life [75]. Control and
modified atmosphere storages are examples of techniques that have been successfully used to
minimize quantity and quality losses in fresh produce. The use of edible coatings as an enrobing
layer on fresh produce not only provides a semipermeable barrier to gases and water vapor, but
also serves an alternative to modified atmosphere storage. It therefore modifies and controls the
internal atmosphere resulting in an extension in shelf life [75, 80] through reduced water loss and
respiratory activity.
Edible films serve as barrier to gases and water vapor by being positioned either on the
surface of food as protective coating, or within the food to separate components with different
water activities thereby preserving the textural properties of each component [81]. Edible
coatings also function to improve the structural integrity of food products like that of pizza
topping during distribution, and also provide physiological and physical protection to products
like fruits and vegetables which are susceptible to injury during processing and transportation
[80]. Incorporation of food additives such as antioxidants, antimicrobials, among many others
into edible packages allow localized and controlled release rate of these additives into the
packaged product. The two main advantages of coating fruit have been identified by Banks et al.
[54] as:
1.

Coatings by virtue of being in close contact with the fruit surface have the same

temperature as the fruit surface. This eliminates condensation, arising from high humidity
conditions, which causes rotting and is limiting to the application of MA packaging technologies.

51

2.

Coatings serve as a form of packaging and can reduce deterioration. This holds an

advantage in markets which are green-conscious in minimizing food packaging waste [81].
According to Donhowe and Fennema [81], hydrocolloids, lipids, and composites are the
three main categories of edible films and coatings with their possible functions listed in Table
2.2. Hydrocolloid films possess good barrier to oxygen, carbon dioxide and lipids, but not to
water vapor due to their hydrophilic nature. Films and coatings from hydrocolloids can be either
carbohydrate- (natural or chemically modified starches, gums, etc.) or protein- (gelatin, casein,
gluten, etc.) based. Water solubility of hydrocolloid films become advantageous when the film
will be consumed with a product heated prior to consumption [81].
Films composed of lipids are often used as barriers to water vapor, and as gloss additives
to products. Lipid films as stand-alone are limited because of the lack of sufficient structural
durability; they are therefore used in conjunction with supporting matrixes [81]. Composite films
are composed of both hydrocolloids and lipids, and combine the advantages of each while
minimizing the distinctive disadvantages of each. They can exist as a bilayer or a conglomerate
where the components are interspersed throughout the coating [81].
Just like any product edible packages are not without the negatives. According to Park
[75] consumers tend to be wary of commodities for which they detect waxy coatings and
therefore suggests the developments of coating that impart minimal detection of a waxy taste.
This wariness stems from the association of waxes with non-food uses [72]. Low O2 and high
CO2 disorders as a result of the modification in the internal atmosphere have been linked to
edible coatings. This observation makes it vital for the identification of edible coatings that
produce a favorable modified internal atmosphere with desired quality. This can be achieved by
selecting the appropriate coating that gives the selective desired internal gas composition specific

52

for a product [75] thereby ensuring the achievement of the possible optimal positive benefits
derived from using edible coatings. The gas permeation properties of the coating, internal gas
compositions of the coated fruits, and the coating effect on quality changes must be studied to
effectively select the appropriate coating for a specific produce [75].

Table 2.2. Possible Uses of Edible Films and Coatings
USE

APPROPRIATE TYPES OF FILM

Retard moisture migration

Lipid, composite

Retard gas migration

Hydrocolloid, lipid, or composite

Retard oil and fat migration

Hydrocolloid

Retard solute migration

Hydrocolloid, lipid, or composite

Improve structural integrity or handling properties

Hydrocolloid, lipid, or composite

Retain volatile flavor compounds

Hydrocolloid, lipid, or composite

Convey food additives

Hydrocolloid, lipid, or composite

a

a

A composite film consists of lipid and hydrocolloid components combined to form a bilayer or
conglomerate.
Source: Donhowe and Fennema [81]

Since the package is what attracts the consumer and it’s the consumer’s first point of
interaction with a product, it is not enough in the fruit industry for edible coatings to successfully
serve the function of preservation. It is highly important that it is also able to efficiently
communicate “quality” to the consumer. Waxes have been applied to tree fruits for generations
and in recent times coating fruits prior to marketing has become standard practice with the
‘Delicious’ being the key variety for the development of coatings [43]. An emphasis on coating
development is controlled gas exchange with the attempt to create a modified atmosphere within
the fruit to delay ripening and senescence similar to the more costly CA storage [80].
Experiments on storage and fruit weight loss of waxed and unwaxed apples conducted by Drake
53

[29] have shown that waxing is important in reducing weight losses when fruits are stored for
longer periods. Thompson et al. [20] report about one-third reduction in water loss by waxing
fresh produce. These reports also showed that equally good quality attributes were derived from
using either shellac or carnauba. The amount of coating, however, applied to fruits is negligible
compared to the natural wax present [77]. Studies on the amount of wax deposited on apples
using 5 different apple coating formulations by Kolattukudy [77] showed that the amount added
by the waxing process was insignificant such that the increase over that naturally present was
negligible. The amount of wax (ppm) on the untreated fruit was 994 which reduced to 973 when
washed. The application of wax, after washing the fruit, only increased it by 3units to 978.
Several studies have also been conducted on the coating of apples with natural occurring
waxes to achieve an improvement in the visual appeal and marketability through an extension of
shelf life with the appropriate internal atmosphere modification and maintenance of desired
quality attributes. An earlier study of commercially available coatings reports shellac-coated
fruits to have a low permeability to gases with the lowest and highest internal O2 and CO2
respectively [50, 57], and also a significantly less weight loss compared to other treatments [50].
Shellac of all the coatings provided the most gloss with the least quality changes; this depicts the
direct relationship between gloss, respiration rate and changes in weight and firmness. Carnauba
wax without the problems of whiting of the finish provided a lesser gloss compared to shellac
[72]. In view of this a lower gloss may compensate for the risk of higher gloss and whitening in
humid environments. However, it has been found by horticulturists that shellac coating is
excellent for dark Red Delicious apples as it “impacts gloss, hides bruises and forms a modified
atmosphere condition that tends to preserve firmness and prolong shelf life for this variety” [43].

54

2.6.3. Rheological profile of edible fruit coating
Gloss and related quality attributes are linked to the wax viscosity as discussed in
previous sections, 2.5.1 and 2.5.3. Waxes have different physical states ranging from the solid to
the liquid form, and different chemical properties ranging from the homogenous pure state to the
multiphase emulsion system. The differences in the various properties indicate different flow
behaviors. For the focus of this research it was important to study the rheological properties of
the apple coatings by determining the flow behaviors and spreadabilty in relation to its
interaction with the surface of the fruit under different simulated waxing conditions.
Rheology deals with the science of the deformation and flow of matter, and is the study
of the behavior of materials under applied stress or strain. All materials have rheological
properties which can differ for materials within the same product category. It’s the rheological
properties that govern the behavior of materials in multiphase systems and in processes like
spraying, spreadability, and adhering, just to mention a few [82, 83]. Flow properties are
described by a fluid model which is a mathematical equation derived from fitting experimental
data to a statistical curve. Fluid models are either described as Newtonian or non-Newtonian
characterized mainly by their relationship between shear stress and shear rate.
The shear stress is the force required to maintain velocity/flow of the coating per unit
area, and the shear rate is the velocity relative to the available distance of flow. The behavior that
is independent of time exhibiting a linear relationship between stress and strain and has no yield
stress is described as Newtonian. All other fluids not showing the ideal viscous behavior are
termed non-Newtonian [82, 83]. Rheological data conforming to Newtonian flow show the
relationship between viscosity, shear stress and shear rate as [82]:-

55

.

  

(1)

Where σ = shear stress (Pa)
µ = absolute viscosity (Pa.s)
.

-1

 = shear rate (s )

The power law model has been successfully used to describe the behavior of majority of
non-Newtonian fluids [82]:.

  K

n

(2)
n

Where K = consistency coefficient (Pa.s )
n = dimensionless flow behavior constant
.

-1

 = shear rate (s )
When n=1 the power model becomes a Newtonian model and K = µ.
In the case n > 1, the flow behavior is described as shear-thickening or dilatant. The flow is
considered shear-thinning or pseudoplastic when 0 < n < 1.
Viscosity modeled after the power law is described as the apparent viscosity (ɳ):



.n

K
 
.
.



. n 1

 K

(3)



Apparent viscosity is dependent on the numeric value of K and n, and changes with shear rate
[82]. Non-linearity can therefore be said to be an indication of changing viscosity with shear rate
and point to non-Newtonian behavior.

56

According to Steffe et al. [82], the power law model is appropriate for most nonNewtonian flow and provides a good estimation for the flow behavior of materials with a
significant yield stress. However for highly viscous materials with substantially large yield
stresses more complex rheological models such as the Bingham, Herschel-Bulkley and Casson
equations may be used. Materials described by the Newton and power law models have
rheological properties that are unaffected by the mechanical effects of the systems used and are
thus considered as time-independent [82]. A classic example is seen in the case of measuring the
viscosity of water which remains constant irrespective of the duration of stirring. A weak gel in
the other scenario may record a reduction in its consistency with mixing as a result of the
destruction of certain structures within its formulation. Time-dependent fluids are classified as
thinning (thixotropic) or thickening (rheopectic) with time [82].
Numerous devices are available for determining the flow behavior of materials.
Rheometers measure rheological properties whereas viscometers are more limited to measuring
only viscosity. The most common instruments capable of measuring fundamental rheological
properties are categorized as the rotational and tube types; the former includes the parallel plate,
cone and plate, concentric cylinder and mixer; and the latter includes the glass capillary, high
pressure capillary and pipe. Rotational systems are more suited to measure time-dependent
behavior as the tube system permits only one passage of the material through it [83].

57

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65

CHAPTER 3
The Effects of Packing Line Washing Treatments and Waxing Conditions on
the Surface Cleanliness and the Gloss Development of ‘Red Delicious’ Apples
(Malus domestica)

1

______
1

To be submitted to Postharvest Biology and Technology
66

3.1. Introduction
Apples are a popular produce to which a number of postharvest practices are applied.
These include controlled atmosphere (CA) storage, washing, sorting, waxing, drying and
packaging prior to being delivered to the consumer. Apples have to be cleaned in preparation for
waxing. The deposition of numerous elements including soil, and chemicals used to control pests
and diseases caused by fungus, which not only have to be removed for health reasons, may
interfere with the way the applied coatings interact with the fruit peel for an appealing coverage.
As practiced in the industry, waxes derived from natural sources are applied to cleaned
fruit to improve appearance and quality. Appearance through the impartation of gloss and quality
through replacement of the removed natural wax to control transpiration, the reduction of water
loss, delayed ripening and senescence, and retention of color (Drake et al., 1990; Bai et al., 2003;
Cisneros-Zevallos et al., 2005). A variety of materials are used in the formulation of commercial
coatings which include but is not limited to lipids, proteins, carbohydrates, plasticizers and
solvents like alcohols and water. The different properties of these materials provide a wide range
of possible behavior of coating systems (Cisneros-Zevallos et al., 2005). In addition, the type of
fruit and coating, coating thickness and permeability, fruit-coating surface coverage, and
environmental conditions such as temperature and humidity play a significant role in the
performance of the coating system (Paull, 1999; Cisneros-Zevallos et al., 2005).
Like all other tree fruits, apples are prone to pre- and postharvest physiological disorders
that require that the fruit be treated while in the field and prior to storage – e.g. preharvest
application of calcium to control bitter pit and drenching with diphenylamine (DPA) to control
superficial scald in storage (Watkins et al., 2004). The use of fungicides in drench tanks is also
practiced to reduce the survival and activity of Botrytis cinerea and Penicillium expansum, the

67

agents of the two most important postharvest wound diseases, grey and blue mold respectively
(PTRIC, 2002). Apples also have a film of natural wax which increases in thickness with growth
and maturity (Kolattukudy, 2003) and serves as a function of protection against pests and fungi
in addition to prevention of water loss. The residues that have to be removed from the skin for
efficient cleaning are therefore predominantly chemical sprays, natural waxes, pest larvae and
eggs, dirt and other external matter that may have been deposited at any time prior to packaging.
Once on the packing line, the objective is to clean the surface of the residue outlined
earlier through chemical and mechanical actions. Detergent brush washes followed by clean
water rinse are used to remove soil and other contaminants from the surface of the fruit
(Thompson et al., 2002). Some commercial apple packing houses use cold water wash-rinse
systems, while others use heated water and detergent to clean their surface prior to wax
application. It is essential that cleaning treatments do not damage the apples but rather maintain
or enhance quality. In view of this, the effects of hot water in the washing system on the quality
of fruits have been described by Bai et al. (2006); Spotts et al.(2006); Hansen et al. (2006); and
o

Neven et al. (2006). These serial studies documented that a wash temperature of 50 C caused
thermal fruit injury to pears (Bai et al., 2006) but not to apples (Hansen et al., 2006; Neven et al.,
2006); high pressure sprays (400kPa and greater) contributed significantly to the removal and
hatching of arthropod eggs on apple surfaces and did not cause any external or internal disorders
(Neven et al., 2006); and the use of either soft or firm brushes did not affect apple quality
(Hansen et al., 2006) unlike in pears with a more sensitive peel where the use of firm brushes
caused friction discoloration (Bai et al., 2006). Chlorine and water ozonization are also
frequently used to control decay-causing microbes.

68

Surface gloss is one of the many quality grading attributes detected by visual perception
and is therefore used to describe the appearance of a material (Jha et al., 2002; Silvennoinen et
al., 2008). It is “a function of spectral reflectance, and is dependent on source and intensity of the
coincident light” (Jha et al., 2002), signifying the interaction of light with the surface of interest.
A rough surface has a matt appearance, compared to a smooth surface which is glossy, as a result
of reduced intensity of the specular reflectance (Silvennoinen et al., 2008). Therefore a uniformly
coated fruit with an intact smoother coating will have higher gloss as a result of the intensity of
the reflection, and also have decreased gaseous exchange as a result of the reinforced peel
barrier. Whereas surface gloss may not be valued for certain commodities, it adds value to apples
(Szczesniak, 1983; Nussinovitch et al., 1996) increasing consumer appeal and economic value
(Bai et al., 2002a).
Environmental conditions of temperature and relative humidity (RH) mainly during the
dewatering and drying process may influence the interface between the apple surface and the
applied coating, compromising the interaction between the two. Studies on the effects of RH on
apples have often focused on the relationship between coating permeability and storage RH, with
the resultant effect on quality (Hagenmaier et al., 1991; Bai et al., 2003; Cisneros-Zevallos et al.,
2005). Baldwin et al. (1995) and Bai et al. (2002a) also studied the effect of temperature during
storage on the quality attributes of fruits. None to our knowledge have studied the effect of
temperature and RH conditions during coating application on gloss development. It was
therefore deemed necessary to investigate the effect of environmental temperature and RH on the
waxing process and ensuing apple quality.
Several other studies have also looked at enhancing gloss and other quality attributes
such as firmness, total soluble solids, titratable acidity and controlled respiration through the use

69

of new wax formulations. A zein formulation by Bai et al. (2003) improved the quality of the
fruit and depending on the concentration of ingredients used, provided comparable results to
commercial shellac formulation. An earlier study by the same research group also documented
the effectiveness of experimental polyvinyl acetate, starch and carnauba-polysaccharide coatings.
These new wax formulations usually involved varied amounts of the different components which
changed the solid content. For example, Bai et al. (2002b) studied the effect of 19% shellac
against a 13.3% with 3% whey protein isolate or 9.5% with 8.3% carnauba. An experimental
formulation of candelilla wax was also included in their study. AvenaBustillos et al. (1997) also
studied the effect of an edible caseinate-acetylated monoglyceride coating, at different solid
contents of the major components, on the quality of produce. Varying solid content directly
affects the viscosity and flow properties of the coatings which directly influences the coating
characteristics by virtue of the spreadability on the fruit and its resultant effect on fruit quality
such as firmness and gloss.
The purpose of this study was to determine the washing line conditions that result in
optimum fruit surface cleanliness and ‘dryness’ for an efficient wax application and a resultant
appealing gloss. There is a need to evaluate the efficiency of the different cleaning treatments
used but currently no standard detection systems for the degree of cleanliness exist though there
are several complex methods for the determination of the presence of residual crop fungicides
and their extraction (García-Reyes et al., 2006). Most of these methods are however destructive,
comprising fruit homogenization, and pre-concentration and extraction of the residues using
organic solvents (Di Muccio et al., 1999; Saad et al., 2004). Research by Ong et al. (1996) using
chlorinated and ozone washes is the only non-destructive procedure reported to the author’s
knowledge that evaluates residual chemicals on whole fresh apples after they have been cleaned

70

by various treatments. Based on the information garnered, we developed a non-destructive
method for assessing the efficiency of the cleaning treatments used in this study, detailed in
section 3.2.4, to allow the selection of the most effective cleaning treatment for waxing
evaluation.
The ability to efficiently measure gloss for the evaluation of the waxing process has been
a challenge since most commercial glossmeters are not designed for uneven curved surfaces like
that of apples. The quest for the evaluation of the gloss and quality of fresh produce has been on
the rise in recent years, and many studies have developed non-destructive but laborious
techniques to determine the quality indices of produce (Jha et al., 2004). To the author’s
knowledge it was not until a little over a decade ago that Nussinovitch et al. (1996) and Ward et
al. (1996) reported the construction of the first device capable of non-destructively measuring
gloss on non-flat surfaces. Non-destructive measurement is a desired feature as this will make
possible the evaluation of the same fruit unit for subsequent measurements during a study. It is
also a desirable feature on a packing line as a quality control tool since it minimizes the number
of fruit destroyed from quality testing and allows evaluated fruit to be packaged (i.e. if deemed to
meet quality standards) for revenue. These challenges led to the development of a nondestructive gloss device by the research group for the evaluation of gloss that developed on
apples from using a simulated packing line wax application process.
This study was to evaluate the effect of surface treatments in achieving fruit surface
cleanliness in preparation for an efficient wax application and hence an appealing gloss.
Investigating the effects of the wax application process conditions on surface gloss development
was another objective for which a non-destructive gloss device was developed to enable gloss
measurements of curvatures like that of apples.

71

Apples
from CA
storage

Dump Tank
Washing
Section

Evaluation of cleanliness
Washing Brushes

Cleaned fruit

Dryer

Surface Characteristics
Temperature
Moisture

Dried fruit

Waxer

Gloss
Evaluation

90, 300 s
in
Environmental Conditions
50 oC
25, 60% RH

Dryer
Coated fruit

3 shellac formulations
in
Environmental Conditions
15, 35 oC
60, 85% RH

90, 300 s
in
Environmental Conditions
50 oC
25, 60% RH

Figure 3.1. Flow diagram for the evaluation of cleanliness and gloss development on ‘Red
Delicious’ apples
72

The flow chart outlines the processes for the evaluation of the level of cleanliness and
gloss attained in this study.

Drying in this study corresponds to the sections of the packing line where (1) the washed apple
surfaces are dewatered by means of blowing fans and brushes on a conveyor belt prior to
coating, and (2) coatings applied to the apples are dried and solidified through solvent
evaporation by circulating heated air. However in Michigan drying is carried out at high RH
due to the climatic conditions. Drying as a process is the removal of moisture or humidity, but
the high humidity condition used in this study was unable to achieve a dry surface. Therefore the
effect or result of the drying process on the packing line is denoted in quotation marks: “dried”,
“dryness”.

3.2. Materials and Methods
3.2.1. Fruit, Detergents and Coatings

1

‘Red Chief Red Delicious’ apples harvested in Michigan, in 2006, 2007 and 2008, were
DPA drenched and stored under controlled atmosphere (CA) at a commercial packing house. In
the later part of the last quarter of each year respectively, the fruit were transported to Michigan
State University where they were portioned into storage units with continued storage under CA
o

of 1 C, 1.5% O2 and 3% CO2.
Commercially available alkaline (Field Clean and Fruit Cleaner 395) and neutral
(EpiClean) fruit detergents used in this study were donated by Pace International, LLC. (Yakima,
WA), and FMC Technologies, Inc. (Lakeland, FL). These two companies and Cerexagri, Inc.
(Monrovia, CA) donated commercially available shellac fruit coatings (Shield-Brite AP-40,
73

Fresh-Cote 214 and Premium Apple Lustr) all of which have different volatile contents. The
predominant volatile compositions were isopropanol (≤ 20% to 12%) and morpholine (≤ 5% to
3%). Isopropyl alcohol, for the evaluation of the efficiency of cleaning treatments, was obtained
from Sigma-Aldrich Chemical Co. (St. Louis, MO).
1

The mention of a product/company is for identification purposes only and does not imply an

endorsement by the author.

