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This is to certify that the

dissertation entitled

A Study of Blackspot Bruising in Potatoes

presented by

Habibur R. Chowdhury

has been accepted towards fulfillment
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MSU IoAn Nam-five ActIon/Equd Opportunlty Institution
Wm:

A STUDY OF BLACKSPOT BRUISING IN POTATOES

BY

HABIBUR R. CHOWDHURY

A DISSERTATION

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

DOCTOR OF PHILOSOPHY

Department of Agricultural Engineering

1995

ABSTRACT
A STUDY OF BLACKSPOT BRUISING IN POTATOES

By

Habibur R. Chowdhury

Blackspot bruising is caused by internal tissue failure due to stresses induced by
dynamic contact pressure during free fall impact of potatoes. A technique was developed
to identify potato bruising rapidly. Dynamic contact pressure was measured by dropping
potatoes onto a pressure sensitive film attached to a steel surface. A threshold dynamic
contact pressure that causes internal bruising was found for each variety. The dynamic
contact pressure was affected by the mechanical properties of potatoes. The study
revealed that dynamic contact pressure was highly correlated with yield stress of
potatoes. There existed a high correlation between drop height and the
polyphenoloxidase activity in a susceptible variety. Therefore, bruise susceptible variety
shows higher polyphen ol oxidase activity upon impact. Potato samples were
characterized as moderately resistant to moderately susceptible based on the optical
density. Varietal differences in measuring bruise susceptibility was found to be

significant. The mineral contents was affected by the variety. Stress distributions in

Habibur R.Chowdhury

a loaded potato model were studied using finite element method. von Mises contour
stress band was used to locate failed elements in the loaded potato model. The von
Mises stress was found to concentrate at the area of failed elements below the surface

at the stem-end of the potato model where most blackspot bruising occurs.

/

        

Approved: fiflwm/

Department Chairperson

DEDICATION

This dissertation is dedicated to my beloved father late Dr. Luthfor Rahman
Chowdhury, LMF, MBBS, and my beloved mother Mrs. Shamsun Nahar Chowdhurani
who always encouraged for higher studies and gave strong moral support towards my

successful accomplishment.

iv

AC KNOWLEDGENIENTS

I begin with the name of Allah, the most merciful and beneficient. All praises
and thanks due to Him, for his divine direction in striving me for this long study. I
express my sincere thanks to my honorable major professor Dr. Ajit K. Srivastava for
his guidance, providing Assistantship and editorial help in writing this manuscript.

I am indebted to Dr. Roger C. Brook, Agricultural Engineering department for
providing three years of financial support with whom I started the project. His
personal help in collecting fresh potatoes from the growers and setting experiments have
greatly enhanced my experience, research capability and endurance. His pleasant
personality and friendliness have encouraged me and enhanced my effort that eventually
drove me to the end of the project.

I am grateful to Prof. Larry Segerlind, Agricultural Engineering department for
his suggestions about using Finite Element Method to solve the contact problem. His
suggestions in developing potato model and analysis were invaluable.

My deepest gratitude is extended to Prof. Jerry N. Cash, Food Science
department who served as a member of my committee and allowed me to use his
laboratory. His advice from time to time in determining and analyzing chemical

properties of potatoes were extremely helpful.

I am also expressing appreciation to Dr. Robert C. Hemer, Horticulture
department for his constructive suggestions.

I am extending thanks to Prof. Gary Cloud, Mechanics department for his serving
as a member of my committee. His suggestions and critics regarding the experiments
were appreciable.

Finally, a special thank to Dr. von Bernuth, Chairperson, Ag.Engineering
department for his financial support when there was no support available around. His
pleasant friendliness and generosity are invaluable input in completing my study and
dissertation.

I am thankful to Soil Testing Lab, Ag Enginnering machineshop, USDA
personnel and the graduate students who helped me in various ways to crOss this
dissertation bridge.