3.2.2. Laboratory Packing Line
A pilot packing line was customized by Michigan Orchard Supply (MOS, South Haven,
MI) for the purposes of this study, to simulate that of a commercial line. The washing brushes
(WB) made from tufted brushes of 0.010” level nylon consisted of four rows; each brush was
18” long and had a 3.5” outside diameter rotating at a speed of 99 RPM. The system was
enhanced by attaching a dump tank (DT), detergent and water-wash tanks with a heating system
and associated piping. A Norpro instant immersion heater was used to construct the heating
system (Everett, WA) which was equipped with blue spirit laboratory thermometers (Fisher
Scientific, Waltham, MA) for temperature control. Separate pressure pumps with valve units
were also attached for the detergent and water wash tanks. Mounted 28 mm above the WB
system were TeeJet 6504 and 8005 nozzles (Spraying Systems Co., Wheaton, IL) for the
detergent and rinse water systems respectively. At pressures of 30 and 60 psi, the 6504 nozzle
discharged 0.35 and 0.49 GPM of detergent respectively whilst the 8005 nozzle discharged 0.43
and 0.61 GPM of water respectively.
The waxing brushes made from waxlon® consisted of four rows; each brush was 18”
long and had a 3.5” outside diameter rotating at a speed of 99 RPM. The unit was placed in an
74

o

environmental chamber with an accuracy of ± 0.3 C and ± 2.5% RH (model SM-32S-SH;
o

Thermotron Industries, Holland, MI) to generate specific temperature and RH, of 15 and 35 C,
and 60% and 85%, during waxing. The shellac coating was dispensed at 10 psi through an XRTeeJet 8003VK nozzle (Spraying Systems Co., Wheaton, IL) fitted to a customized 400 ml tube,
which was connected to an air pressure system.
The drying section of the waxing line was an environmental chamber (model SM-8-SH;
o

Thermotron Industries, Holland, MI), with convectional heating capability, set to 50 C. It
equipped with a humidifier controller system with a ± 0.3% RH accuracy to monitor and control
selected RH of 25% and 60%.

3.2.3. Cleaning Treatments
The apples were washed and “dried” under different conditions using the pilot packing
line. Below in Table 3.1 is the compilation of the factors, and their levels, of the cleaning
treatments used. In the DT section, the effects of alkaline and neutral detergents, DT water
o

temperature of 18 and 40 C, and fruit dwell time (which is the amount of time the fruit is in
contact with the content of the DT) of 30 and 60 s were assessed. In the WB section a constant
o

rinse water temperature of 40 C at rinse pressures of 30 and 60 psi were used, with fruit dwell
times of 60 and 120 s. Thirty two cleaning treatments per detergent were done and 16 evaluated
when no detergent was used, and the factors were studied to evaluate the efficacy of the packing
line in cleaning apples prior to waxing.

75

Table 3.1. Factors (detergent time, temperature dwell time, temperature and spray rinse pressure)
on different sections of the washing line affecting the efficacy of the packing line in
achieving optimal surface cleanliness on apples
Section of washing line

Factor

Level

Detergent

Alkaline
Neutral

Temperature

o

18 C
o

40 C
DUMP TANK
Dwell Time

30 s
60 s

Detergent

Alkaline
Neutral
o

Temperature

40 C

Dwell Time

60 s
120 s

Rinse Pressure
WASHING BRUSHES

30 psi
60 psi

3.2.4. Surface Assessment after the Different Cleaning Treatments
A method to evaluate the level of cleanliness that resulted from subjecting the fruit to
different combinations of washing conditions was developed. The method constituted using a
solvent extraction system to determine the amount of residue left on the apple surface after each
treatment, and correlated to surface cleanliness. This novel system involved rinsing the surface

76

of the cleaned apple fruit (ca. 190 – 250 g) with 200 ml of isopropyl alcohol for 2 min (30 s on
each axis). The rinse process was repeated three times with new fruit and fresh isopropyl alcohol.
The three rinses were combined and the residue from evaporation of the alcohol was weighed as
an index of surface cleanliness. This was replicated 8 times, averaging 24 apples per cleaning
treatment with 8 three-combined washes for evaluation; more residue extracted indicated a lesser
cleaning power of the cleaning treatment used. A total of 80 surface treatments with 1920 apples
were used for this part of the study.

3.2.5. Assessment of Surface Temperature, Dryness, and Wax Viscosity Prior to Waxing
o

Cleaned fruit from section 3.2.3 were “dried” at 50 C for either 90 or 300 s at 25% or
60% RH. The temperature of the fruit after emerging from each drying condition was measured
o

using an infrared thermometer gun, with ± 2 C accuracy at less than 80% RH (Catalog No.
S90202, Fisher Scientific, Hampton, NH), at a distance of ca. 30 – 35 in. The average and
standard deviation of 6 replicates were reported. Surface moisture, remaining after drying, was
quantified by absorbing the moisture on the surface with absorbent #2 filter paper (Whatman
Inc., Florham Park, NJ) and recording the difference in weight. This was repeated for 6 fruits; the
average and standard deviation were reported.
The rheological profile of the coatings were studied by measuring their viscosity at 1, 2.5,
5, 10, 20, 50 and 100 RPM using a Brookfield viscometer (model 1/2RV TDV-I; Middleboro,
MA). Spindle no.1 at a constant depth in a 600 ml beaker was used. The experiments were
conducted at room temperature and the flow properties measured for wax at ambient temperature
o

o

(20 C) or heated to 40 C; the two temperatures were applied to both the original coating and

77

after solvent evaporation (7% w/w) to a more viscous consistency. Models described by Steffe et
al. (2006a) were used to determine the shear rate and stress from which apparent viscosity was

.n

calculated using the power law model   K  .
Nine replicates were conducted for each coating, fluid consistency and spindle speed.

3.2.6. The Waxing Process
To study the effect of different factors of the application process such as coating
formulation, drying time, temperature and RH with their interactions on the efficiency of the
waxing process, cleaned and “dried” fruit were coated with shellac coatings in an environmental
chamber (detailed in section 3.2.2) using a combination of the factors listed in Table 3.2.

Measurement of Fruit Gloss
Coated fruit were left overnight to allow the wax to set prior to gloss measurements.
Gloss was measured using a non-destructive customized glossmeter. Three apples were
measured from each treatment, and three measurements were taken per apple; 130 images were
obtained per measurement making a total of 1170 images per treatment. The average gloss and
standard error per coating treatment were reported.

Construction of a customized non-destructive gloss device
A prototype glossmeter (Figure 3.2) was successfully designed and built specifically for
the purposes of this study to be able to non-destructively measure the gloss on whole fruit. The
TM

non-destructive glossmeter was constructed with a Fire-i

Digital Camera, and two 60 W

incandescent light bulbs serving as the light source. A black box housed the device to prevent
78

o

extraneous light sources from seeping in. The apple fruit automatically rotated at 360 on a
rotary jig fruit holder while 130 images were taken. In operation the glossmeter checks the pixels
for the red, green and blue channels. The blue channel was chosen because it gave consistent low
and high values when measuring a matte and glossy spheres respectively. The threshold was set
at 230 and all blue pixels above the threshold were recorded. The average number of saturated
blue pixels for all images was calculated as an index of fruit gloss (Gloss Units) using
MATLAB. The device was calibrated against standard glossy and non-glossy black and red
spheres before the collection of each data set. Gloss as determined by the glossmeter was
correlated to human perception of gloss; a consumer sensory panel using three different levels of
gloss, and measuring the gloss of the same fruit on the glossmeter was conducted.

Consumer Sensory Evaluation
A cohort of 50 untrained panelists comprising students, faculty and staff, spanning a wide
range of ages, at Michigan State University was recruited. A ranking test with a 3-level scale was
used in a single session conducted under white incandescent light at room temperature. Apples
were coated using three different waxing conditions a day prior to the evaluation to allow
efficient drying and setting of the coating. Random 3-digit codes were assigned to each treatment
and presented on trays to the panelists in a random fashion on the evaluation day. The ranking
used was as 1 (least glossy); 2 (moderately glossy); and 3 (highly glossy). The gloss on the same
fruit evaluated by the panel was measured on the customized glossmeter.
This study has Institutional Review Board (IRB) approval (#X07-469) as exempt.

79

Table 3.2. Factors of the wax application treatments evaluated for their effects on gloss
development
Factor
Wax Viscosity

Level

Description

Original Volatile content (w/w) evaporated
7%

Waxer Temperature

o

15 C
o

Temperature in wax application chamber,
simulating extreme conditions in packing

35 C
house
Waxer Relative Humidity

Drying Time

Humidity in the wax application chamber,

85%
1

60%

typical of environmental conditions

90 s

50 C when the fruit come out of the waxer

o

300 s
1

Relative Humidity in the Dryer

25%
60%

Conditions in the drying oven for both
washed and waxed fruit

1

The drying time and relative humidity in the dryer represent the different levels of surface
dryness and temperature

80

Fire-I Camera

Light Source
Sample
Sample Holder

Figure 3.2. Customized glossmeter for non-destructive apple gloss measurement

3.2.7. Statistical Analyses
While a full factorial design was used to assess the effect of washing treatment on surface
cleanliness, a fractional factorial design was used to assess the effect of coating conditions on the
development of gloss. The designs provided an 84% and 100% power respectively for
identifying significant differences in the treatments used, and also allowed the evaluation of the
main effects with their 2-way interactions.
The collected data was analyzed using the Statistical Analysis System (SAS Version 9.1)
software (SAS Institute, Cary, NC). Analysis of variance (ANOVA) at confidence interval of
95% (α = 0.05) was evaluated and Tukey’s multiple comparison test for the means was used to
determine significant differences in the detergent type and the levels of the other factors used.
Multiple comparison adjustment for the means using Bonferroni’s correction was used to analyze
the effect of the waxing conditions on the quality attribute of gloss. The Spearman rank

81

correlation was used to find the correlation between the human perception of gloss and the
glossmeter measurements.

3.3. Results and Discussion
3.3.1. The effect of surface treatments on cleanliness
Critical points in the cleaning section of the packing line were identified in order to
minimize their variability, and to optimize the packing line to improve surface cleanliness and
ensuing gloss. A simulation of existing washing treatments used in the industry was employed;
several controllable factors such as the type of detergent, water temperature, rinse pressure, and
dwell time were identified to contribute to the surface cleanliness of apples. The fruit detergents
were used at concentrations according to the manufacturers’ specifications. A detergent was
applied to a section of the washing line at a given time such that when it was applied in the DT,
only water was applied on the WB and vice versa. The control was no detergent in either section
of the washing line.

Effects of the dump tank temperature and detergents
o

A DT temperature of 40 C resulted in significantly lower residue recovered from the
o

fruit surface, indicative of a greater cleaning power compared to 18 C (Figure 3.3). The higher
temperature was better able to remove the natural wax and the residual fungicides present on the
o

fruit, recovering 0.037 ± 7.1E-04 g in comparison to 0.042 ± 6.8E-04 g recovered using 18 C in
the DT. Hot water systems not only produce cleaner fruit, but have been shown with various heat
treatments by many studies to control insects and diseases in produce as well as maintaining and

82

enhancing quality (Smith et al., 2000; Fallik et al., 2001; Spadaro et al., 2004; Bai et al., 2006;
Wang et al., 2006). It is essential that these heat treatments are closely monitored to prevent heat
injury to the fruit.

0.05
a
Residue (g) left on apple

0.04

b

0.03

0.02

0.01

0.00

18

40

Dump tank temperature (oC)
o

Figure 3.3. Effect of 18 and 40 C dump tank temperature on apple surface cleanliness
Bar columns with the same letter are not significantly different (means ± SE; n = 672; p ≥ 0.05)
There was no significant difference in the cleaning power of the alkaline detergent and
when no detergent (i.e. only water) was used (Figure 3.4), irrespective of where the detergent
was applied, either in the DT or on WB. The rubbing action of manually cleaning apples
accounts for the observed similar cleaning power of systems with no detergent to that with
detergent (Kenney et al., 2002); in this study this would be attributed to the scrubbing action of

83

the brushes. Overall the use of fruit detergents is reported by Alvindia et al. (2007); Hansen et al.
(2006) and Kenney et al. (2002) to impact treatment efficacy.
The alkaline detergent produced significantly cleaner apples than the neutral detergent.
Differences in their formulation could be the reason for the significant difference observed. The
alkaline detergent used in this study contained sodium hydroxide and/or potassium hydroxide
and/or their phosphate salts, together with anionic, cationic and non-ionic surfactants. In the
formulation of the neutral detergent however, only an anionic component was listed as a major
ingredient. Surfactants act to reduce surface tension of the solution which is required for
adsorption of the surfactants at the interface. Interactions across an interface are stronger for
molecules of the same nature and ions of opposite charges (Rosen et al., 2002). Thus the alkaline
detergent having different charged ends is better able to attract, remove and disperse oppositely
charged molecules of the residue, compared to the neutral detergent which is limited to the
removal of predominantly the positively charged residues. Also, when neutral detergents are
used in an immersion cleaning system, as in the DT, the dirt rises to the surface and is redeposited on the part of the substrate which is pulled out through the top layer of dirt (Moore,
2003). This could be due to the absence or low concentration of dispersants such as silicates in
the neutral formulation. The alkaline detergent by having a caustic base has a strong dissolving
power by penetrating the residue and facilitating its separation from the fruit (Moore, 2003). By
virtue of possessing different charged surfactants, the alkaline detergent also had the advantage
of synergy. Synergy of a surfactant mixture of different charge types is an important method to
enhance performance of detergents (Rosen et al., 2002).

84

Effects of the dwell time in the dump tank and on the brushes, and rinse pressure
No significant differences were recorded for the level of cleanliness that resulted from
subjecting the fruit to dwell times of either 30 or 60 s in the DT. The 60 s dwell time produced a
residue recovery of 0.039 ± 7.1E-04 g whilst that of 30 s produced a recovery of 0.040 ± 7.0E-04
g. Walker et al. (1999) reported that increasing dwell time from 10 to 23 s in a pressure wash for
citrus produced a significant change in the level of cleanliness by decreasing the number of pests
by 6.1-fold. The 13 s increase in dwell time which produced a significant change in the level of
cleanliness could be attributed to the peel morphology being different from that of an apple, and
also the differences in the variables studied.

0.05
b
0.04

a

No Detergent

Residue (g) left on apple

a

Alkaline

0.03

0.02

0.01

0.00
Neutral

Type of detergent

Figure 3.4. Effects of no detergent vs. using an alkaline vs. neutral detergent on the
surface cleanliness of apples
Bar columns with the same letter are not significantly different (means ± SE; n = 672; p ≥ 0.05)

85

Fruit surface cleanliness resulting from the dwell times of 1 and 2 min on the WB were
also observed not to be significantly different. Similar amounts of residue, 0.039 ± 6.9E-04 g and
0.039 ± 7.2E-04 g respectively were recovered. Thus increment in the dwell times in both the DT
and on the WB were observed not to be significantly different in achieving a cleaner surface.
Also, the power of the rinse pressure used had no significant effect on cleanliness. The 30
psi rinse recovered a residue of 0.039 ± 7.1E-04 g, whilst that of 60 psi rinse recovered 0.040 ±
6.9E-04 g. High pressures washes have been recorded to enhance the cleaning efficiency of
systems used for produce. According to Neven et al. (2006) the use of high pressure proved
effective in cleaning (removal of moth and eggs) than hot water sprays and dips. Walker et al.
(1999) reported that high pressure significantly cleaned the surface of citrus fruit. The
observational dissidence in this study and Walker’s was probably because of the differences in
the pressure levels, 30 and 60 psi rather than 325 and 150 psi since such high pressures would
have damaged the apple.

Interaction of temperature, detergent, dwell time and pressure on the level of cleanliness
Out of the ten possible two way interactions, the significant pairs were
detergent*temperature of DT, detergent*dwell time on WB, temperature of DT*dwell time on
WB, and dwell time in DT*rinse pressure. It is interesting to note that neither detergent nor DT
temperature were significant in a two way interaction with either the dwell time in DT or the
rinse pressure, while the interaction between dwell time in DT and rinse pressure was significant.
Graphs of the interactions demonstrate the effect of one factor on the other. In the
detergent*DT temperature interaction (Figure 3.5A), increasing the DT temperature not only
resulted in a cleaner surface but its effect was improved with the use of the alkaline detergent. At

86

A

0.048

Residue (g) left on apple

0.046
0.044
0.042
0.040
0.038
0.036
Alkaline
Neutral

0.034
0.032

18

40
Dump tank temperature (oC)

B

0.046

Residue (g) left on apple

0.044
0.042
0.040
0.038
0.036
0.034

60 s
120 s
18

40
Dump tank temperature (oC)

o

Figure 3.5. Two way interactions of 18 and 40 C dump tank temperature with (A) alkaline and
neutral detergents and (B) 60 and 120 seconds dwell time on the washing brushes on
the surface cleanliness of apples (means ± SE; n = 512 for A, 672 for B; p ≤ 0.05)
87

o

18 C there was no significant difference in the level of cleanliness regardless of the detergent
o

type, but at 40 C there was a significant difference with the alkaline producing a significantly
cleaner surface. Temperature greatly influences cleaning efficiency as an increase in temperature
increases the activity of the cleaning agents (Chateau et al., 2004) by enhancing the formation of
micelles with the dissolution of residue and its ultimate removal and dispersion at the interface
(Rosen et al., 2002). Unlike the synergistic influence of the surfactants with temperature in the
alkaline detergent to produce a cleaner surface, the neutral detergent recorded no significant
difference in the residue recovered at both temperatures.
For the interaction of DT temperature*dwell time on WB (Figure 3.5B) increasing the
temperature together with increasing the dwell time on the WB produced a cleaner fruit. An 18
o

C wash with a dwell time of 60 s resulted in cleaner fruit, while the 120 s dwell time produced
o

o

cleaner fruit at 40 C. Since the higher temperature of 40 C was more efficient at removing
extraneous variables on the surface of the fruit, an increase in exposure time will facilitate the
cleaning process and produce a cleaner surface. Considering the dwell time in DT*rinse pressure
interaction (Appendix B), both 30 and 60 psi rinse pressures at 30 s in the DT produced the same
level of cleanliness. At 60 s in the DT, the 30 psi recovered the lower amount of residue. Since
the action of the rinse pressure is on both the fruit and brushes, the reason that the 60 psi rinse
produced the less cleaner fruit could be due its more powerful force dislodging residue that had
been removed, from the fruit, by the brushes back onto the fruit. This would be more pronounced
with exposure time and could account for the higher residue recovery from fruit rinsed at 60 psi
for 60 s. In the interaction of detergent*dwell time on WB (Appendix B), it was observed that 60
s on the washing brushes was what produced the cleaner fruit with the alkaline detergent whereas

88

in the case of the neutral detergent it was the 120 s dwell time on the brushes. At 60 s with the
alkaline detergent, most of the extraneous variables had been removed such that an increase in
dwell time did not contribute significantly to surface cleanliness. But with the neutral detergent,
which is less aggressive, an increase in exposure time was required to observe its effectiveness.

3.3.2. The effect of dewatering on surface dryness
Dewatering and drying are necessary after cleaning to complete the surface treatment
phase required for waxing. Surface dryness and temperature are critical to achieving optimal wax
application in terms of a uniform and even coverage, and humidity plays a vital role in the
dryness of the fruit surface. Dew point, the temperature at which condensation occurs at a
specific vapor pressure, is an important factor to consider in order to make provision for possible
condensation on the surface of the apples. Moisture will condense on the fruit when the vapor
pressure at a set temperature exceeds the maximum for that temperature (Wills et al., 2007). To
avoid condensation, it is therefore critical to keep the temperature above the dew point by
maintaining a warm fruit surface, and if possible controlling the humidity of the processing
o

environment. For this study, fruit were dewatered and “dried” at 50 C in 25 or 60% RH which
o

have dew point temperatures of 24.56 and 40.07 C respectively. In either condition, the fruit
were “dried” for 90 or 300 s. The purpose of varying the humidity and drying time was to
simulate different climatic conditions and lengths of the dewatering tunnel respectively. The
control was fruit that had just emerged from the washing brushes and had not been “dried”. Prior
to entering the dryer, the surface temperature of the fruit was above the dew point for the 25%
condition but below that for 60% RH. Upon exiting the dryer the surface temperature was still
below the dew point for the 60% RH conditions (Figure 3.6).
89

0.8

0.4

0.2

d
c

36
34

c
a

a

0.0

32

b

b

a

30
28

b

a
l
tro
on
C

%
25
s,
90

38
o
Temperature ( C)

Moisture (g)

0.6

40

Surface Moisture
Surface Temperature
Dew Point Temperature at 25% RH
Dew Point Temperature at 60% RH

26
RH

%
25
s,
0
30

RH

%
60
s,
90

RH

%
60
s,
0
30

Drying Conditions (50 oC)
o

RH

24

o

Figure 3.6. Surface moisture (g) and temperature ( C) of cleaned apples “dried” at 50 C for
90 and 300 seconds in RH conditions of 25% and 60%
Data symbols with different letters are significantly different (mean ± SD; n = 30; p ≤ 0.05)
o

The average fruit surface temperature was 25.95 C, upon exiting the washing section of
the line and prior to entering the dryer. After drying in the oven in 25% RH, the surface
temperature increased from 27.67 to 30.50

o

C for the drying duration of 90 and 300 s
o

respectively. There was an increase from 31.17 to 37.17 C in the 60% RH drying conditions
(Figure 3.6). It is obvious that the longer duration and more humid conditions produced the
warmest fruit surface. As a result of the surface temperature of fruit from the 60% RH drying
conditions being below the dew point, there was significantly more condensation of moisture on

90

the fruit surface compared to those from the 25% RH treatment. There was an observed minimal
decrease of 31.06% in surface moisture at 25% RH, with a maximum increase of 210.61% at
60% RH (Figure 3.6). Once the fruit have gone through the dewatering and drying phase, they
are conditioned for waxing, but the presence of moisture would prevent an even and intact
coating disposition of the wax.

3.3.3. Changes in apparent viscosity of the coatings with solvent concentration and
temperature
The rheological characteristic of the wax is important as it influences the outcome of the
application process. Temperature plays a critical role in the rheological properties of fluids
(Steffe et al., 2006b) just as changes in formulation. Viscosity, a controllable factor, is
particularly important since flow properties influence the spreadability. It is related to
spreadability such that a high viscosity will reduce flow properties and therefore affect the
distribution and coverage on the fruit surface.
Assessment of the rheological profile of the coatings revealed them to exhibit timeindependent non-Newtonian behavior (data not shown). The slope of shear stress against rate
.n

using the power law model   K 

was calculated as the apparent viscosity of the coatings

(Steffe et al., 2006b). The apparent viscosity decreased with an increase in temperature from 20
o

or 40 C, which resulted in a fluid with a more rapid/increased flow (data not shown). Studies on
the effect of temperature on the rheological properties of solid shellac reported a sharp decrease
in melt-viscosity with temperature (Goswami et al., 2003). This shows that solid shellac and
solution of shellac have the same behavior pattern with increasing temperature. According to

91

Goswami et al. (2003), the decrease in viscosity with increasing temperature is due to changes in
the conformation of the shellac molecules as a result of decreased molecular chain entanglement.
It was hypothesized that viscosity would increase as the solvent matrix evaporates, and an
increase of 4.6 – 11.4% in apparent viscosity was observed when 7% (w/w) of the solvent was
evaporated. If the solvent is acting as a plasticizer or diluent then the increase could be explained
by an increase in entanglement and/or decrease in flexibility of the molecular chains of the
coating. Figure 3.7 shows the changes in shear stress and apparent viscosity with increasing
volatile vaporization. During processing, there are chances of volatile vaporization with
improper handling and storage of the coating, which could lead to an increase in viscosity.

0.08

1.8
1.6

Shear Stress (Pa)

1.4

0.06

1.2

0.05

1.0

0.04

0.8
0.6

0.03

0.4
0.2
0.0
-0.2

0

5

0.02
Original Formulation Shear Stress
7% (w/w) Evaporated Shear Stress
0.01
Original Formulation Apparent Viscosity
7% (w/w) Evaporated Apparent Viscosity
0.00
10
15
20
25

Apparent Viscosity (Pa.s)

0.07

Shear Rate (s-1)
o

Figure 3.7. Apparent viscosity and shear stress of shellac fruit coating at 20 C with varied shear
rates (mean ± SD)

92

Changes in viscosity, with accompanied changes in flow behavior, will affect the uniformity and
thickness of the coating layer and hence the gloss and barrier properties, which can influence
quality such as changes in weight and firmness. Information garnered in this section provided
insight into the flow characteristics and behavior of the coatings in the waxing system, hence the
effect of viscosity in the wax application process on gloss. There was however the importance of
a valid means to measure ensuing gloss to effectively study the influence of the wax application
variables on it.

3.3.4. Correlation of the customized glossmeter to human perception of gloss
The effects of wax application variables namely viscosity, the temperature and RH during
the waxing and drying process, as well as surface dryness on the development of gloss were
studied. Gloss determined by the gloss device had to be correlated to gloss perception by
humans. This was necessary to identify and develop a scale based on the human ability to
perceive differences in gloss, and it involved several sensory evaluations with modifications to
the gloss device. The first correlation study involved fifty untrained panelists who evaluated and
rated the gloss on apples as “slightly”, “moderately” and “highly” on a 15-point scale using
standards marked as “non-glossy” and “glossy” to understand the aesthetic scale. The gloss on
the fruit evaluated by the panel was measured on a customized glossmeter, which was a modified
Ocean Optic Spectrometer mounted over an inverted cup in a black box. The panel could not
consistently distinguish between the “moderately” and “highly” gloss fruit but the glossmeter
was able to distinguish between the different intensities of gloss though with high variability.
This led to the redesign and the development of a new device to improve precision using a Firei

TM

1394 Firewire camera over a rotary jig fruit holder described in the methodology section.

93

Fifteen of the 50 panelists who were able to successfully distinguish and identify the
sequence of the degree of gloss from the first study were selected to evaluate another set of fruit
to establish a correlation. They were presented with 3 test sets of 4 apples each, and gloss ranks
were determined by both the sensory panel and the customized glossmeter. Spearman rank
correlation coefficients were computed between the gloss ranks produced by the sensory panel
and that obtained from the glossmeter measurements. Rank correlation coefficients were
computed for the overall dataset, for each panelist and for each test set of apples evaluated. There
was evidence for a significant association between the ranking yielded by the glossmeter and the
sensory panel (P < 0.0001). The rank correlation was estimated to be equal to 0.38. From a
validation standpoint, it was concluded that the estimated rank correlation between the sensory
panel and the glossmeter was fairly low. Subsequent analyses showed that the low correlation
may be due to the test sets and the experimental design model used. Running an Analysis of
Covariance (ANCOVA) showed that approximately 88% of the variation in glossmeter readings
was explained by a model that included the effects of sample set and the average assessment of a
sensory panel. This led to the design of a new consumer evaluation test recruiting another 50
panelists and described in the methodology section 3.2.6.
The Spearman rank correlation coefficient between the ranking of the new data set
yielded by the glossmeter and the sensory panel was significantly different (P = 0.0024) but with
a fairly low rank correlation of 0.247. The frequency table (Table 3.3) indicated a 45.6% (sum of
the diagonal percentages: 22.45 + 14.29 + 8.84) agreement between the panel ranking and the
glossmeter measurement; there was a mismatch of 54.4% (sum of the off-diagonal percentages)
between the sensory panel and the glossmeter. The chi-square test for independence indicated a
significant association between the panel ranking and the glossmeter measurement (P<0.0001).

94

Table 3.3. Frequency table of panel ranking by glossmeter measurement
Glossmeter Measurement*
Total
Panel Ranking*

1

2

3

Frequency
Percent

1

33

5

10

48

22.45

3.40

6.80

32.65

4

21

26

51

2.72

14.29

17.69

34.69

12

23

13

48

8.16

15.65

8.84

32.65

Frequency

49

49

49

147

Percent

33.33

33.33

33.33

100.00

2

3

Total

* 1 = least glossy, 2 = moderately glossy, and 3 = highly glossy

Glossmeter rankings of 2 (moderately glossy) and 3 (highly glossy) were not consistently
distinguishable by the panel. Therefore rankings 2 and 3 were collapsed into one category and
the correlation association re-analyzed. The rank correlation coefficient between the ranking
yielded by the glossmeter and the sensory panel was significantly different (P = 0.0024) but this
time with a rank correlation of 0.523. Identifying the gloss levels imperceptible to the human

95

senses enabled the setting of the glossmeter scale within that of human perception. A frequency
table (Table 3.4) indicated complete agreement between the panel ranking and the glossmeter
ranking for 78.9% of the test samples; there was a mismatch of 21.1%. The chi-square test for
independence indicated a significant association between the panel ranking and the glossmeter
ranking (P<0.0001).

Table 3.4. Frequency table of panel ranking by glossmeter measurement after merging the
moderately and highly glossy fruit into one category
Glossmeter Measurement*
Total
Panel Ranking*

A

B

Frequency
Percent

A

33

15

48

22.45

10.20

32.65

16

83

99

10.88

56.46

67.35

Frequency

49

98

147

Percent

33.33

66.67

100.00

B

Total

* A = least glossy, B = moderately glossy + highly glossy

A Spearman correlation coefficient of 0.773 was estimated between the glossmeter
rankings and its measurements. Given that the glossmeter rankings were based on customized
96

gloss measurements, this correlation may be considered as an upper limit to compare agreement
between the panel and the glossmeter. When the gloss on a coated fruit was compared to that of
an uncoated fruit, there was a 100% agreement between the panel and glossmeter rankings.
The prototype gloss device therefore provided an efficient and accurate means to nondestructively measure gloss from different waxing conditions.

3.3.5. The effects of wax application process conditions on gloss prior to refrigerated
storage
The performance of the coating system is an interaction not only with the type of fruit but
also with the wax properties (Cisneros-Zevallos et al., 2005). The effect of changes in apparent
viscosity of the coating, as a result of varying wax temperature, was studied on gloss. Though
there were significant differences in the apparent viscosities with varying the wax temperature,
preliminary studies showed no significant differences in the gloss that developed when the wax
o

temperature was either at 20 or 40 C. This was attributed to heat transfer during the spray
application which may have rendered the wax temperature the same, and hence similar viscosity,
by the time it made contact with the apple peel. However significant differences in gloss were
observed with varying the viscosity of the wax at the same temperature. Thus for the remainder
of the study only the wax viscosity was evaluated, excluding its temperature.
o

Prior to wax application cleaned fruit were “dried” in 25 or 60% RH at 50 C condition,
which is commonly found in northeastern packing houses. “Dried” fruit from the 60% RH
treatment had condensation as a result of the surface temperature being below that of the dew
point (Figure 3.6), and thus entered the wax applicator wet. This meant a dilution of the applied
coating. The conditions in the wax applicator used in this study were a temperature and RH of 15
97

o

or 35 C, and 60 or 85% respectively. No visual differences in the wax distribution were
o

observed when the wax was applied in 15 or 35 C conditions. However there was a difference
observed between the two temperatures when the humidity was set to 85%. Fruit waxed in the 35
o

C, 85% RH exited the wax applicator with a thin coverage of coating which appeared to be

diluted; the surface temperatures prior to entering the wax applicator under the stated conditions
o

were below that of the dew point. Drying coated fruit at 50 C in 60% RH visibly washed off
the coating as a result of condensation, and the wash-off was more pronounced in the longer
duration. The drying conditions before and after applying wax influenced surface temperature
and moisture, thereby dictating the final gloss on the fruit by way of dew point temperature and
condensation.
A relationship between how dry the surface of the apples needs to be, and what surface
temperature has to be attained to give the best gloss was determined. As the humidity and
duration in the dryer increased, and the moisture on the surface of the fruit increased as a result,
there was a decrease in the gloss that developed. The effect of surface temperature and moisture
on gloss could therefore be said to be influenced by the dew point temperature such that if the
surface temperature was below the dew point, condensation prior to and after coating application
diluted the applied coating and decreased the gloss that developed.
Only the humidity in the drying oven played a significant role in the gloss that evolved.
This was obviously due to dilution and removal of the coating in the presence of condensation.
However, the effect of the factors evaluated on each other resulted in significant effects on the
resulting gloss. Significant interaction effects were identified between wax viscosity*RH in

98

drying oven, temperature in wax applicator*drying time, RH in wax applicator*drying time and
RH in wax applicator*RH in drying oven on the gloss on apples.
An increase in RH from 25% to 60% while drying fruit coated with the original
formulation (0.051 – 0.062 Pa.s) resulted in a significant difference in the gloss whereas for the
higher viscous coating (0.057 – 0.066 Pa.s) no significant differences were recorded (Figure
3.8A). It was observed that the more viscous wax did not spread readily and evenly on the fruit,
and was therefore unable to produce a uniform coating coverage as a result of the evaporation of
solvents which concentrates the solid content thus producing a system with high viscosity and
reduced spreadability. The difficulty in the utilization of viscous formulation is also reported by
Rojas-Argudo et al. (2009) in a study on locust bean gum-based edible coatings. Under the 25%
RH condition in the dryer, the higher viscous coating produced a significantly higher gloss, and
at 60% RH the higher gloss was produced by the original formulation. The author hypothesize
that drying in 25% RH (without the incident of condensation) provided conditions that did not
interfere with flow behavior. Therefore sections of the fruit peel that were coated had relatively
more wax (compared to that of the less viscous coating) setting in those areas. Preliminary
studies with wax volume showed that coating with a greater amount of wax result in higher
gloss; hence though fewer areas were visually observed to be covered with wax from the use of
the more viscous coating, the greater amount of wax in the covered areas translated into higher
gloss. In the case of the original formulation, the author hypothesize that the 60% RH influenced
spreadability and thus an enhanced uniform coverage translating into greater gloss.
An interesting interaction of the higher RH (85%) in the wax applicator with the higher
RH (60%) in the drying oven producing the higher gloss was observed (Figure 3.8B). In the 25%
drying oven RH condition, fruit coated in the 60% wax applicator RH developed more gloss

99

whereas it was those coated in 85% wax applicator RH that had the higher gloss in the 60%
drying oven RH. Under the wax applicator humidity of 60% no significant differences were
observed in the gloss level when “dried” in either 25 or 60% drying oven RH, but at the 85%
wax applicator RH the fruit “dried” in 60% RH developed a significantly higher gloss compared
to those “dried” in 25%. The moisture content in the higher humidity conditions of both the wax
applicator and drying oven environments could have possibly acted as a plasticizer freeing the
molecular chains and therefore increasing flow properties with the results of a coating coverage
of greater surface area, leading to the recorded higher gloss. Gloss increased with an increase in
drying time but it significantly decreased when the RH in the wax applicator was increased from
60% to 85% (Figure 3.8C). The longer duration in the “drying” oven made it possible for an
effective curing of the coating for better gloss, and the lower gloss at 85% wax applicator RH
could be explained by the effect of dew point and dilution of applied wax with condensation. The
o

gloss level that evolved from the lower wax applicator temperature of 15 C decreased with
o

increasing drying time. For 35 C wax applicator temperature, the gloss increased with increased
duration in the drying oven (Figure 3.8D). The observed trends were however not significantly
different. How the microstructure of the fruit peel during the wax application process impacts
gloss levels is discussed in Quist et al. (2011).

100

Figure 3.8. Interactions of varied coating conditions of viscosity, temperature, RH, and time on
gloss development (estimate ± SE; n = 414; Bonferroni’s adjustment p ≤ 0.05) prior
to regular atmosphere refrigerated storage

101

Figure 3.8 (cont’d)
A

0.027

b, y

0.026
b, x
G s in e
lo s d x

0.025
0.024

a, x

0.023
a, x

Original formulation
7% (w/w) evaporated
Interaction P < 0.0001

0.022
0.021

25

60
Dryer RH (%)

(a,b) Comparing Viscosity within each Dryer RH: Vertically comparison
(x,y) Comparing Dryer RH within each Viscosity: Horizontally comparison along each line
B

0.026

a, y

G In ex
loss d

0.025

a, x

a, x

0.024

0.023

a, x
60% RH
85% RH
Interaction P = 0.039

0.022

25

60
Dryer RH (%)

(a,b) Comparing Waxer RH within each Dryer RH: Vertically comparison
(x,y) Comparing Dryer RH within each Waxer RH: Horizontally comparison along each line
102

Figure 3.8 (cont’d)
C

0.027
0.026

b, y

a, x

G
loss Index

0.025
0.024
a, x
0.023
a, x
60% RH
85% RH
Interaction P = 0.0005

0.022
0.021

90

300
Drying Time (s)

(a,b) Comparing Waxer RH within each Drying Time: Vertically comparison
(x,y) Comparing Drying Time within each Waxer RH: Horizontally comparison along each line
D

0.027

G
loss In ex
d

0.026

0.025

0.024

15 oC
35 oC

0.023

Interaction P = 0.032
0.022

90

300
Drying Time (s)

103

3.4. Conclusion
o

Based on the factors used in this study a 40 C water-alkaline fruit detergent solution in
the DT for at least 60 s produced a significantly cleaner fruit surface. Rinse conditions of at least
o

30 psi with 40 C water for at least 60 s in combination with the above contributed to ensuring a
clean fruit surface. If alkaline fruit detergent is used on the washing brushes, then fruit should be
rinsed thoroughly on exiting the brushes with the same rinse conditions as when exiting the
dump tank.
This study successfully met the challenge of developing a non-destructive gloss device
for the evaluation of gloss development on apples using a combination of different simulated
packing line coating application processes. High gloss values were recorded on fruit that attained
surface temperature above that of the dew point. Therefore we recommend dry and warm fruit
surface for a good gloss. In addition, the utilization of coatings that exhibited good flow
properties and provided more coverage on the fruit surface resulted in a good coating finish.
These findings can be extrapolated to other apple varieties and regional packing houses.
Gloss could therefore be used as an indicator of the optimization of the packing line, and
a non-destructive glossmeter will not only complement but facilitate the assessment and
consequently the development of an optimal packing process.

3.5. Acknowledgements
This study was supported by the Michigan Apple Committee and project GREEEN (Generating
Research and Extension to meet Economic and Environmental Needs). This study was supported
by the Michigan Apple Committee (A-19-08) and Project GREEEN (Generating Research and

104

Extension to meet Economic and Environmental Needs) #GR06-068 at Michigan State
University.

105

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Spotts, R.A., Serdani, M., Mielke, E.A., Bai, J., Chen, P.M., Hansen, J.D., Neven, L.G.,
Sanderson, P.G., 2006. Effect of high-pressure hot water washing treatment on fruit
quality, insects, and disease in apples and pears: Part II. Effect on postharvest decay of
d'Anjou pear fruit. Postharvest Biology and Technology 40(3):216-20.
Steffe, J.F., Daubert, C.R. 2006a. Pipeline Rheology. Bioprocessing Pipelines: Rheology and
Analysis. East Lansing: Freeman Press. pp 30-3.
Steffe, J.F., Daubert, C.R. 2006b. Rheological properties of biological fluids. Bioprocessing
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Thompson, J.F., Mitcham, E.J., Mitchell, F.G. 2002. Prepartion for Fresh Market. In: Kader AA,
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110

CHAPTER 4
The Effect of Wax Application and Storage on the Gloss and Quality of ‘Red
Delicious’ Apples (Malus domestica)

1

______
1

To be submitted to the Journal of Agricultural and Food Chemistry

111

4.1. Introduction
Coating apples is used to improve appearance and quality; appearance through the
impartation of gloss and quality through the reduction of water loss, delayed ripening and
senescence (1, 2). The efficacy of the coating system is influenced by more than the type of fruit
and coating. It is also dictated by the coating thickness and permeability, surface coverage, and
environmental conditions such as temperature and humidity (2, 3). Critical to gloss development
are environmental conditions and other wax application factors such as drying time and wax
formulation which affect the thickness and uniformity of the coating layer. The coating layer
reinforces the barrier properties of the peel influencing respiration and transpiration, thereby
controlling weight loss and firmness retention during storage. It is therefore important to
investigate how environmental conditions affect the waxing process and thereby influence
quality which is key to the successful commercialization of produce. A recent study by Quist et
al. (4) expounds on how these conditions affect the initial gloss quality but further studies are
needed to learn how gloss and other quality attributes, weight and firmness, are affected by
storage.
For red colored apples, the intensity and characteristics of the red skin promotes the
perception of quality (5). Surface quality, characterized by gloss, is one measure of the quality of
an object, and gloss is one of the most important properties considered when evaluating visual
properties. Surface gloss is crucial to human perception of quality, and gloss measurements are
routine in assessing quality. Gloss is therefore used to describe the appearance of a material and
is basically the interaction of light with the surface as a function of specular reflectance (6, 7).
Specular gloss is an important factor considered in the estimation of the smoothness or roughness
of surfaces. A smooth surface appears glossy whereas a rough surface has a matt appearance as a

112

result of reduced intensity of the specular reflection. Gloss therefore describes a surface’s ability
to reflect light in the specular direction. It’s the superposition of the topography of the surface,
the angle of the incident light and the refractive index of the surface that affect gloss, and hence
influence the surface appearance (6). Unlike certain commodities, surface gloss adds value to
apples (8, 9) by increasing both its consumer appeal and market value (10); especially for ‘Red
Delicious’ apples which naturally have a low gloss compared to other varieties, and according to
Bai et al. (1) is the reason that led the apple industry to apply shiny coatings to ‘Red Delicious’.
However, the complete reliance on appearance to judge quality sacrifices critical
attributes like firmness, color and aroma. Gloss, weight and firmness preservation are the quality
attributes of interest in this study; these are important parameters in judging the quality and
freshness of produce (7). Some studies have investigated and used surface gloss as a potential
indicator to determine the quality of eggplants (7, 11) and bananas (12) but none so far is
explicitly documented for apples. None to the author’s knowledge has also investigated the
potential effect of coating conditions on quality during storage of apples.
The objective of this research was to study the effect of gloss on the changes in weight
and firmness during storage, as well as how these are impacted by waxing conditions. It also
investigated the effect of the peel surface roughness and coating thickness on gloss intensity. The
findings from this study enabled the establishment of a relationship between the quality attributes
of gloss, weight and firmness per unit time with waxing conditions.
Figure 4.1 details the work plan for this study.

113

Cleaned fruit

Dryer

90, 300 s
in
Environmental Conditions
50 oC
25, 60% RH

Dried fruit

Waxer

3 shellac formulations
in
Environmental Conditions
15, 35 oC
60, 85% RH

Dryer

90, 300 s
in
Environmental Conditions
50 oC
25, 60% RH

Coated fruit

Weight

4 oC
95% RH

Fortnightly

Quality
Evaluation

Refrigerated Storage

Gloss

Firmness

Figure 4.1. Flow diagram of the work plan
114

Drying in this study corresponds to the sections of the packing line where (1) the washed apple
surfaces are dewatered by means of blowing fans and brushes on a conveyor belt prior to
coating, and (2) coatings applied to the apples are dried and solidified through solvent
evaporation by circulating heated air. However in Michigan drying is carried out at high RH
due to the climatic conditions. Drying as a process is the removal of moisture or humidity, but
the high humidity condition used in this study was unable to achieve a dry surface. Therefore the
effect or result of the drying process on the packing line is denoted in quotation marks: “dried”,
“dryness”.

4.2. Materials and Methods
4.2.1. The Packing Line and Wax Application Process1
‘Red Chief Red Delicious’ apples harvested in 2007 and 2008 were stored under
controlled atmosphere (CA) at a commercial packing facility in Sparta, MI. In the later part of
the last quarter of each year respectively, the fruit were transported to Michigan State University
o

where they were portioned into storage units with continued storage under CA of 1 C, 1.5% O2
and 3% CO2. Commercially available shellac fruit coatings (Shield-Brite AP-40, Fresh-Cote 214
and Premium Apple Lustr) with different volatile contents, isopropanol (≤ 20% to 12%) and
morpholine (≤ 5% to 3%), were donated by Pace International, LLC. (Yakima, WA); FMC
Technologies, Inc. (Lakeland, FL); and Cerexagri, Inc. (Monrovia, CA).
The set-up of the packing line used in this study, consisting of a washer and waxer
component and an environmental chamber, is described in detail in Quist et al. (4). Cleaned and
“dried” fruit were coated with shellac coatings of different spreadability in an environmental
o

o

chamber of 15 or 35 C and 60 or 85 % RH. A drying oven set at 50 C was used to dry the
115

coated fruit for 90 or 300s under 25 or 65 %. The temperature and RH were carefully monitored
to facilitate the study of the effect of diverse environmental and process conditions on wax
application and the resultant fruit quality as detailed in Quist et al. (4). These varied conditions
for the remainder of the study will be referred to as treatments.
1

The mention of a product/company is for identification purposes only and does not imply an

endorsement by the author.

4.2.2. Microscopy Assessment of Fruit Peel Surface
To evaluate the surface of the coated and uncoated apples, different microscopy
techniques, detailed below, were used. For a controlled investigation on the effect of fruit surface
microstructure on gloss, varying amounts (0.5, 1.0 and 2.0 ml) of shellac coating were hand
coated on uniformly sized fruits. These amounts in terms of gloss levels were designated low,
medium and high respectively as preliminary studies showed the gloss value to increase with the
amount of coating.

4.2.2.1. Atomic Force Microscope (AFM), and Scanning Electron Microscope (SEM)
Surface roughness of the apple peel was measured using a Nanoscope IV Multimode
AFM (Veeco, Santa Barbara, CA). Exposed fruit flesh was dipped in paraffin wax to coat the
tissue (and thus prevent water loss and shrinkage which affect the surface structure). The surface
morphology was analyzed in contact mode using an NP20 probe obtained from Veeco. The
scanning parameters were optimized at scan rates of about 3 Hz and integral and proportional
gains of about 1 and 2 kHz respectively. The mean roughness, Ra, arithmetic average of
deviations from the center plane, of four sub-samples were reported.

116

Cross sectional samples were freeze dried in an EMS750X Freeze Drier (Electron
Microscopy Science; Hatfield, PA) prior to analysis on the SEM. The freeze dried samples were
then mounted on aluminum stubs using high vacuum carbon tabs (SPI Supplies, West Chester,
PA), and coated with Osmium (≈ 10 nm thickness) in an NEOC-AN osmium coater (Meiwa
Shoji Co. Ltd, Osaka, Japan). Four samples per treatment were micrographed in a scanning
electron microscope JEOL JSM - 6400V (Japan Electron Optics Laboratories, Tokyo, Japan)
with a lanthanum hexaboride electron emitter, using analySIS Pro software (Version 3.2;
Olympus Soft Imaging Solution Co., Munster, Germany), at an accelerating voltage of 10 kV.

4.2.3. Assessment of Fruit Quality
o

Fruit were stored in a temperature- and humidity-controlled chamber, at 4 C and 95%
RH, for two months, and quality assessments performed fortnightly. The quest to be able to nondestructively measure the gloss on the whole fruit over the duration of the study led to the
development of a non-destructive prototype gloss device. Gloss was measured using the
customized device described elsewhere (4); the same fruits were evaluated for the entire study,
and the gloss average and standard error per storage time were reported.
Fruit firmness of the stored apples was measured using a texture analyzer (TA-XT2i,
Texture Technologies Corp., Scarsdale, NY) with an 11 mm diameter probe. Three skin discs 20
mm equidistant from each other were removed from the fruit and the probe driven into the
exposed flesh to a 9 mm depth. Three fruits per treatment per storage time were evaluated, and
the average of 3 force values required to penetrate the preset depth was used as an index for fruit
firmness per fruit. The average and standard error of the 3 replicates were reported.

117

Five randomly selected apples from each treatment were weighed from the start to the end of the
study on an electronic laboratory precision balance with 0.01g sensitivity (Adventurer ARC120;
Ohaus, Pine Brook, NJ). The results were expressed as percent weight loss of the initial weight,
using the average and standard error.

4.2.4. Statistical Analyses
A fractional factorial design was used in studying the effect of environmental conditions
on the efficiency of the waxing process; the design provided a 100% power for identifying
differences. Data were analyzed using the Statistical Analysis System (SAS Version 8, SAS
Institute, Cary, NC). Analysis of variance (ANOVA) at confidence interval of 95% (α = 0.05)
and multiple comparison adjustment for the means using Bonferroni’s correction was used to
analyze the effect of the waxing conditions on the quality attribute of gloss, weight loss and
firmness; the main effects and their 2-way interactions were assessed.

4.3. Results and Discussion
4.3.1. The relationship between gloss, peel rugosity and coating thickness
To investigate the relationship between the appearance of the fruit in terms of its
glossiness (or lack thereof) and the apple peel surface structure, a study measuring the surface
roughness by AFM was conducted. It was observed, as expected, that the Ra values decreased
(from 31.2 to 0.4 nm) as the amount of coating applied to a uniform surface area increased. This
was a clear indication that a layer of coating led to a corresponding decrease in surface
roughness. The shellac was able to fill in the valleys as a result of increasing lateral layer growth
on the surface as more coating was applied (13). There was a considerable decrease in the

118

surface roughness of the low gloss fruit (0.6 ± 0.1 nm) compared to the control (31.2 ± 2.4 nm),
but as the coating was increased to the medium gloss fruit, the surface roughness increased
slightly to 1.0 nm, changing the trend (Figure 4.2a, b and c); this according to Salvadori et al.
(13) is the stage where the “tops of the hills grow faster than the valleys are filled”. Other studies
on membrane casts (14) have reported a similar trend of decrease in surface roughness with
thickness growth to a certain limit after which it begins to increase.
Based on the observations of the surface roughness with the amount of coating, and
preliminary studies that increasing the amount of coating increased the gloss index,
investigations to study the relationship between the amount of coating, coating thickness and
gloss were carried out. Microstructural analyses of the apple coated surface using the SEM
showed the influence of the coating thickness on gloss. The amount of coating used did not only
have a direct impact on the gloss level alone but also on the thickness, as expected, such that as
the thickness of the coating increased the gloss index also increased (Table 4.1).

Table 4.1. Comparison of gloss index to coating thickness and surface roughness of
unwaxed, and low, medium and high gloss apples
Gloss Level
Control (unwaxed)

Gloss Index
0*

Thickness (nm)
-

Roughness, Ra (nm)
31.2 ± 2.4

Low

4.1E-5 ± 2.1E-5

2242.6 ± 784.5

0.6 ± 0.1

Medium

2.5E-4 ± 0.5E-4

4663.9 ± 1264.3

1.0 ± 0.0

High
5.2E-4 ± 0.9E-4
8562.5 ± 911.5
* The glossmeter scale was adjusted to zero for the control

0.4 ± 0.0

It could therefore be inferred that as the surface got smoother with a corresponding
increase in the coating thickness (Figures 4.2 and 4.3) the gloss level also increased (Figure 4.4).
119

Figure 4.2. AFM images of the apple surface with different amounts of coating
(A) Control, (B) 0.5 ml (low gloss), (C) 1.0 ml (medium gloss), and (D) 2.0 ml (high gloss) of coating
x 1.000 µm/div, z 20.000 µm/div except z 200.000µ/div for A.

120

Figure 4.2 (Cont’d)
B
A

µm

µm
4

3
2

1

4

3
2

1

C

D

1

2

3

4

µm

1
121

2

3

4

µm

Figure 4.3. Scanning Electron Microscope micrographs of the apple surface with different
amounts of coating
(A) 0.5 ml, (B) 1.0 ml, and (C) 2.0 ml of coating classified as low, medium and high gloss
respectively

122

Figure 4.3 (Cont’d)
A

1212.98 nm

B

3044.60 nm

C

9328.02 nm

123

The nature of a rough surface induces light scattering which causes the irradiance distribution to
be shorter and wider as the surface roughness increases. As a result not only is the intensity of
the specular reflection weak but also all of the specularly reflected light may not be collected by
the photodetector leading to a decreased gloss value as surface roughness increases (6). Thus as
the valleys are filled with the evolution of a smoother surface, reflection is optimized.

10000

0.0006
Thickness

8000

0.0005

Gloss

7000

0.0004

6000
5000

0.0003

4000
0.0002

3000
2000

Gloss Index

Coating thickness (nm)

9000

0.0001

1000
0

0
Low

Medium

High

Gloss Level

Figure 4.4. Relationship between coating thickness and gloss at low, medium and high gloss
levels
o

4.3.2. The effects of waxing conditions and storage, at 4 C in 95% RH, on gloss
o

On storing the coated fruit at 4 C in 95% RH, gloss was observed to decrease over time
which is

consistent with studies on bananas (Musa acuminate) (12), eggplants (Solanum

melongena) (11), apples (Malus domestica) (15) and mandarins (Citrus clementina Hort. ex Tan.
× Citrus tangerine Hort. ex Tan.) (16) among other commodities which also recorded the decline
in peel gloss with storage. According to Jha et al. (11), gloss levels decline with storage as a
124

result of the decrease in the quantity and structure of surface waxes as observed with bananas
and eggplants. Damage of epidermal cells during storage contributes to an increase in surface
roughness which translates as decreasing gloss with storage (7, 11).
The surface gloss during storage was significantly affected by the RH conditions in the
wax applicator, drying time and RH in the drying oven, and obviously the time in storage. There
was evidence of an interaction between each of the significant factors and storage time,
indicating that the effect of each of these factors on gloss on apples varied across storage times.
The effect of the RH in the drying oven during storage, independent of the other coating process
factors, showed the fruit “dried” under the 25% RH to have a higher gloss than those under the
60% RH. This observation was similar to that reported by Quist et al. (4) 24 hours after coating,
prior to refrigerated storage. Fruit “dried” under 25% RH achieved a surface temperature of
o

o

30.50 C, which was above the dew point of 24.56 C, while those from 60 % RH treatment with
o

o

a surface temperature of 37.17 C was below the dew point of 40.07 C (4). The latter therefore
had condensation and entered the wax applicator wet, as a result of the surface temperature being
below that of the dew point. This meant a dilution of the applied coating and the reason for the
lower gloss (4). A diluted coating will translate into a thinner layer on the apple, which has been
shown in the SEM studies (figures 4.3 and 4.4) to have decreased gloss.
There was an interaction between the RH in the drying oven*drying time (Figure 4.5A)
and RH in the wax applicator*drying time (Figure 4.5B). The effect of RH in the drying oven on
gloss on apples depended on drying time such that 90 s of drying produced a significantly higher
gloss compared to 300 s under 60% RH (Figure 4.5A). The significantly lower gloss at 300 s was
because the relatively longer exposure aggravated the effect of condensation due to the surface

125

A

0.027
0.026

G s I dx
lo s n e

0.025

a, x
a, x

b, x

0.024
0.023
0.022
a, y

90 s
300 s
Interaction P = 0.004

0.021
0.020
25

60
Dryer RH (%)

(a, b) - Comparing Drying Time within each Dryer RH: Vertically comparison
(x, y) - Comparing Dryer RH within each Drying Time: Horizontally comparison along each line
B

0.027
0.026
a, x
G s I dx
lo s n e

0.025

a, x

a, x

0.024
0.023
60% RH
85% RH
Interaction P = 0.019

0.022
0.021

90

b, y

300
Drying Time (s)

(a, b) - Comparing Waxer RH within each Drying Time: Vertically comparison
(x, y) - Comparing Drying Time within each Waxer RH: Horizontally comparison along each line
o

Figure 4.5. Interactions of coating conditions on gloss with storage at 4 C in 95% RH (mean ±
SE; n = 2031; Bonferroni’s adjustment p ≤ 0.05)
126

temperature being below the dew point. As a result there was pronounced removal of the applied
coating (4) with the effect of increased surface roughness and a thinner coating layer, which
culminate to reduced gloss. Under the 25% RH in the drying oven it was also the 90 s that
developed the higher gloss but it was not significantly different from that developed at 300 s
(Figure 4.5A) since the environment was already relatively dry and a prolonged drying did not
influence the setting of the applied coating.
The RH in the wax applicator * drying time interaction is an indication that the moisture
conditions in the applicator on the apple surface gloss depended on drying time. Fruit coated
under 60% RH developed a significantly higher gloss compared to those coated under 85% RH
when “dried” for 300 s. At 90 s of drying however, the fruit coated under 85% RH were those
with the higher gloss but the gloss level was not significantly different from those coated under
60% RH (Figure 4.5B). Once again the effect of condensation with the consequence of wax
removal and dilution, which sum up into the effect of a thin coating on a rough surface, is the
reason for the significantly lower gloss observed under the 85% RH in the applicator with
prolonged duration in the drying oven.

o

4.3.3. Quality characteristics of apples during storage at 4 C in 95% RH
There was a direct relationship between decreasing attributes of gloss, weight and
firmness (Figure 4.6). Numerous studies over the years have recorded significant decreases in
weight loss with coating (17-19). According to Yaman et al. (17) the primary mechanism for
moisture loss is the establishment of a vapor pressure gradient at different locations on the
produce which causes vapor-phase diffusion. The permeability of the coating as well as its

127

A

0.0252

236
Gloss
Weight

0.0250
0.0248
0.0244

233

0.0242

232

0.0240

231

0.0238

230

0.0236

229

0.0234
0.0232

Weight (g)

234

0.0246
Gloss Index

235

0

2

4

228

6

Storage time (weeks)
B

0.0252

76
Gloss
Firmness

0.0250
0.0248
0.0244

70

0.0242
68

0.0240
0.0238

66

0.0236

64

0.0234
0.0232

Firmness (N)

72

0.0246
Gloss Index

74

0

2

4

6

62

Storage time (weeks)
Figure 4.6. Relationship between gloss and (A) weight, and (B) firmness of fruit in 6 weeks of
o
storage at 4 C in 95% RH (mean ± SE)
128

thickness is important in determining how much moisture is lost; permeability is directly
proportional to thickness. The temperature and RH also play a critical role as they affect the
vapor pressure difference between the atmosphere and the fruit (3, 17, 20), hence the rate of
transpiration with effects on weight loss and firmness preservation. However it is the nature of
the surface of a commodity that has a major influence on the rate of evaporation of moisture (3).
A commodity with larger pores and lenticels in the peel will have a greater surface area for
moisture loss. Respiration also causes weight reduction in fruit as a result of the loss of a carbon
atom from the fruit in each respiration cycle when CO2 is produced and released, into the
atmosphere, from an absorbed O2

(17, 20). The treatments which exhibited longer gloss

retention with storage were those that retained firmness with a minimal weight loss. Since weight
loss in fruit is mostly a result of moisture loss, this observation can therefore be attributed to the
fact that the coating treatments with greater gloss retention, by means of a thicker coating layer
(Figure 4.4), maintained barrier properties that minimized the loss of moisture and hence
controlled weight loss. This finding is similar to those reported by Rojas-Argudo et al. (16) in a
study with mandarins and Jha et al. (7, 11) in a study with eggplants where a linear correlation
between the decrease in gloss and weight during storage were observed.
The weight of the fruit decreased with time. Overall the percentage weight loss decreased
by waxing the fruit, shown in figure 4.7. The coated fruit lost 1.79% in weight at the end of 8
weeks of storage compared to 5.11% in the uncoated control. During the first month of storage,
fruit that were treated to lower humidity in the drying oven exhibited slightly more controlled
weight loss. At 6 weeks of storage it was the fruit “dried” in the higher humidity conditions that
showed a slightly more controlled loss in weight (Figure 4.7), bringing them to par by the second
month of storage. The coating was effective in reducing weight loss such that the percentage
129

weight loss of coated fruit at 8 weeks was similar to the weight loss of uncoated fruit at week 2.
Coating decreased the weight loss by almost three-fold.

6
High gloss, 25% RH dryer
Low gloss, 60% RH dryer
Uncoated

Weight loss (%)

5
4
3
2
1
0

2

4

6

8

Storage time (weeks)
o

Figure 4.7. Weight loss in apples during storage (mean ± SE) at 4 C in 95% RH for 8 weeks

The wax formulation, and RH conditions in the wax applicator had significant effects on
the change in weight during storage. The higher viscose formulation (0.057 – 0.066 Pa.s)
recorded a more pronounced loss in weight at the onset of storage but had evened out with that of
the lesser viscose formulation (0.051 – 0.062 Pa.s) by the end of the study. The more viscose
coating had reduced flow properties and spreadability. Visual observations showed a patchy and
non-uniform coating coverage (4). With more surface area not uniformly covered, the apples
coated with the more viscose formulation had a compromised barrier to moisture with more
130

o

weight loss. However, during the prolonged storage in the high RH (94% in 4 C), the coating
integrity was reduced (2). The effectiveness of the less viscose coating barrier during storage is
hypothesized to have been reduced to that of the more viscose formulation. The latter by having
relatively more wax (thicker setting) in the areas covered (4) may not have been compromised at
the same rate. This could be the reason for the similar weight loss for both formulations by the
8th week. Also, the deposit of a thicker layer supposedly protects the fruit from the rigors of
temperature and humidity during application and storage. This effect is the function of an
increase in the solid content of the coating which better covers the pores and fills in the cracks on
the fruit peel (16). Coverage of the pores and filling in of the cracks will result in a decrease in
surface roughness per unit area (Figure 4.2) which will enhance barrier to moisture loss, resulting
in reduced weight loss.
Retention of firmness is another benefit derived from application of coatings to apples.
Firmness decreased with storage for both uncoated and coated fruit, with the coated lot retaining
firmness longer to almost three-fold that of the uncoated at the end of storage (Figure 4.8). Just
as observed in the investigation on weight loss, loss of firmness was accompanied by loss in
surface gloss (Figure 4.6B). Ward et al. (12) in a study on bananas, also found gloss to decrease
with the ripening of the fruit. Loss of firmness is a result of fruit ripening (21) and retention of
firmness is attributed to delayed degradation of insoluble protopectins to the more soluble pectic
acid and pectin (17). The activity of pectinesterase and polygalactronase increase during ripening
with an increase in the deploymerization of pectic substance (18, 21, 22) but during storage, the
low levels of O2 and high levels of CO2 derived from coatings (23) impeded the activities of
these enzymes resulting in the retention of firmness (17). Coating fruit introduces a barrier that
inhibits water loss resulting in the maintenance of tugor pressure and the cell wall and middle
131

lamella, all of which diminish with ripening (18, 22, 24). The type of coating used however
plays a significant role in the maintenance of firmness. Shellac coated apples among other types
of coatings used on apples were reported to have the least loss of flesh firmness (15).

80

a a a

a a a

a

a
b

60
Firmness (N)

a

a

b

b

40

c
c

20

0

0

2

4

6

8

Storage time (weeks)
High gloss, 25% RH dryer
Low gloss, 60% RH dryer
Uncoated

o

Figure 4.8. Loss in fruit firmness with storage at 4 C in 95% RH for 8 weeks
Grouped columns with the same letter are not significantly different (mean ± SE; n = 684; p ≥
0.05)

132

In a study on mangoes, a climacteric fruit exhibiting rapid ripening after harvest just like
apples, the firmness of coated fruit was found to be significantly different from that of the
uncoated (25). The researchers found that coated fruit required 7.0 – 5.3 N to compress a 2 mm
distance compared to the significantly different 2.8 N need to compress the same distance for
uncoated fruit. Yaman et al. (17) in a study on cherries (Prunus avium) found that fruit stored at
o

o

cold temperature, 0 C, had higher firmness than that stored at ambient temperature, 30 C. The
effect of cold temperature storage on the retention of firmness was reinforced by Meng et al. (26)
study on peaches (Prunus persica L. cv. Jiubao) where they found firmness to decrease rapidly at
o

higher storage temperature; fruit stored at 10 C softened rapidly with a significant decrease in
o

firmness compared to 5 C storage with an effective maintenance of firmness. There is a
significant increase in oxygen requirement for respiration at higher storage temperatures. As
temperature increases there is an increase in metabolism accompanied by an increase in the
respiratory demand and the utilization of stored carbohydrates that lead to loss of firmness (27).
Only the RH factors (wax applicator and drying oven) had significant effect on firmness.
The effect of coating and RH in the dryer on firmness shown in figure 4.8 revealed that applying
coating under low humidity conditions in the dryer slowed down the loss of firmness. The
observation of the latter could be the effect of the comparatively more intact and even coating
layer derived as a result of the “drier” environment. This reinforced the peel barrier properties
which reduced the rate of water loss and culminated in an effective maintenance of firmness.
o

High RH (94% – 98% in 1 C), however, in prolonged storage reduces coating integrity with a
loss in barrier properties (2) which causes an increase in the loss of weight and flesh firmness in
fruit which many not have had pronounced differences at the beginning of storage (16). This
133

could be the reason for the significant loss in firmness at week 6 of the fruit “dried” under 60%
RH (Figure 4.8). Also if the fruit temperature is not near that of the air temperature a high RH
will not prevent moisture loss (3), as the fruit will have to lose moisture till the vapor pressure
around it is in equilibrium with that of the atmosphere.
Clearly, reduced gloss decay contributed to maintaining weight and preserving firmness
during storage. A study on bananas (12) report an increase in peel roughness and a decrease in
gloss with ripening. They attributed the increase in the roughness of the peel to the damage of
epidermal cells which is partly the effect of moisture loss resulting in a shriveled surface. Since
peel rugosity is directly proportional to the coating thickness, a rougher surface would have a
thinner coating layer with reduced barrier properties which promotes more rapid water loss and
respiratory exchange; the result of which is pronounced weight and firmness loss. It could also
be hypothesized that at high RH the top layer of the coating is dissolved leaving the
compromised valleys filled with moisture. The presence of moisture in the valleys would
facilitate further dissolution of the remaining coating resulting in reduced barrier properties. The
accompanying lower gloss levels, with the loss in weight and firmness, would be the result of the
increasing optical scattering as a result of increasing surface roughness.

4.4. Conclusion
Fruit with higher gloss retention recorded the lowest weight loss and reduction in
firmness which indicated the effect of coating and its barrier properties on enhancing and
maintaining quality respectively. The gloss level and barrier properties are highly dependent on
the nature of the fruit surface, and the thickness and uniformity of the coating layer. The
smoother surface and thicker layer recorded the highest gloss value and in turn exhibited better

134

quality retention. The gloss, weight and firmness decreased during storage, as the apples ripened,
and this could be attributed to increasing surface roughness with a loss in coating thickness and
barrier properties respectively. Monitoring RH in the dryer during the coating process is critical
as it was observed to have significant effects on the loss in firmness. Low humidity settings in
the drying oven would be recommended as its effect slowed down the loss of firmness during
storage. The effects of the environmental conditions in the wax applicator, however, on fruit
quality during storage were varied, showing neither a systematic decrease nor increase.
The influence of surface roughness and coating thickness on gloss can be successfully
studied using the AFM and SEM and used as an indicator of fruit quality. Just as knowledge of
the surface roughness and coating thickness can potentially help predict the quality by means of
gloss levels, knowledge of the initial gloss at the onset of storage can also be potentially used as
an index to predict the quality, via weight and firmness, at any given time during storage.

4.5. Acknowledgements
This study was supported by the Michigan Apple Committee (A-19-08) and Project
GREEEN (Generating Research and Extension to meet Economic and Environmental Needs)
#GR06-068 at Michigan State University.

135

REFERENCES

136

1.
Bai, J.; Alleyne, V.; Hagenmaier, R. D.; Mattheis, J. P.; Baldwin, E. A., Formulation of
zein coatings for apples (Malus domestica Borkh). Postharvest Biology and Technology 2003,
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2.
Cisneros-Zevallos, L.; Krochta, J. M., Internal modified atmospheres of coated fresh
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3.
Paull, R. E., Effect of temperature and relative humidity on fresh commodity quality.
Postharvest Biology and Technology 1999, 15, (3), 263-277.
4.
Quist, E. E.; Rubino, M. I.; Guyer, D. E.; Harte, J.; Lu, R.; Auras, R.; Bello, N. M., The
effects of packing line washing treatments and waxing conditions on the surface cleanliness and
the gloss development of ‘Red Delicious’ Apples. Submitted to Postharvest Biology and
Technology 2011.
5.
Iglesias, I.; Echeverría, G.; Soria, Y., Differences in fruit colour development,
anthocyanin content, fruit quality and consumer acceptability of eight 'Gala' apple strains.
Scientia Horticulturae 2008, 119, (1), 32-40.
6.
Silvennoinen, R.; Peiponen, K.-E.; Muller, K., Specular gloss. In Specular Gloss,
Elsevier: Amsterdam, 2008; pp 79-101.
7.
Jha, S. N.; Matsuoka, T.; Miyauchi, K., PH - Postharvest Technology: Surface Gloss and
Weight of Eggplant during Storage. Biosystems Engineering 2002, 81, (4), 407-412.
8.
Nussinovitch, A.; Ward, G.; Mey-Tal, E., Gloss of Fruits and Vegetables. LebensmittelWissenschaft und-Technologie 1996, 29, (1-2), 184-186.
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Szczesniak, A. S., Physical properties of foods: what they are and their relation to other
food properties. In Physical Properties of Foods, Peleg, M.; Bagely, E. B., Eds. AVI Publishing
Co.: Westport, CT, 1983; pp 11 - 12.
10.
Bai, J. H.; Baldwin, E. A.; Hagenmaier, R. H., Alternatives to shellac coatings provide
comparable gloss, internal gas modification, and quality for 'Delicious' apple fruit. Hortscience
2002, 37, (3), 559-563.
11.
Jha, S. N.; Matsuoka, T., Changes in Electrical Resistance of Eggplant with Gloss,
Weight and Storage Period. Biosystems Engineering 2004, 87, (1), 119-123.
12.
Ward, G.; Nussinovitch, A., Peel gloss as a potential indicator of banana ripeness.
Lebensmittel-Wissenschaft und-Technologie 1996, 29, (3), 289-294.
13.
Salvadori, M. C.; Martins, D. R.; Cattani, M., DLC coating roughness as a function of
film thickness. Surface and Coatings Technology 2006, 200, (16-17), 5119-5122.
14.
Khulbe, K. C.; Matsuura, T.; Noh, S. H., Effect of thickness of the PPO membranes on
the surface morphology. Journal of Membrane Science 1998, 145, (2), 243-251.
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15.
Bai, J.; Hagenmaier, R. D.; Baldwin, E. A., Coating selection for 'Delicious' and other
apples. Postharvest Biology and Technology 2003, 28, (3), 381-390.
16.
Rojas-Argudo, C.; del Río, M. A.; Pérez-Gago, M. B., Development and optimization of
locust bean gum (LBG)-based edible coatings for postharvest storage of `Fortune' mandarins.
Postharvest Biology and Technology 2009, 52, (2), 227-234.
17.
Yaman, Ö.; Bayondrl, L., Effects of an Edible Coating and Cold Storage on Shelf-life
and Quality of Cherries. Lebensmittel-Wissenschaft und-Technologie 2002, 35, (2), 146-150.
18.
Zhou, R.; Mo, Y.; Li, Y.; Zhao, Y.; Zhang, G.; Hu, Y., Quality and internal
characteristics of Huanghua pears (Pyrus pyrifolia Nakai, cv. Huanghua) treated with different
kinds of coatings during storage. Postharvest Biology and Technology 2008, 49, (1), 171-179.
19.
Drake, S. R.; Nelson, J. W., Storage quality of waxed and non-waxed 'Delicious' and
'Golden Delicious' apples. Journal of Food Quality 1990, 13, 331-341.
20.
Pan, J. C.; Bhowmilk, S. R., Shelf life of mature green tomatoes stored in controlled
atmosphere and high humidity. Journal of Food Science 1992, 57, 948-953.
21.
Wakabayashi, K., Changes in Cell Wall Polysaccharides During Fruit Ripening. Journal
of Plant Research 2000, 113, (3), 231-237.
22.
Brummell, D. A., Cell wall disassembly in ripening fruit. Functional Plant Biology 2006,
33, (2), 103-119.
23.
Hagenmaier, R. D.; Baker, R. A., Reduction in gas exchange of citrus fruit by wax
coatings. J. Agric. Food Chem. 1993, 41, (2), 283-287.
24.
Fraeye, I.; Knockaert, G.; Buggenhout, S. V.; Duvetter, T.; Hendrickx, M.; Loey, A. V.,
Enzyme infusion and thermal processing of strawberries: Pectin conversions related to firmness
evolution. Food Chemistry 2009, 114, (4), 1371-1379.
25.
Baldwin, E. A.; Burns, J. K.; Kazokas, W.; Brecht, J. K.; Hagenmaier, R. D.; Bender, R.
J.; Pesis, E., Effect of two edible coatings with different permeability characteristics on mango
(Mangifera indica L.) ripening during storage. Postharvest Biology and Technology 1999, 17,
(3), 215-226.
26.
Meng, X.; Han, J.; Wang, Q.; Tian, S., Changes in physiology and quality of peach fruits
treated by methyl jasmonate under low temperature stress. Food Chemistry 2009, 114, (3), 10281035.
27.
Beaudry, R. M.; Cameron, A. C.; Shirazi, A.; Dostal-Lange, D. L., Modified atmosphere
packaging of blueberry fruit: effect of temperature on package O2 and CO2. Journal of the
American Society for Horticultural science 1992, 117, 436-441.

138

CHAPTER 5
The Effect of Edible Coating Application Process and Storage Temperature
on the Respiration and Quality of ‘Red Delicious’ Apples (Malus domestica), and
the Transmission of Gases through the Coating

1

______
1

To be submitted to the Journal of Agricultural and Food Chemistry

139

5.1. Introduction
Edible coatings provide a barrier to respiratory gases and water vapor which slow down
respiration, transpiration and related physiological responses and decay. However edible coating
interaction with temperature and relative humidity is crucial to the maintenance of quality. The
application of edible coatings also imparts gloss, which together with the barrier properties are
highly dependent on the thickness and uniformity of the coating layer such that thicker and
smoother coatings exhibit higher gloss and enhanced barriers (1).
By enrobing the fruit, edible coatings serves as a package providing the function of a
packaging material such as modified atmosphere (MA) through changes in the permeability of its
skin and its internal atmosphere. A MA relies on the modification of gases through the film by
providing a low O2 and a high CO2 environment with the benefit of decreased respiratory rate,
delayed compositional changes and slowed physiological deterioration. These benefits cumulate
into slowing down ripening with its associated changes (2). In view of the internal atmosphere
of fruit being different from that of their external atmosphere, fruit are said to be MA packages
for their tissues (3). According to Banks et al. (4), it is the modification of the fruit’s internal
composition that achieves all MA effects. The internal composition is mainly affected by
respiration and diffusion. While respiration tends to reduce the internal O2 partial pressure, gas
diffusion exerts the opposite effect. The change in the atmospheric composition is dictated by the
permeability of gases through the coating, and the respiration rate and gas diffusion through the
produce. The ability to reduce respiratory activity under MA conditions is a benefit only if the
levels of O2 and CO2 are within that tolerated by the commodity of interest (2). Physiological

140

benefits of MA accrue from a decline in processes linked to respiration and ethylene synthesis
and action (4).
Disorders associated with low O2 and high CO2, as a result of the modification of the
internal atmosphere, have been linked to edible coatings. Oxygen concentration dropping too low
and/or CO2 concentration rising too high will induce anaerobic respiration due to decreased O2
uptake and its reduced concentration in the fruit’s internal atmosphere (2, 5, 6). Gaseous
exchange occurs through the pores and cuticle of the fruit peel. The application of surface
coatings increase the resistance to gaseous exchange by covering the cuticle and pores of the skin
which culminates in decreases in the transmission rates of gases between the internal and
external atmospheres. Thus surface coatings, through these effects, have the tendency to modify
the composition of the internal atmosphere, suppress the respiration rate and reduce transpiration
in fruits (4).
Respiration, dependent on O2 concentration and temperature, generally decreases in low
levels of O2, and increases with high temperature (7, 8). There is a two to three-fold increase in
biological activity with every 10

o

C rise in temperature within the temperature range

encountered in the commercial chain (9, 10). Yearsley et al. (8) found the respiration rate of
both ‘Cox’s Orange Pipping’ and ‘Braeburn’ to increase with temperature increases from 0 – 24
o

C. Poor temperature control can accelerate deterioration with the induction of anaerobic

respiration (2). For normal respiration, O2 and CO2 must enter and escape respectively through
the peel of the commodity (11) with the permeability of the peel influencing the transfer of
respiratory gases. Permeability is influenced by temperature, relative humidity and air movement

141

around the film package (2). Temperature dependence of film permeability and fruit respiration
is determined by the film type and the physiology of the commodity respectively (7). The
resistance of O2 uptake by many produce is at the skin, and Beaudry et al. (7) discovered that the
rate of O2 consumption by the commodity increased more rapidly than the skin permeance to O2
o

as temperature increased from 0 - 25 C. Consequently O2 levels in both the external and
internal atmosphere decrease while that of CO2 rises due to continued respiration (8).
Fruit of the same strain have shown variations in their internal gas concentrations (11);
these variations are the results of differences in skin permeance, respiration rates, and in the fruit
surface area to mass ratio (3, 12). But the temperature and the physiological stage of the
commodity in relation to their climacteric will determine the magnitude of the differences (8,
12). In general the external atmosphere is more influenced by temperature than the internal. This
is because the internal atmosphere is more dependent on the intercellular spaces and/or cell
matrix, and the composition of the external atmosphere, rather than the skin permeance (8) which
is dependent on surrounding environmental conditions. The skin permeance is also influenced by
the coating integrity and its thickness. A uniform coating allows the diffusion of gases to the
same degree on all areas of the peel whereas inconsistencies in the coating may introduce
different gas diffusion properties in different sections of the commodity’s peel. A thick coating
layer will provide a greater barrier to the entry and exit of respiratory gases which translates to
decreased normal respiration, and the opposite occurs for a thin coating layer.
The objective of this study was therefore to investigate the effect of barrier properties
through coating application, and storage temperature on the respiration of ‘Red Delicious’
apples. It also aimed at establishing a relationship between gloss and the barrier properties and

142

hence the effect of gloss on changes in respiration. The study, outlined in Figure 5.1, determined
how temperature alters the respiration rate of both coated and uncoated ‘Red Delicious’. The
transmission of respiratory gases through the coating were carried out at 2 levels of coating
thickness and 3 levels of temperature to determine how these factors affect the diffusion of CO2
and O2 through the coating layer. The findings of this study enabled the evaluation of coating
performance with storage, and the relationship between coating, permeation of respiratory gases,
respiration rates, and weight loss, and how they affect and are affected by changes in gloss
intensity.

Drying in this study corresponds to the sections of the packing line where (1) the washed apple
surfaces are dewatered by means of blowing fans and brushes on a conveyor belt prior to
coating, and (2) coatings applied to the apples are dried and solidified through solvent
evaporation by circulating heated air. However in Michigan drying is carried out at high RH
due to the climatic conditions. Drying as a process is the removal of moisture or humidity, but
the high humidity condition used in this study was unable to achieve a dry surface. Therefore the
effect or result of the drying process on the packing line is denoted in quotation marks: “dried”,
“dryness”.

143

Polyethylene
Film
Cleaned fruit

1.0 ml shellac wax
Uncoated Film
Uncoated fruit Hand coated fruit
Dryer
50 oC
25, 60% RH
90 s

1.0 ml shellac wax
Coated Film
4, 10 µm wet
thickness

Dryer
50 oC
300 s

Uncoated Film
Uncoated
Low Gloss High Gloss
fruit
fruit
fruit

Coated Film
4, 10 µm wet
thickness

Refrigerated storage
Permeability cell
4 oC

10 oC

20 oC

Respiration Rates

10 oC

Correlation

Figure 5.1. Flow diagram of the work plan
144

15 oC

CO2 & O2
Transmission Rates

20 oC

5.2. Materials and Methods
5.2.1. Surface Treatment and Coating of fruit1
‘Red Chief Red Delicious’ apples harvested in 2008, stored under controlled atmosphere
(CA) at a commercial facility in Belding, MI, were transported to Michigan State University in
May 2009 where they were portioned into storage units with continued storage under CA of 1
o

C, 1.5% O2 and 3% CO2. Fruit from CA were equilibrated to room temperature overnight prior

to surface treatment in preparation for coating and subsequent wax application. The surface
treatment section of the packing line, consisting of a heated dump tank and heated pressure rinse,
is described in Quist et al. (13). The fruit were cleaned, using commercially available alkaline
detergent donated by Pace International LLC. (Yakima, WA), employing the optimum identified
surface treatment described in Quist et al. (13). Using the method of Bai et al. (6) with
modifications, 1.0 ml of commercially available shellac fruit coating, donated by Cerexagri, Inc.
o

(Monrovia, CA), was evenly spread on the fruit by hand. The fruit were “dried” at 50 C in 25%
or 60% RH for 90 s prior to a month’s storage. The control was uncoated fruit subjected to the
same procedure.
1

The mention of a product/company is for identification purposes only and does not imply an

endorsement by the author.

5.2.2. Microscopy of Fruit Peel Surface - Scanning Electron Microscope (SEM), Atomic
Force Microscope (AFM), and Digital Optical Microscope (DOM)
Shellac coating of varied amounts (0.5, 1.0 and 2.0 ml) were hand coated on uniformly
sized apples, for a controlled investigation on the effect of coating thickness on gloss. Cross
sectional samples were freeze dried and micrographed in a scanning electron microscope JEOL

145

JSM - 6400V (Japan Electron Optics Laboratories, Tokyo, Japan) with a lanthanum hexaboride
electron emitter, using analySIS Pro software (Version 3.2; Olympus Soft Imaging Solution Co.,
Munster, Germany) as previously described in Quist et al. (1). Four samples per treatment were
evaluated (1). A Keyence digital microscope (Keyence Corp., Woodcliff Lake, NJ), at 1000x
magnification, was used to capture 3D images of the surface of each treatment for an evaluation
of the surface structure.

5.2.3. Analysis of Respiration Rate and Fruit Quality
The flow-through system was used in determining the respiration rates. Coated and
o

uncoated fruit were stored at 4, 10 and 20 C in 1 L capacity glass jars and analyzed for
respiratory gases for a month, using the experimental setup described by Beaudry (14). Two
experimental units were setup in the refrigerated chambers. One set was used to study the
changes in gloss and weight at the start and end of the respiration study, week 1 and 4, which
will be referred to as ‘Study I’ in the results and discussion section. The second set, referred to
as ‘Study II’, subjected to the same treatment and storage conditions, was used for weekly
respiratory measurements. The jar lids were fitted with ports for atmospheric gas inlet and outlet
to allow the attainment of steady state. A glass sampling septum was attached to the fruit, in the
second experimental setup, using silicone sealant; this allowed sampling of internal gases. Three
jars, each containing a weighed fruit, per coating treatment and experimental setup were placed
in each refrigerated chamber.
Gas samples (100 µl) were withdrawn from the headspace and glass septum (external and
internal) weekly with a 0.5 ml insulin plastic syringe and analyzed for CO2 using an infrared gas
analyzer (ADC-225-MK3, The Analytical Development Co. Ltd., Hoddesdon, England). The gas

146

sample was injected into the gas analyzer which uses N2 as the carrier gas at a flow rate of 200
ml/min, and connected to a chart recorder. Accumulated CO2 from the headspace and core of the
fruit were calculated relative to a standard (Matheson Gas Products Inc., Montgomeryville, PA)
containing 4.85% CO2 and 1.95% O2. The respiration rate was modeled after (9):
 y out  y in   F
 CO
CO2 
2

RCO  
2
100  M
-1

-1

Where RCO2  Respiration rate, CO2 produced (mlCO2 kg h )
y  Volumetric concentration, %v/v
-1

F  Flow rate (ml h )
M  Mass of the fruit (kg)

Two determinations were made per jar, and the average of three jar replicates was reported.
Methods described in Quist et al. (1, 13) using a laboratory precision balance and a nondestructive prototype glossmeter are what were used to measure weight and gloss respectively.

5.2.4. Evaluation of CO2 and O2 Transmission Rates
To determine the coating barrier, a polyethylene (PE) film of thickness of 50 µm was
used as the carrier. One ml of fruit coating was uniformly deposited on the film to a wet
thickness of 4 and 10 µm using a K-hand coater bar no. 0 (white) and US5 (grey) respectively
o

(RK Print Coat Instruments Ltd., Litlington, UK). The film was dried at 50 C for 5 min, and
thickness measured at three positions around the film and in the center using a digital micrometer
(Model CD-6”BS, Mitutoyo Corp., Japan) to verify the differences in the coating thickness. The

147

CO2 and O2 transmission rates (CO2TR and O2TR) through the coated and uncoated PE were
o

evaluated at 10, 15 and 20 C using ASTM D1434-82 (15), and the activation energy
determined. The CO2TR and O2TR were measured using the Permatran-C 4/41 (Mocon,
Minneapolis, MN) and the 8001 Oxygen Permeation Analyzer (Illinois Instruments, Inc.,
-4

2

-3

Johnsburg, IL) respectively. The tested film areas were 3.24 x 10 m for CO2TR and 5.0 x 10
2

m for O2TR. Uncoated film was used as control, and the average of four replicates was
reported.

5.2.5. Statistical Analyses
A full factorial design was used to assess the effect of coating, peel transmission
properties and storage temperature on respiration rates. Data was analyzed using the Statistical
Analysis System (SAS Version 8) software (SAS Institute, Cary, NC). Analysis of variance
(ANOVA) and Tukey’s multiple comparison test for the means were used to determine
significant differences in respiration rates, gloss and weight changes at the different temperatures
which enabled a correlation to peel permeation.

5.3. Results and Discussion
An efficient application of fruit coating is dependent on the surface preparation. The
surface preparation in this study was achieved by simulating a commercial line but the
application of coating was conducted in a controlled environment to achieve a predetermined
level of gloss. This allowed the investigation of the respiratory behavior of low vs. high gloss
apples. Two experimental setups were used in this study. Setup one to evaluate changes in

148

respiration, weight and gloss at the start and end of the study, to minimize the influence of
human tampering with the storage jars. Setup two was solely for respiratory and internal CO2
measurements. Both setups were stored at 4

o

C, to simulate storage conditions prior to

o

marketing, and at both 10 and 20 C to simulate marketing conditions. Information on the
transmission of gases, from the transmission rate studies, will provide additional insight into the
changes in gloss, weight and respiratory behavior of the apples under marketing conditions.

5.3.1. The effect of coating thickness and gloss levels on respiration rate and quality
Previous studies on coating conditions and gloss showed that apples coated and “dried”
in a humid environment produced lower gloss fruit. This was attributed to the effect of the
interaction of attained surface temperature and dew point temperature with a pronounced effect
of condensation which resulted in the dilution of the applied wax (13). Reduction in the amount
of applied coating on the fruit also translated into a thinner coating layer with increased surface
rugosity, all of which impacted gloss (1). Thus the study was started with the prior knowledge
that the apples processed in the 25% RH would have a higher gloss and thicker coating layer
than those in 60% RH which would have a lower gloss with thinner coating. The two levels of
gloss will be referred to as low and high gloss respectively for the remainder of the study.

Study I
The first experimental setup focused on changes in respiration, gloss and weight. The
o

gloss was significantly affected by the drying treatment (50 C in 25% or 60% RH for 90 s)
given to the apples and the duration of storage (1 and 4 weeks). Gloss was higher for the fruit

149

“dried” under the 25% RH and both gloss levels were observed to decrease over time (Figure
5.2), confirming the observation of previous studies where the fruit were waxed in a
conventional packing line (1, 13). Previous studies on gloss and coating thickness using the
SEM, showed a thicker layer with a corresponding increase in gloss as the amount of applied
coating was increased (1). A smooth surface appears glossy and light scattering is reduced by the
smoothening of a rough surface. This results in a stronger and less dispersed specular reflection
(16) and is the reason for the observed higher gloss when a higher amount of coating and
smoother surface is present. The remarkable evolution of a thinner surface with decreasing
amounts of coating applied, observed with SEM images (1) together with digital images of the
surface of coated fruit (Figure 5.3), could account for the decreasing gloss intensity with storage.
This observation infers that with storage, there was progressive thinning and evolution of a
decline in the smoothness of the finish which resulted in the observed decreasing gloss with time.
With regards to the respiration rate and weight of the apples, it was also the drying
treatment together with the storage temperature that showed significant effects. The rate of
respiration was lower, just as the loss in weight was slower in the higher gloss fruit (Figure 5.4).
Figure 5.5 and SEM micrographs from previous studies (1) show the progressive increase in
coating thickness and gloss with the application of more coating. The higher gloss fruit by virtue
of a thicker coating therefore reduced the rate of transpiration and respiratory exchange as a
result of an enhanced barrier to gases, translating into reduced respiration rates and weight loss.
Park et al. (17) in a study on sucrose polyester coatings on apples also observed the thicker
coatings to have reduced respiration rates and delayed changes in quality as a result of higher gas
barriers. Banks et al. (4) explain the resistance of the skin to the diffusion of gases to be

150

dependent on what proportion of the pores are blocked by the coating and also the extent to
which the coating adheres to the fruit surface especially along the edges of the pores.

0.03

a

1 week

Gloss Index

0.025

a

a

4 weeks

b

0.02
0.015
0.01
0.005

a
b

0
Control

Low

High

Coating Treament (gloss level)

Figure 5.2. Changes in gloss of unwaxed, low and high glossed apples at 1 and 4 weeks of
storage
Gloss level group columns with the same letter are not significantly different (mean ± SE; n =
72; p ≥ 0.05)
Microstructural analyses of the apple coated surface using the SEM showed air spaces in
the coating layer of the low gloss fruit. As more coating was applied, with a corresponding
increase in gloss, the air bubbles decreased in size and progressed to a dense and thicker coating
in the high gloss fruit (1). A loosely adhering coating offers the opportunity for gaseous
exchange in the space between the fruit and the coating, whereas the opposite applies in the
situation of a tightly adhering coating (4). Thus the air spaces in the low gloss fruit allowed more

151

A

B

C

Figure 5.3. Digital optical images of the apple surface with different amounts of coating
(A) Control, (B) 0.5 ml, low gloss, and (C) 2.0 ml, high gloss, of coating

152

4.5
4

4C
10C

Weight Loss (%)

3.5

20C

3
2.5
2
1.5
1
0.5
0
Control

Low

High

Coating Treatment (gloss level)

o

Figure 5.4. Weight loss in unwaxed, low and high gloss apples stored at 4, 10 and 20 C (mean ±
SE)

10000

Coating thickness (nm)

9000
8000
7000
6000
5000
4000
3000
2000
1000
0
Low

High
Coating treatment (gloss level)

Figure 5.5. Thickness of shellac coating on low and high gloss apples (mean ± SD)
Data exported from SEM studies in Quist et al. (1)

153

gaseous exchange and give a reason for the lower respiration rate observed in the high gloss fruit
with smaller and fewer air spaces in a denser and thicker coating layer.
Increase in the temperature of the refrigerated chamber resulted in increases in the rate of
respiration and weight loss. Temperature is identified as a critical extrinsic factor influencing
respiration; low temperatures are generally known to reduce biochemical reaction rates and vice
versa (9). The association of weight loss with higher respiration rates could be due to the
utilization of stored carbohydrates in the fruit tissues for respiration (8) thereby decreasing the
amount of tissue composition which translates to weight loss. Increasing temperatures are
associated with ripening, while ripening is associated with decreasing gloss and increasing peel
roughness (18). Since the peel gloss is directly proportional to the coating thickness, low gloss
fruit would have thinner coatings. This would translate into reduced barrier properties which
promotes more rapid water loss and respiratory exchange resulting in a pronounced weight loss
(1), as observed in the fruit stored at the higher refrigerator temperatures.

Study II
In the second experimental setup, all three factors – storage temperature (4, 10 and 20
o

C), time (1, 2, 3 and 4 weeks), and gloss treatment (thin and thick coated) - under study

significantly affected both the respiration rates and internal CO2 concentrations in the fruit. The
reason the duration of storage was significant in this setup and not the first could be due to
variability in individual fruit response to the coating treatment and/or due to the opening of the
jars for internal CO2 sampling. In the case of the latter, this would result in infiltration of the
surrounding’s respiratory gases which would offset the conditions in the jar weekly.

154

Increases in storage temperature were once again shown to cause an increase in
respiration rates (Table 5.1) as well as internal CO2 concentrations (Figure 5.6). These
observations are consistent with the findings of increasing respiration rates of apples as storage
o

temperature was increased from 0 – 24 C (8). The drying treatments, resulting in different levels
of gloss, applied to the fruit were not shown to significantly affect respiration and this could
again be due to variability in individual fruit. The variability in the fruit response to a coating
treatment is due to inherent variability in the fruit skin resistance to gas diffusion and hence
respiration rate. The variation in the proportion of blocked pores, by coating, also accounts for
differing fruit response to coating treatments (4). There were observed differences, though not
significantly different, in the uncoated and coated fruit (Table 5.1). The uncoated fruit recorded
higher rates of respiration due to the lack of an enhanced barrier allowing a faster and unimpeded
exchange of respiratory gases. Also as coating was applied, the valleys which may serve as a
source of entry and exit for respiratory exchange were filled up, as observed from the optical
images (Figure 5.3), and therefore reduced the gaseous exchange rate with a corresponding
decrease in respiration rates. Overall, though differences were observed in the respiration rates of
the coated and uncoated apple, the fact that the differences were not significantly different
implies that the exchange of respiratory gases occurs predominately through the pores and not
the cuticle, confirming the findings of Banks and his team (4).
o

The coated fruit recorded the higher concentrations of internal CO2. At 4 C storage, the
internal CO2 concentration among the coated samples were not significantly different as a result
of the low physiological activity at low temperatures but as the storage temperature increased
with an increase in respiration and metabolism, significant differences were progressively

155

o

Table 5.1. Respiration rate* of apples with varied coating thickness stored at 4, 10 and 20 C for
4 weeks
-1 -1

Respiration (mgCO2kg h )
Coating Treatment (gloss level)
o

Storage Temperature ( C)

Control

Low

High

4

0.47a,~

0.03a, ~

0.30a, ~

10

1.55b, ~

1.42b, ~

1.39b, ~

20

2.67c,~

2.13b,~,^

2.13c,^

4

0.16a,~

-0.04a,~

0.19a,~

10

1.51b,~

1.36b,~

1.28b,~

20

2.60c,~

2.10b,~

2.18c,~

4

0.35a,~

0.06a,~

0.27a,~

10

1.48b,~

1.40b,~

1.33b,~

20

2.74c,~

2.73c,~

2.68c,~

4

0.33a,~

0.22a,~

0.32a,~

10

1.62b,~

1.55b,~

1.43b,~

20

Storage Time (Week)

2.89c,~

2.66c,~

2.92c,~

1

2

3

4

*Residual for the respiration rate was not normal, so the data was log transformed
Numbers in the same category column, specific to a storage time with the same letter; and
numbers in the same row, specific to a storage temperature and time, with the same symbol are
not significantly different (α = 0.05)

156

identified between the different levels of gloss. Though the coated fruit produced less CO2 by
virtue of the lower respiration rate, the presence of the coating, which enhances the peel barrier,
reduced the rate at which gases escaped from the interior of the fruit. Hence, a more concentrated
amount over time and higher in the high gloss fruit with a more dense and thicker layer of
coating (Figure 5.6).

Internal CO2 Concentration (%)

At the end of the study, the fruit were cut open and no CO2 damage was observed.

8
7
6

c

Control
Low Gloss
High Gloss

b

5
4
3

b

2
1

b

b

b

a

a

a

0
4

10

20

Storage Temperature (oC)
o

Figure 5.6. Internal CO2 of coated apples stored at 4, 10 and 20 C for 4 weeks
Group columns with the same letter are not significantly different (mean ± SE; n = 144; p ≥ 0.05)

5.3.2. Respiratory gas transmission properties of the wax coating
Respiration is not only O2 dependent but also influenced by peel permeation (11); the
more available O2 is, the higher the respiration rate. The coating on an apple dictates how O2 and

157

CO2 diffuse in and out of the fruit respectively. Understanding the permeation characteristics of
the coating will provide an explanation for the influence of the fruit peel on respiration. For
explicit analyses of the influence of the fruit coating and its thickness on the exchange of
respiratory gases, shellac coating of 4 and 10 µm wet thicknesses were bar-coated onto a 50 µm
PE film and dried. The dried films recorded thicknesses of 53.98 µm and 60.33 µm respectively.
Both the CO2TR and O2TR through shellac, increased with increasing temperature. The CO2TR
was higher than that of O2 for all testing parameters, thickness and temperature (data not shown).
o

At 10 C, CO2TR decreased from 667.1 ± 132 to 301.5 ± 20.3 mg/m2.h for the 4 and 10
µm coated thickness respectively, compared to 123.5 ± 11.1 to 56.2 ± 1.2 mg/m2.h for the O2TR;
a similar trend was observed for the other temperatures (Figure 5.7). The relatively low barrier of
the coating to CO2 was ideal for the release of CO2, produced during respiration; such release
prevented anaerobic respiration and the breakdown of the internal tissue (or formation of
byproducts). The O2TR indicated how much O2 each coating thickness and consequently gloss
level allowed to permeate its peel for respiration. Based on the transmission rate data, the high
gloss fruit by having a thicker coating should have had a decreased gaseous exchange and a
corresponding significant lower respiration rate. In the same vein, the low gloss apple should
have had a higher respiration rate, and a rate significantly higher than that of the latter should
have been observed for the uncoated. This once again goes to prove that the exchange of
respiratory gases occurs mainly through the pores.

158

Transmission Rate (mg/m2.h)

200

4um wet thickness
10um wet thickness

180
160
140
120
100
80
60
40
20
0

10

15

20

Temperature (oC)

Figure 5.7. O2 transmission rate through shellac coating of 4 and 10 µm wet thicknesses at 10,
o

15 and 20 C (mean ± SD)
Temperature increases also resulted in increases in the transmission rate, as expected, in
both of the shellac thicknesses evaluated (Figure 5.7). As the temperature was increased, the
chains of the shellac polymer gained more energy which resulted in an increase in their
segmental mobility. The mobility of the chains expanded the intercellular spaces allowing a
greater amount of gases to be transmitted. However, as the thickness increased a thicker barrier
over the intercellular spaces was created which was why the 10 µm coating recorded less
significant transmission rates than the 4 µm (Figure 5.7). The observation of the effect of
temperature on changes in respiration rates (Figure 5.4 and Table 5.1) is concurrent with the
observation in the changes in O2TR with temperature (Figure 5.7); the lower storage temperature
recorded lower respiration rates as well as reduced gaseous transmission rates. A reduction in
O2 to less than 10% (by volume) is what is required to achieve a reduction in respiration.

159

However this reduction is temperature dependent such that when temperature is lowered the
oxygen requirement is reduced (7, 9, 19). Hence as more O2 permeates the peel with temperature
increases, the respiration rate is higher with the availability of more O2, and a response in the
increased production of CO2.
The activation energy, Ea, of O2 transmission rate were 17.5 ± 5.2 and 17.4 ± 2.8 kJ/mol
for the 4 and 10 µm coated thickness respectively, and 38.6 ± 6.9 and 33.3± 5.7 kJ/mol
respectively for the CO2 transmission rate. The Ea was greater for CO2, compared to that of O2,
indicating that the CO2TR underwent the most change with temperature increases which is
evident from the data (Figure 5.7).

5.4. Conclusion
Reduced gloss occurred when coated apples were “dried” under high humidity (25%
compared to 60% RH). Also gloss was observed to decrease over time, as a result of changes in
peel roughness (18), and confirmed earlier studies (1, 13). Coating the apples reduced weight
loss. This was more pronounced in the higher gloss fruit, with an enhanced thicker barrier to
minimize the loss of moisture. Higher storage temperatures increased the O2 demand for
respiration (7) which was evident in the increased O2TR with higher temperatures and increased
respiration rates by means of CO2 production. Respiration rates were observed to be lower for
coated fruit and much lower for the higher gloss fruit. Therefore the reinforced barrier reduced
the permeation of respiratory gases.

160

The gloss intensity, as influenced by the thickness of the coating layer and the roughness
of the apple surface, could be potentially used as an indicator of the permeation of respiratory
gases. Therefore gloss levels could be used as a potential index to predict respiration rates and
changes in weight during storage.

5.5. Acknowledgements
This study was supported by the Michigan Apple Committee (A-19-08) and Project
GREEEN (Generating Research and Extension to meet Economic and Environmental Needs)
#GR06-068 at Michigan State University.

161

REFERENCES

162

1.
Quist, E. E.; Rubino, M. I.; Lu, R.; Guyer, D. E.; Auras, R., The effect of wax application
and storage on the gloss and quality of ‘Red Delicious’ Apples. Submitted to the Journal of
Agricultural and Food Chemistry 2011.
2.
Kader, A. A.; Zagory, D.; Kerbel, E. L., Modified atmosphere packaging of fruits and
vegetables. Critical Reviews in Food Science and Nutrition 1989, 28, 1 - 30.
3.
Dadzie, B. K.; Banks, N. H.; Cleland, D. J.; Hewett, E. W., Role of Skin Resistance to
Gas Diffusion in the Response of Fruits to Modified Atmospheres. Acta Horticulturae 1993, 343,
129 - 134.
4.
Banks, N. H.; Dadzie, B. K.; Cleland, D. J., Reducing gas exchange of fruits with surface
coatings. Postharvest Biology and Technology 1993, 3, (3), 269-284.
5.
Baldwin, E. A.; Nisperos-Carriedo, M.; Shaw, P. E.; Burns, J. K., Effect of Coatings and
Prolonged Storage Conditions on Fresh Orange Flavor Volatiles, Degrees Brix, and Ascorbic
Acid Levels. J. Agric. Food Chem. 1995, 43, (5), 1321-1331.
6.
Bai, J.; Hagenmaier, R. D.; Baldwin, E. A., Coating selection for 'Delicious' and other
apples. Postharvest Biology and Technology 2003, 28, (3), 381-390.
7.
Beaudry, R. M.; Cameron, A. C.; Shirazi, A.; Dostal-Lange, D. L., Modified atmosphere
packaging of blueberry fruit: effect of temperature on package O2 and CO2. Journal of the
American Society for Horticultural science 1992, 117, 436-441.
8.
Yearsley, C. W.; Banks, N. H.; Ganesh, S., Temperature effects on the internal lower
oxygen limits of apple fruit. Postharvest Biology and Technology 1997, 11, (2), 73-83.
9.
Fonseca, S. C.; Oliveira, F. A. R.; Brecht, J. K., Modelling respiration rate of fresh fruits
and vegetables for modified atmosphere packages: a review. Journal of Food Engineering 2002,
52, (2), 99-119.
10.
Zagory, D.; Kader, A. A., Modified atmosphere packaging of fresh produce. Food
Technology 1988, 42, (9), 70-77.
11.
Hagenmaier, R. D., A comparison of ethane, ethylene and CO2 peel permeance for fruit
with different coatings. Postharvest Biology and Technology 2005, 37, (1), 56-64.
12.
Yearsley, C. W.; Banks, N. H.; Ganesh, S.; Cleland, D. J., Determination of lower
oxygen limits for apple fruit. Postharvest Biology and Technology 1996, 8, (2), 95-109.
13.
Quist, E. E.; Rubino, M. I.; Guyer, D. E.; Harte, J.; Lu, R.; Auras, R.; Bello, N. M., The
effects of packing line washing treatments and waxing conditions on the surface cleanliness and
the gloss development of ‘Red Delicious’ Apples. Submitted to Postharvest Biology and
Technology 2011.
14.
Beaudry, R. M., Effect of carbon dioxide partial pressure on blueberry fruit respiration
and respiratory quotient. Postharvest Biology and Technology 1993, 3, (3), 249-258.

163

15.
ASTM, D1434-82(2009)e1: Standard Test Method for Determining Gas Permeability
Characteristics of Plastic Film and Sheeting. In American Society for Testing and Materials
(ASTM) International: West Conshohocken, PA, 2009.
16.
Silvennoinen, R.; Peiponen, K.-E.; Muller, K., Specular gloss. In Specular Gloss,
Elsevier: Amsterdam, 2008; pp 79-101.
17.
Park, H. J., Bunn, J. M., Vergano, P. J. , and Testin, R. F, Gas permeation and thickness
of the sucrose polyesters, semperfreshTM coatings on apples. Journal of Food Processing and
Preservation 1994, 18, (5), 349-358.
18.
Ward, G.; Nussinovitch, A., Peel gloss as a potential indicator of banana ripeness.
Lebensmittel-Wissenschaft und-Technologie 1996, 29, (3), 289-294.
19.
Wills, R. B. H.; McGlasson, W. B.; Graham, D.; Joyce, D. C., Postharvest - An
introduction to the physiology and handling of fruit, vegetables and ornamentals 5th ed.; CAB
International Wallingford, UK 2007; p 11, 163 - 166.

164

CHAPTER 6
SUMMARY, CONCLUSIONS & RECOMMENDATIONS FOR FUTURE
STUDIES

165

Commercial cleaning practices were evaluated in an attempt to identify cleaning factors
that produced significantly cleaner apple surfaces, as a preparatory step in conditioning the fruits
o

for further studies and analyses. A 40 C water-alkaline fruit detergent solution in the DT for at
o

least 60 s, together with a 40 C – 30 psi rinse for at least 60 s produced significantly cleaner
fruit surfaces. The section for the utilization of the alkaline fruit detergent can be either the dump
tank or the washing brushes. In the case where the detergent is used on the washing brushes, the
fruits should be rinsed thoroughly with the same rinse conditions as when exiting the dump tank.
Simulating a combination of commercial packing line coating application processes,
different gloss intensities evolved that were successfully measured on a customized nondestructive glossmeter. Higher gloss intensities were observed not only with apples that were
“dried” under low humidity but with those that obtained a surface temperature above the dew
point, and were “dry” and warm prior to the application of the fruit coating. The less viscose
coatings with good flow properties were also observed to give a better coverage on the fruits, and
recorded higher gloss values.
Throughout the 8weeks storage study, the fruits with the higher gloss values recorded the
lowest reduction in firmness and weight loss as a result of the function of the barrier properties
of the coating. The uniformity and thickness of the coatings were also observed to influence the
gloss and quality attributes. The smoother and thicker coatings recorded higher gloss values with
greater firmness and weight retention. Respiration rates were also observed to be lower for
coated fruits and much lower for the higher glossed fruit, as a result of the reinforced barrier of
the thicker (higher gloss) coating minimizing the permeation of respiratory gases.
As the apples continued to ripen during storage, the gloss, weight and firmness were
observed to decrease as expected. These changes were accompanied by decreases in coating
166

thickness and increases in the surface roughness, as evaluated with the SEM and AFM
respectively. Storage in high temperature environments increased the respiration rates, by means
of CO2 production, since the demand for O2 increases with temperature. The changes in gloss
with its associated changes in weight, firmness and respiration, were attributed to the changes in
peel coating roughness and thickness which culminated a corresponding decline in the coating’s
barrier properties.
Knowledge of the initial gloss at the onset of storage can be potentially used as an index
to predict the fruit quality, via weight, firmness and respiration, at any given time during storage.
In that vein, gloss could therefore be used as an indicator of the optimization of the packing line
and processes. Complementing the packing line with a non-destructive glossmeter will facilitate
the development and assessment of an optimal packing process. Information on the surface
roughness and coating thickness, using the AFM and SEM respectively, can also be potentially
used as an indicator of fruit quality by means of gloss levels and its association with respiratory
gases transmission rates, and changes in the quality attributes of interest in this study. Thus while
visual perception is key to judging the quality of produce, scientifically the microstructure of the
surface can also be used as a tool to judge and predict quality.
Recommendations for improving the packing line based on the findings from the pilot
and scale-up studies are outlined in the report submitted to the Michigan Apple Committee
(Appendix A). A good suggestion for future studies would be to expand the study on the
environmental conditions used to evaluate the wax application process and ensuing gloss. This
would involve testing different wax formulations to identify which gives the highest gloss
intensity for specific sets of temperature and relative humidity. In doing so, a range of
environmental conditions that influence gloss to similar but insignificant extents could be
167

identified, allowing the formulation of wax coatings for specific range of conditions. It would be
very good for apple packing houses to have coatings formulated for use in high RH regions such
as that of Michigan and New York. This way, packing houses can easily change the waxes on
their packing lines as the seasons change with their accompanying changes in temperature and
relative humidity. A study on the turbidity of the water in the dump tank and how it affects gloss
should be conducted. This would make possible the determination of the turbidity at which the
dump tank water interferes with the deposition of the coating and therefore with the development
of gloss. The acquisition of such knowledge will facilitate the establishment of a point at which
the dump tank water has to be changed during processing.
It would also be ideal to incorporate a glossmeter as a non-destructive testing unit in the
packing line. This would minimize errors and losses as fruits will not have to be moved from one
unit to the other before evaluating the efficiency of the system in producing the desired gloss. It
would ensure efficiency of the system by possibly shutting down the line automatically or
issuing a sound signal when the gloss value is below the set point. This would provide an ongoing evaluation on the line and therefore allow adjustments of the line to ensure acceptable
gloss intensities whenever gloss is detected to be lower than the set value.
Since packing houses handle different varieties of apples, it is highly recommended that
the findings from this study be tested on apples other than ‘Red Delicious’, to verify the
adaptation to all apples. This will ensure consistency and universality in the implementations of
the findings from this study. Therefore packing lines could be enhanced to pack different
varieties of apples on the same settings and adjustments.

168

APPENDICES

169
 

APPENDIX A
Copy of the Final Report for the Michigan Apple Committee

170

FINAL REPORT
for
“Evaluation and Improvement of the Appearance of MI ‘Red Delicious’ Apples”

RECOMMENDATIONS FOR ACHIEVING A GOOD APPLE GLOSS

Submitted to
The Michigan Apple Committee
February 2010

171

Start Date:

Fall 2006

Completion Date:

Spring 2010

Principal Investigator:

Maria Rubino , PhD

1

1

2

3

Co-Principal Investigators: E. Enyonam Quist , Daniel Guyer , Renfu Lu , Rafael Auras
1

2

1

School of Packaging, Biosystems and Agricultural Engineering,

3

USDA/ARS/SBRU; MSU, East Lansing, MI 48824.

Conducted by:

E. Enyonam Quist

Testing Facility:

School of Packaging
Michigan State University
East Lansing, MI 48824

172

1. BACKGROUND
Pilot packing line components were set up at the School of Packaging, Michigan State
University, to test and evaluate critical points on the packing line that significantly contribute to
achieving a good waxing job. The different factors on the line that were tested, and their
outcomes, were reported in the interim report ‘Guidelines for Achieving a Good Apple Gloss’
(June 1, 2009). This included the effect of (1) having a heated water system on the packing line,
(2) maintaining a warm fruit surface temperature taking into consideration the environmental
temperature and relative humidity (RH), as well as that of (3) using a coating with higher solid
content specially formulated for humid environments. A scale-up was conducted on a
commercial packing line to test and verify the findings from the pilot line reported in the interim
report.

2. OBJECTIVE
The objective of the project was to evaluate and improve the apple packing line, and packing
line operations, to achieve the desirable fruit finish by:
1. Identifying the parameters on the washing section of the line that will ensure efficient
cleaning of the fruit surface in preparation for further processing
2. Evaluating different wax application treatments on apple gloss while maintaining
environmental conditions of temperature and RH common to Michigan
3. Studying gloss decay with storage and correlating such decay to quality attributes of firmness
and weight loss
4. Evaluating different commercially available wax formulations that could provide better apple
gloss when applied under high humidity environments

173

5. Scaling-up: Verify the finding from the laboratory pilot trials by running trials on a
commercial line and evaluate:
a. The extent to which gloss improves when using a heated water system on the packing
line
b. A new wax formulation with high solid content that provided a high gloss on
Michigan apples when applied at high RH during the pilot trial
c. The effect of dew point on gloss development

3. SIGNIFICANCE
The significance of this study is that:


The packing process specific to Michigan will be modified, based on observations in the pilot
plant, ensuring the delivery of appealing apples



Waxes used by the Michigan industry are those recommended by manufacturers for
packinghouses without taking the humidity of the environment into consideration, thus the
observation of lower gloss compared to higher gloss from regions with lower humidity
conditions. This critical observation will foster communication with formulators to develop
new coatings that is adapted for the Michigan climate



The recommended changes require the contribution of both packers and wax formulators to
achieve the desired goal



Reports from MAC indicate that the investment to provide the consumer with appealing
fruits may substantially increase the annual revenue from the sale of fresh ‘Red Delicious’

174

4. SUMMARY OF THE PILOT FINDINGS
4.1. Fruit cleaning
The type of detergent and temperature of the dump tank were the factors with significant
effects on the cleanliness achieved after washing. The use of alkaline detergent proved to do a
better job than the neutral detergent. There was no significant difference where the detergent was
o

applied, be it in either the dump tank or on the washing brushes. The use of 40 C in the dump
tank resulted in lower residue recovery from the fruit surface indicative of a better cleaning
o

power compared to 18 C (Figure A.1). The power of the rinse pressures (30 or 60 psi) used in
this study was not significant in achieving a cleaner fruit surface. The dwell times in both the
dump tank (30 or 60 seconds) and on the washing brushes (60 or 120 seconds) also did not play a
significant role in achieving a cleaner surface.

0.06
Residue Extracted (g)

a
0.05
0.04

b

0.03
0.02
0.01
0
18

40
Dump Tank Temperature (oC)

Figure A.1. Effect of dump tank temperature on surface cleanliness (per 3 apples)
Bar columns with the same letter are not significantly different (means ± SD; n = 672; p ≥ 0.05)

175

4.2. Wax application and drying
The RH in the drying oven significantly affected the gloss that evolved, with the 25% RH
producing the higher gloss fruit (Figure A.2). There was also an effect of interactions of factors
used in the waxing process that resulted in significant gloss level differences; these included the
wax viscosity and drying time.
The longer drying time (300 compared to 90 seconds) produced drier fruit surfaces, as
o

expected, at the lower RH of 25% compared to 60% when applied at 50 C. For example when
comparing fruits that had not been dewatered to fruits emerging from dewatering (just before
waxing), there was a decrease of 31% and an increase of 210% in surface moisture in the 25%
and 60% RH drying environments respectively. This will have a significant effect on the wax
coating thickness and uniformity on the entire fruit and eventually affect the gloss. Drier apples
will have a uniform thick coating and consequently a higher gloss. Apples with a wet surface
when the wax is applied will not have a uniform coating and patches of wax will be observed
throughout the apple surface. As a consequence the gloss will be very low.

4.3. Wax formulation
A preliminary study with high solid content waxes, while keeping all other ingredients
the same, performed better when applied in a high humidity environment and gave higher gloss
apples during our initial pilot trials. We refer to these waxes as ‘new’ formulation for the
remainder of this report. A subset of the data obtained during our initial pilot trial is shown in
figure A.3. It is, however, important to keep in mind that the interaction with other packing line
factors, such as drying time (that relates to line speed and length of drying tunnel), temperature
and RH in the drying tunnel, impact the solidification of the wax on the surface of the apple
176

which during the pilot trial were very easy to control and maintain. The solidification of the wax
in the surface is critical for gloss development.
0.026
0.0255

a

Gloss Index

0.025
0.0245
0.024

b

0.0235
0.023
0.0225
0.022
0.0215
25

60

Relative Humidity in the Dryer (%)

Figure A.2. Effect of humidity in the drying oven on gloss level of coated fruits
Bar columns with the same letter are not significantly different (means ± SD; n = 2031; p ≥ 0.05)

b

0.03

Gloss Index

0.025

a

0.02
0.015
0.01
0.005
0
Old

New
Wax Formulation

Figure A.3. Effect of coating formulation on gloss level of fruits dried in 60% RH
for 90sec during the pilot trial
Bar columns with the same letter are not significantly different (means ± SD; n = 18; p ≥ 0.05)

177

5. SCALE-UP: TESTING ON THE COMMERCIAL LINE
Both the ‘old’ (current formula) wax and the ‘new’ formulation were tested on the day of
the scale-up with MI ‘Red Delicious’ fruits. The fruits were tested using both heated and cold
dump tanks. The cold dump tank was simulated using a plastic bucket filled with unheated water
from the dump tank on the packing line. Due to challenges posed by the amount of wax and
using a hand sprayer, the team resorted to dipping fruits into a tub of shellac coating and
transferring coated fruits to the waxing brush bed by hand. This application was used for both the
‘old’ and ‘new’ formulations to ensure efficient comparison of the gloss that developed from the
two formulations. The brush bed was cleaned between formulations by running fruits until it
seemed to be dry and clean.

5.1. FINDINGS/RESULTS FROM THE COMMERCIAL LINE
5.1.1. Temperature of the water system and surface temperature of the apples
o

Based on the findings in the pilot study, it was recommended to maintain a 40 C (104F)
o

temperature in the dump tank together with a 40 C (104F) rinse. These conditions on fruits,
equilibrated to room temperature prior to entering the packing line, with dwell times of at least
one minute in both the dump tank and on the washing brushes ensured a surface temperature of
o

24 - 28 C (75.82 – 82.4F) entering the waxing component. The dwell time, temperature, and RH,
in the dewatering section contributed to the surface temperature of the apples before they entered
the wax applicator.
However, during the scale-up, there was a challenge with testing the theory of having a
heated water system on the packing line as the rinse water was cold. Our preliminary testing on

178

o

the line showed the temperature on the fruits to drop from 30 C (86F), upon exiting the dump
o

tank, to 18 C (64.4F) upon exiting the cold rinse on the washing brushes. Since the spreadability
of applied coatings is highly influenced by the surface temperature and moisture content on the
fruit in addition to the dew point temperature, a decision was made to bypass the washing brush
bed. Thus fruits were held in mesh bags for 4 minutes in the dump tank and delivered to the
dewatering brush bed. The surface temperature of the apples transitioning through the packing
line is shown in figure A.4. The dwell time on the dewatering brushes was an average of 0.5
minutes.

35

Warm Dump Tank

Temperature (oC)

30

Cold Dump Tank

25
20
15
10
5
0
Bin

Dump Tank

Dewattering Fans

Section of packing line

Figure A.4. Surface temperature of apples transitioning through the packing line
(means ± SD; n = 24)

179

5.1.2. Surface Moisture of the apples
o

o

The ambient (15 C, 70% RH) and the dewatering (18 C, 61% RH) environmental
o

conditions had dew point temperatures of 9.6 and 11 C respectively. The surface temperature of
the fruits treated to the warm dump tank was well above the dew point temperature while that of
those treated to the cold was below it (figure A.4). As a result of condensation on the fruits
treated to the cold dump tank, higher surface moisture content was recorded (Figure A.5). The
higher surface moisture also impacted gloss development as the presence of moisture on the fruit
in the wax applicator would dilute the wax applied as will be shown in the next section.

0.45

Surface moisture (g)

0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
Warm

Cold

Conditions of the dump tank

Figure A.5. Surface moisture on apples, treated in either a warm or cold dump tank
o
under 15 C and 70% RH, just prior to entering the wax applicator
(means ± SD; n = 12)

180

5.1.3. Impact of wax formulation and processing on gloss
Fruits treated to the warm dump tank, waxed with either the ‘old’ or ‘new’ shellac
formulation recorded significantly higher gloss values than those treated to the cold dump tank
(Figure A.6). This observation could be attributed to the surface temperature of the warmer fruits
being above the dew point temperature, and thus minimal surface moisture to significantly dilute
the applied wax.

1.40E-04
1.20E-04

a

Gloss Index

1.00E-04
8.00E-05

Warm

b

6.00E-05

Cold

4.00E-05
2.00E-05
0.00E+00
Warm

Cold

Conditions of the dump tank

Figure A.6. Comparing gloss on apples treated in either a warm or cold dump tank,
and waxed with the 'old' formulation
Bar columns with the same letter are not significantly different (means ± SE; n = 31; p ≥ 0.05)

In the warm dump tank however, just as in the cold, it was the ‘old’ formulation that
recorded the higher gloss values as shown in figure A.7. The observations on the commercial
line contradict that of the pilot. This could be attributed to better control of the experiments and
factors, such as drying time and temperature, in the latter compared to the former where minimal
adjustments were made on the line during the scale-up. A scale-up will have more variability
181

than a controlled pilot trial. The ‘new’ formulation with higher solid content may require more
time to solidify compared to the current formulation in which the coating seemed to be
completely dry and less sticky with the evaporation of the volatile solvents.
Although the low performance of the ‘new’ wax was unexpected it is possible that factors
that produced a positive outcome in the pilot trials do not perform in the same manner on the
commercial line, which is the reason why it is critical to run a scale-up to validate the results
obtained in the pilot trials.
As a result of the scale-up we are confident that a close control of the dump tank
temperature and apple surface temperature and moisture content will have a positive impact on
the gloss development on a commercial line since these were validated.

1.40E-04

b

1.20E-04

Gloss Index

1.00E-04
8.00E-05
6.00E-05
4.00E-05

a

2.00E-05
0.00E+00
New

Old

Type of fruit coating formulation

Figure A.7. Comparing gloss on apples treated in a warm dump tank and waxed with
either the 'old' or 'new' formulation
Bar columns in the same group with the same letter are not significantly different (means ± SE; n = 31; p ≥ 0.05)

182

6. RECOMMENDATIONS
These recommendations outlined below are based on both the findings in the pilot and
commercial scale packing lines:
1. To ensure efficient cleaning of apples and a warm surface before being introduced into the
o

wax applicator, it is recommended to use at least 40 C water in the dump tank together with
o

at least 30psi, 40 C rinse water. Though the heating systems for both the dump tank and
washing brushes may be equipped with a digital regulator-display, we recommend the use of
a stick thermometer on a regular basis to check that the systems are heating efficiently. The
key is to have an efficient heating system to enable attaining fruit surface temperature above
the dew point.
2. We recommend that the surface moisture and temperature of the fruits be critically monitored
as they transition through the packing line. Fruit surface temperature above the dew point
will prevent condensation of moisture and ensure a good gloss. Knowledge of the dew point
temperature (Table A.1) will enable appropriate adjustment of the temperature of the water
system and dwell times in the dump tank and/or washing brushes to maintain surface
temperatures above the dew point. The surface temperature of the apples can be monitored
using a hand held infra-red thermometer gun and surface moisture can be measured by
absorbing with an absorbent paper.
a. The temperature and dwell time in the drying tunnel may also need to be adjusted
based on the environmental conditions and surface temperature the fruits attain prior
to being dried. These adjustments will be specific to each packing line as a result of
the different sizes and lengths of individual lines. A monitoring system is suggested
as a means to either control the speed of fruit through the drying zone and/or sound a
183

warning based on the dew point falling near or below expected fruit surface
temperatures.
b. Surface moisture was 0.1 – 0.2g for an average sized apple (ca. 250-300g) with
surface temperature above the dew point. Therefore we recommend that the surface
moisture on fruit with surface temperature above the dew point does not exceed 0.2g
to ensure efficient application and drying of the applied coating.
3. We recommend the use of a coating that has a minimum Zahn viscosity of ~45 Zahn seconds
when using a 0.078” orifice Zahn cup. Using a viscosity below that is likely to drip off the
fruit while that above it may not provide an efficient and uniform spread on the fruit, both of
which impact gloss. Since the initial assumption of changing the solid content to provide a
higher gloss has not shown consistent results we highly recommend more work to be done on
coating formulations by the companies to improve formulations suited for Michigan.
4. The effect of the type of detergent on surface cleanliness was not assessed during the scaleup so as not to interfere with the main objective of the scale-up. Therefore this
recommendation is based solely on observations during the pilot:We recommend the use of an alkaline detergent in either the dump tank or on the washing
brushes. In the case where the detergent is used on the washing brushes, it is recommended to
thoroughly rinse the fruits with the rinse conditions mentioned in “Recommendation 1”
above. We also recommend the use of a sanitizing agent such as chlorine or chlorine dioxide
(as is practiced in WA) in the dump tank to eliminate bacteria and kill spores on the fruit
surface. This can be coupled with the alkaline detergent in either the dump tank or on the
washing brushes, together with another sanitizer on the washing brushes prior to rinsing the
fruits.

184

Table A.1. Temperature, Relative Humidity and Dew Point Chart
Relative Humidity / Dew Point Temperature (F)
Temperature (F)

40%

45%

50%

55%

60%

65%

70%

85%

90%

95%

100%

78

51.75 54.96 57.87 60.54 63.00 65.29 67.43 69.43 71.33 73.12

74.83

76.45

78.00

79

52.64 55.87 58.79 61.47 63.94 66.23 68.38 70.40 72.30 74.10

75.81

77.44

79.00

80

53.53 56.77 59.71 62.39 64.88 67.18 69.34 71.36 73.27 75.08

76.80

78.44

80.00

81

54.43 57.68 60.62 63.32 65.81 68.13 70.29 72.33 74.24 76.06

77.79

79.43

81.00

82

55.32 58.58 61.54 64.25 66.75 69.08 71.25 73.29 75.22 77.04

78.77

80.42

82.00

83

56.21 59.49 62.46 65.18 67.69 70.02 72.20 74.25 76.19 78.02

79.76

81.42

83.00

84

57.10 60.39 63.37 66.10 68.62 70.97 73.16 75.22 77.16 79.00

80.75

82.41

84.00

85

57.99 61.29 64.29 67.03 69.56 71.91 74.12 76.18 78.13 79.98

81.73

83.40

85.00

86

58.88 62.20 65.20 67.96 70.50 72.86 75.07 77.15 79.10 80.96

82.72

84.40

86.00

87

59.77 63.10 66.12 68.88 71.43 73.81 76.03 78.11 80.07 81.94

83.71

85.39

87.00

88

60.67 64.01 67.03 69.81 72.37 74.75 76.98 79.07 81.05 82.92

84.69

86.38

88.00

89

61.56 64.91 67.95 70.73 73.31 75.70 77.93 80.04 82.02 83.89

85.68

87.38

89.00

90

62.45 65.81 68.86 71.66 74.24 76.64 78.89 81.00 82.99 84.87

86.66

88.37

90.00

91

63.34 66.71 69.78 72.59 75.18 77.59 79.84 81.96 83.96 85.85

87.65

89.36

91.00

92

64.22 67.62 70.69 73.51 76.12 78.54 80.80 82.92 84.93 86.83

88.64

90.36

92.00

93

65.11 68.52 71.61 74.44 77.05 79.48 81.75 83.89 85.90 87.81

89.62

91.35

93.00

94

66.00 69.42 72.52 75.36 77.99 80.43 82.71 84.85 86.87 88.79

90.61

92.34

94.00

95

66.89 70.32 73.44 76.29 78.92 81.37 83.66 85.81 87.84 89.77

91.59

93.34

95.00

185

75%

80%

Table A.1 (cont’d)
96

67.78 71.22 74.35 77.21 79.86 82.32 84.62 86.78 88.81 90.75

92.58

94.33

96.00

97

68.67 72.13 75.26 78.14 80.79 83.26 85.57 87.74 89.79 91.72

93.57

95.32

97.00

98

69.56 73.03 76.18 79.06 81.73 84.21 86.52 88.70 90.76 92.70

94.55

96.32

98.00

99

70.44 73.93 77.09 79.99 82.66 85.15 87.48 89.66 91.73 93.68

95.54

97.31

99.00

100

71.33 74.83 78.00 80.91 83.60 86.09 88.43 90.63 92.70 94.66

96.52

98.30

100.00

101

72.22 75.73 78.92 81.83 84.53 87.04 89.38 91.59 93.67 95.64

97.51

99.30

101.00

102

73.11 76.63 79.83 82.76 85.47 87.98 90.34 92.55 94.64 96.62

98.50

100.29 102.00

103

73.99 77.53 80.74 83.68 86.40 88.93 91.29 93.51 95.61 97.60

99.48

101.28 103.00

104

74.88 78.43 81.65 84.61 87.33 89.87 92.24 94.47 96.58 98.57 100.47 102.27 104.00

186

7. REQUESTED COMPLEMENTARY STUDY: GLOSS DECAY WITH TIME
During a progress report session on the pilot findings, the Michigan Apple Committee
requested a study to evaluate gloss decay over time. The research team complemented the decay
study with quality attributes of weight and firmness, and also correlated that to gloss thickness
using scanning electron microscopic techniques.
The weight of the fruits decreased with time and coating the fruits decreased the weight
loss by almost 3-fold (Figure A.8); the coated fruits lost 1.79% in weight at the end of the 8
weeks storage compared to 5.11% in the uncoated. Firmness also decreased across time (Figure
A.9), and in both evaluations of weight and firmness it was observed that the higher gloss fruits
were able to maintain quality longer.

6

Weight loss (%)

5

Higher gloss
Lower gloss
Uncoated

4
3
2
1
0

2

4

6

8

Storage time (weeks)
o

Figure A.8. Weight loss in apples during storage at 4 C, 95% RH (estimate ± SE)

187

80

a a a

a a a

a

a

a

a
b

60
Firmness (N)

High gloss
Low gloss
Uncoated

b

b

40

c
c

20

0

0

2

4

6

8

Storage time (weeks)
o

Figure A.9. Loss in fruit firmness with storage at 4 C, 95% RH
Grouped columns with the same letter are not significantly different (estimate ± SE; n = 684; p ≥ 0.05)

Gloss, just like weight and firmness also decreased over time. A similar trend of decay is
observed in all three (Figure A.10); the fruits with longer gloss retention during storage were
those that retained firmness with minimal weight loss.

188

A
0.0252

236

0.0250

Gloss
Weight

0.0248

Gloss Index

233

0.0244
0.0242

232

0.0240

231

0.0238

W
eight (g)

234

0.0246

230

0.0236

229

0.0234
0.0232

235

0

2

4

6

228

Storage time (weeks)

B
76

0.0252
0.0250

Gloss
Firmness

0.0248

74

Gloss Index

0.0244

70

0.0242
68

0.0240
0.0238

66

0.0236
64

0.0234
0.0232

0

2

4

6

62

Storage time (weeks)

Figure A.10. Relationship between gloss and (A) weight, and (B) firmness of fruits
o
during storage at 4 C, 95% RH (estimate ± SE)

189

Firm
ness (N)

72

0.0246

Studies on the microstructure of the coating showed that the thickness of the coating translates to
gloss (Figure A.11). Since the higher gloss fruits had thicker coatings, it could be said that the
barrier properties were enhanced which is the reason for the slower deterioration of quality in the
higher gloss fruits.

0.0006

10000
Thickness

8000

0.0005

Gloss

7000

0.0004

6000
5000

0.0003

4000
0.0002

3000
2000

0.0001

1000
0

0
Low

Medium

High

Gloss Level

Figure A.11. Relationship between coating thickness and gloss (mean ± SD)

190

Gloss Index

Coating thickness (nm)

9000

APPENDICES
Appendix A – Method to measure water temperature


Use a thermometer stick at predetermined intervals in various sections of the dump tank and
rinse spray.



Immerse the thermometer into the body of water (for the rinse spray, collect a sample in a
beaker) and wait till the dial (or in the case of a digital stick, the display) is stable.



Use the average of about three measurements each for the dump tank and rinse spray.

Appendix B – Method to measure surface temperature
Measure the fruit surface temperature just prior to entering the wax applicator.


o

Use an infrared thermometer gun with ± 2 C accuracy at less than 80% RH (e.g. Catalog No.
S90202, Fisher Scientific, Hampton, NH). (Note: be sure of the IR thermometer cone/area of
incidence/measurement as this varies with distance from apple. Longer distances from the
apple surface may result in measuring an area larger than the apple.)



Measure no fewer than 6 fruit surface temperatures for each batch. Use the average and
standard deviation and compare to the dew point temperature chart.

Appendix C – Method to measure surface moisture
Measure the surface moisture of fruits prior to entering the wax applicator.


Quantify the amount of moisture on the apple surface by absorbing the moisture with
absorbent #2 filter paper (e.g. from Whatman Inc., Florham Park, NJ)



Weigh the absorbent paper on a laboratory precision balance with at least a 0.01g sensitivity
(e.g. from Adventurer ARC120; Ohaus, Pine Brook, NJ). Record the difference in weight of
191

the paper before and after use as the amount of surface moisture. (See Recommendation 2b
under Section 6).


Record the surface moisture of 6 fruits, and use the average and standard deviation.

Appendix D – Method to measure wax viscosity
Measure the viscosity of the fruit coating being used, intermittently throughout the day, with a
0.078” orifice Zahn cup (e.g. from Weschler Instruments, Cleveland, OH) and a timer.


Since temperature affects viscosity, we recommend that the temperature of the coating
should be measured (using a thermometer stick) prior to each viscosity measurement.



Insert the Zahn cup vertically into the coating.



Start the timer when the top of the Zahn cup emerges from the surface of the coating.



Stop the timer when the steady flow of the wax through the orifice ceases.



Record the time and repeat three times.

192

APPENDIX B
The Remainder of the Interactions of Packing Line Factors on the Level of
Cleanliness

193
 

The figure below is complementary to the discussion in Chapter 3, section 3.3.1, on page 88 - 89.

A
0.046

0.044

Residue (g)

0.042

0.040

0.038

0.036

Alkaline Detergent
Neutral Detergent

0.034
60

120

Dwell time on the washing brushes (s)

B
0.042

0.041

Residue (g)

0.040

0.039

0.038

0.037
30s dwell time in DT
60s dwell time in DT

0.036

0.035
30

60

Pressure of rinse spray (psi)

Figure B1. Two way interactions of (A) 60 and 120 seconds dwell time on the washing brushes
with alkaline and neutral detergents and (B) pressure of rinse spray with 30 and 60
seconds dwell time in the dump tank on the surface cleanliness of apples (means ±
SE; n = 512 for A, 672 for B; p ≤ 0.05)
194
 

APPENDIX C
Consumer Panel Consent Form

195

School of Packaging, Michigan State University

Consumer Panel Consent Form
Evaluation of Fresh Apples
Dear Participant:
Michigan State University researchers are working to improve the quality of fresh apples
and asking that panelists participate in a study to evaluate the resulting gloss on the apple after
different treatment conditions. The treatment conditions include cleaning the apples in different
fruit cleaners, varying the time, water temperatures and pressures after which the apples are
waxed with shellac (FDA approved food grade wax) under different environmental conditions.
Each evaluation will take about 10 minutes or less after you receive your samples. You will be
given a coupon or food treat worth $2 or less as an appreciation for your participation and
completion of the questionnaires.
Though none is anticipated, if you have a problem upon touching the samples, notify
the on-site sensory evaluation coordinator and/or principle investigator immediately. You will
be released from participating in this study. Please note if you are injured as a result of your
participation in this research project, Michigan State University will assist you in obtaining
emergency care, if necessary, for your research related injuries. If you have insurance for
medical care, your insurance carrier will be billed in the ordinary manner. As with any medical
insurance, any costs that are not covered or in excess of whatever are paid by your insurance,
including deductibles, will be your responsibility.

Financial compensation for lost wages;

disability, pain or discomfort is not available. This does not mean that you are giving up any
legal rights you may have. Your response is confidential and we will protect your confidentiality
to the full extent of the law.

196

School of Packaging, Michigan State University
Participation in this study is voluntary. You may refuse to grade any of the apples
without penalty, and your decision to refuse participation or discontinue participation during this
study will be honored promptly and unconditionally.
If you have any questions during your reading this consent form, or during or after your
participation, please do not hesitate to contact the on-site sensory evaluation leader and/or the
principle investigator, Dr. Maria Rubino, via phone at 517-355-0172, or regular mail at 130
Packaging Building, East Lansing, MI 48824.

She can also be reached via email at

mariar@msu.edu for any inquiry you might have related to your participation in the study. In
case you have questions or concerns about your role and rights as a research participant, please
feel free to contact Dr. Peter Vasilenko, Ph.D., Director of Human Research Protections, by
phone: (517) 355-2180, fax: (517) 432-4503, email: irb@msu.edu or regular mail: 202 Olds Hall,
East Lansing, MI 48824-1047.

I voluntarily agree to participate in the study.

SIGNED _______________________________________ DATE______________

197

APPENDIX D
Questionnaire for the Sensory Evaluation of the Gloss on Apples - Consumer
Panel Rating

198

School of Packaging, Michigan State University

Evaluation of Fresh Apples
Questionnaire for Sensory Evaluation

Name: _____________________

Email Address: ________________

Date: _________

Type of Sample: Red Delicious Apples
Characteristic Studied: Gloss on apples from different packaging conditions

Instructions:
1. Receive the 5 samples. Two samples are marked as “Non glossy” and “Glossy” while
others are marked with a 3-digit numeric code.
2. Use the “Non Glossy” and “Glossy” samples to understand the aesthetic attribute that we
are asking you to identify. “Non-glossy” represents 2 and “Glossy” represents 13
according to the key provided in instruction # 5.
3. Once you understand the aesthetic attribute that we want you to analyze, go ahead and
note the sample codes for the coded samples in the next section according to its position
on the tray.
4. Visually analyze the samples from left to right and note the intensity of the gloss on the
fruits.
5. Rank the samples according to the following key for degree of glossiness, by drawing a
line through the corresponding grid.

0–3
8 - 11

4–7

Non glossy
Moderately glossy

12 - 15
199

Slightly glossy
Highly glossy

School of Packaging, Michigan State University

Evaluation of Fresh Apples
Code: ________

0

1
Non-glossy

2

3

4

5

6

7

8

9

10

11

12

13

14 15
Highly
Glossy

2

3

4

5

6

7

8

9

10

11

12

13

14

Code: ________

0

1
Non-glossy

15

Highly
Glossy

Code: ________

0

1
Non-glossy

2

3

4

5

6

7

Comments:

200

8

9

10

11

12

13

14 15
Highly
Glossy

APPENDIX E
Selected Panel Consent Form

201

School of Packaging, Michigan State University

Selected Panel Consent Form
Evaluation of Fresh Apples
Dear Participant:
Before you decide to sign this consent form and continue to participate in this study,
please read this document carefully for the information related to the study, ingredients,
packaging material and procedures used in the study. Potential risks and benefits from your
study, assurance of your privacy and your rights as a human subject in our study are also listed.
If you have any questions during your reading this consent form, or during or after your
participation, please do not hesitate to contact the on-site sensory evaluation leader and/or the
principle investigator, Dr. Maria Rubino, via phone at 517-355-0172, or regular mail at 130
Packaging Building, East Lansing, MI 48824.

She can also be reached via email at

mariar@msu.edu for any inquiry you might have related to your participation in the study. In
case you have questions or concerns about your role and rights as a research participant, please
feel free to contact Dr. Peter Vasilenko, Ph.D., Director of Human Research Protections, by
phone: (517) 355-2180, fax: (517) 432-4503, email: irb@msu.edu or regular mail: 202 Olds Hall,
East Lansing, MI 48824-1047.

PLEASE NOTE THAT UPON YOUR SIGNING THIS CONSENT FORM, YOU
VOLUNTARILY AGREE TO PARTICIPATE IN THIS STUDY.

YOUR SIGNATURES

INDICATE YOU HAVE READ ALL THE INFORMATION PROVIDED IN THIS CONSENT
FORM AND THAT YOU HAVE HAD AN ADEQUATE OPPORTUNITY TO DISCUSS
THIS STUDY WITH THE PRINCIPLE INVESTIGATOR AND HAVE HAD ALL YOUR
QUESTIONS ANSWERED TO YOUR SATISFACTION. A COPY OF THIS CONSENT
202

School of Packaging, Michigan State University

Selected Panel Consent Form
Evaluation of Fresh Apples
FORM WITH YOUR SIGNATURE FOR YOUR RECORDS CAN BE PROVIDED UPON
YOUR REQUEST.

I voluntarily agree to participate in the study.

SIGNED _______________________________________ DATE______________

203

School of Packaging, Michigan State University

Selected Panel Consent Form
Evaluation of Fresh Apples
Invitation to Participate: You are invited to participate in the study that assesses the effect of
different packing and environmental conditions on gloss development on fresh apples.

Purpose of the study: We are investigating the effect of different processing conditions on the
resultant gloss on apples. This study would help establish the best processing conditions to
obtain the best sheen on apples and also correlate human perception of gloss to a customized
glossmeter.

Procedure of the study: Each panelist would be served red delicious apples waxed with shellac
under different environmental conditions and would be asked to rank the sample after visual
examination. Each sample would be coded with a unique 3-digit code. We are asking that
panelists participate in this study that will include approximately 5 sensory evaluation sessions.
Evaluations should last about 20 minutes or less.

Sample Preparation: All the materials used in this study are approved by the FDA for contact
with food. The apples will be cleaned with either alkaline or neutral fruit cleaners for different
lengths of time under varied water temperatures and pressures. The cleaned fruits will then be
waxed with shellac under different environmental temperatures and relative humidity to develop
different levels of sheen.

Potential Risks: Since all the materials used to prepare and wax the apples are FDA approved
food grade, these samples pose no adverse health risk, provided the subject has not been identified as
being susceptible to an allergic reaction to apples or surfactants or waxes upon contact. Though none is

204

School of Packaging, Michigan State University

Selected Panel Consent Form
Evaluation of Fresh Apples
anticipated, if you have a problem upon touching the samples, notify the on-site sensory
evaluation coordinator and/or principle investigator immediately. You will be released from
participating in this study. Please note if you are injured as a result of your participation in this
research project, Michigan State University will assist you in obtaining emergency care, if
necessary, for your research related injuries. If you have insurance for medical care, your
insurance carrier will be billed in the ordinary manner. As with any medical insurance, any costs
that are not covered or in excess of whatever are paid by your insurance, including deductibles,
will be your responsibility. Financial compensation for lost wages; disability, pain or discomfort
is not available. This does not mean that you are giving up any legal rights you may have. Your
response is confidential and we will protect your confidentiality to the full extent of the law.

Expected Benefits: This study will enable the researchers understand gloss development under
different packing conditions, and will help validate and correlate readings on a customized
glossmeter to human perception of gloss.

Assurance of confidentiality: Any information obtained in connection with this study that could
be identified with you will be kept confidential by ensuring that all consent forms and response
sheets are securely stored. All data collected and analyzed will be reported in an aggregate
format that will not permit associating subjects with specific responses or findings. Your
privacy will be protected to the maximum extent allowable by law.

205

School of Packaging, Michigan State University

Selected Panel Consent Form
Evaluation of Fresh Apples
Withdrawal from the study: Participation in this study is voluntary. You may refuse to grade
any of the apples without penalty, and your decision to refuse participation or discontinue
participation during this study will be honored promptly and unconditionally.

206

APPENDIX F
Questionnaire for the Sensory Evaluation of the Gloss on Apples Selected Panel Ranking

207

School of Packaging, Michigan State University

Evaluation of Fresh Apples
Questionnaire for Sensory Evaluation

Name: _______________________

Date: _______________

Type of Sample: Red Delicious Apples
Characteristic Studied: Gloss on apples from different packaging conditions

Instructions:
1. Receive the samples and note each sample code in the next section according to its
position on the tray.
2. Visually analyze the samples from left to right and note the intensity of the gloss on the
fruits.
3. Write “1” in the box of the sample which you deem to have the least surface gloss. Write
“2” for the next, “3” for the one next to that and “4” for the sample with the highest
surface gloss.
4. If two samples appear the same, then give them the rank that is average of the ranks. For
example if samples contending for rank # 2 and 3 seem similar, assign both of them
rank= 2.5.

Code:

________

__________

Rating:

208

__________

_________

APPENDIX G
Questionnaire for the Sensory Evaluation of the Gloss on Apples Consumer Panel Ranking

209

School of Packaging, Michigan State University

Evaluation of Fresh Apples
Questionnaire for Sensory Evaluation

Name: _______________________

Email Address: ______________________

Date: ______

Type of Sample: Red Delicious Apples
Characteristic Studied: Gloss on apples from different packaging conditions

Instructions:
1. Receive the samples and note each sample’s code in the next section according to its position
on the tray.
2. Visually analyze the samples from left to right and note the intensity of the gloss (surface
sheen) on the fruits.
3. Rank the samples, relative to each other, according to the following key for degree of
glossiness.

1

Least glossy

Code: ________

2

Moderately glossy

__________

Rating:

Comments:

210

3

Most glossy

__________

APPENDIX H
Rheological Profile of Shellac Coatings Used in this Research

211
 

The table and figures in this appendix complement the discussion in Chapter 3, section 3.3.3.

Table H.1. Apparent behavior of fruit shellac coatings (original formulation and that of 7%
o
weight vaporized) at 20 and 40 C
o

Coating

Formulation

Temperature ( C)

Co. A

Original

20
40
20
40
20
40
20
40
20
40
20
40

7% (w/w) evaporated
Co. B

Original
7% (w/w) evaporated

Co. C

Original
7% (w/w) evaporated

212
 

Consistency
Coefficient, K
0.0165
0.0084
0.0365
0.0142
0.0104
0.0053
0.0138
0.0086
0.0188
0.0103
0.0246
0.0128

Flow Behavior
Index, n
1.4578
1.5975
1.2107
1.4793
1.5789
1.8636
1.4878
1.6302
1.3687
1.5234
1.3796
1.5097

1.6
1.4

Shear Stress (Pa)

1.2
1.0
20 RPM
50 RPM
100 RPM

0.8
0.6
0.4
0.2
0.0
0

1

2

3

4

5

Time (min)
Figure H.1. Time-independent behavior of fruit shellac coating at 20, 50 and 10 RPM (mean ±
SD)

213
 

0.08

Apparent Viscosity (Pa.s)

0.06

0.04

0.02
Original at 20oC
7% at 20oC
Original at 40oC
7% at 40oC

0.00

0

5

10

15

20

25

Shear Rate (s-1)
Figure H.2. Apparent viscosity of fruit shellac coating (original formulation and that of 7%
o
weight vaporized) at 20 and 40 C with varied shear rates

214
 

APPENDIX I
Digital Microscopic Images of Uncoated and Coated Apple Surface

215
 

This figure is complementary to the result and discussion sections in Chapter 4 (section 4.3) and Chapter 5 (section 5.3).

Figure I.1. Digital microscopic images of the apple surface with different amounts of coating at 200x magnification
(A) Control; (B) 0.5 ml, low gloss; (C) 1.0 ml, medium gloss; and (D) 2.0 ml, high gloss

216
 

Figure I.1 (Cont’d)
A

B

100 µm

100 µm
D

C

100 µm

100 µm

217
 

APPENDIX J
CO2 Transmission Rate through Shellac of Varied Thicknesses
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 

218 
 

The figure below is complementary to the discussion in section 5.3.2 of Chapter 5.

Transmission Rate (mg/m2.h)

1800
4um wet thickness
10um wet thickness

1600
1400
1200
1000
800
600
400
200
0

10

15
Temperature

20
(oC)
 

Figure J.1. CO2 transmission rate through shellac coating of 4 and 10 µm wet thicknesses at 10,
o

15 and 20 C (mean ± SD)
 

219