I am very much indebted to my beloved mother Mrs. Shamsun Nahar
Chowdhurani for her continuous encouragement, moral support at the time of my
frustration and her confidence on my successful accomplishment. I am very thankful to
my wife Shirin for her long outstanding patience, sacrifice, moral support, prayers and
encouragement that played a vital role behind this success. I do thankful to my son

Prince, a 5th grader who drew one figure using CANVAS for this dissertation.

vi

TABLE OF CONTENTS

Page
LIST OF TABLES .............................................................................. xiii
LIST OF FIGURES ............................................................................ xv
LIST OF APPENDICES ...................................................................... x
CHAPTER
1. INTRODUCTION ........................................................................... 1
1.1 Economic Significance and Magnitude of the Problem .......................... 1
1.2 Objectives ............................................................................... 3
2. LITERATURE REVIEW .................................................................. 4
2.1 Methods of Inflicting Bruise ......................................................... 4
2.1.1 Falling Mass ................................................................... 4
2.1.2 Pendulum ....................................................................... 9
2.1.3 Penetrometer ................................................................... 10
2.1.4 Simulated Handling ........................................................... 11
2.1.5 Dropping Packaged Potatoes ................................................ 12
2.2 Bruise Identification and Measurement ............................................ 13
2.2.1 Visual Examination ........................................................... l3

vii

Chapter Page
2.2.2 Temperature and Time Treatment .......................................... 14
2.2.3 Oxygen and Temperature Treatment ...................................... 16
2.2.4 Reflectometer .................................................................. 17
2.2.5 Spectrophotometer ............................................................ 17
2.2.6 Chemical Treatment .......................................................... 17

2.3 Bruise Classification .................................................................. 19
2.3.1 External Damage ............................................................. 19
2.3.2 Internal Damage .............................................................. 20

2.4 Mechanical Properties and Measurement ......................................... 23
2.4.1 Quasi-Static Loading ......................................................... 23
2.4.2 Impact Loading ............................................................... 27

2.5 Chemical Reactions and Measurement ............................................ 29
2.5.1 Enzymes"... ................................................................. 29
2.5.2 Enzyme Discoloration of Bruised Tubers ................................ 35
2.5.3 Minerals ....................................................................... 40

3. ESSENTIAL THEORETICAL BACKGROUND .................................... 45

3.1 Hertz Contact Stress Theory ........................................................ 45

3.2 Elastic Impact ......................................................................... 51

3.3 Visco-Elastic Impact ................................................................. 57

3.4 Elasto-Plastic Impact ................................................................ 59

3.5 Plastic Impact ........................................................................ 61

viii

Chapter Page

3.6 Peleg’s Model ........................................................................ 62

3.7 Summary .............................................................................. 68

4. POTATO BRUISING UNDER IMPACT LOADING .............................. 71

4.1 Objectives ............................................................................ 72

4.2 Methodology ......................................................................... 72

4.2.1 Blackspot Bruise Identification Technique ............................. 73

4.2.2 Determination of Dynamic Contact Pressure .......................... 73

4.2.3 Determination of Mechanical Properties of Core Specimen ......... 77

4.2.4 Determination of Dymamic Threshold Pressure ....................... 78

4.2.5 Assay Preparation and Determination of PPO Activity .............. 78

4.2.6 Determination of Optical Density ........................................ 79

4.2.7 Determination of Mineral Contents ...................................... 80

4.2.8 Determination of Bruise Susceptibility .................................. 80

4.3 Results and Discussion ............................................................. 81
4.4 Conclusions .......................................................................... 118
5. POTATO BRUISING BY QUASI-STATIC LOADING ........................... 122
5.1 Methodology ......................................................................... 122
5.1.1 Determination of Quasi-static Contact Pressure ........................ 122
5.2 Results and Discussion ............................................................. 123
5.3 Conclusions .......................................................................... 130

ix

Chapter Page

6 MODELING BASED ON FINITE ELEMENT METHOD ........................ 131
6.1 Theoretical Considerations ......................................................... 132
6.1.1 Stress-Strain Relationship in Elasto—Plastic Domain .................... 132

6.2 Finite Element Formulation ........................................................ 138
6.2.1 Plastic Stress-strain Matrix ............................................... 142

6.2.2 Matrix Formulation ........................................................ 143

6.3 Model Development ................................................................ 146
6.3.1 Core Model ................................................................. 147

6.3.2 Whole Tuber Model ....................................................... 147

6.4 Results and Discussion ............................................................. 148
6.4.1 Core Model ................................................................. 148

6.4.2 Whole Tuber Model ....................................................... 148

6.5 Conclusions .......................................................................... 162

7. SUMMARY AND CONCLUSIONS ........ ' ......................................... 166
7.1 Summary ............................................................................. 166
7.2 Conclusions .......................................................................... 171

8. BIBLIOGRAPHY ........................................................................ 173
9. APPENDICES ............................................................................ 189
A.1: Mechanical Properties of Bruised Superior Potatoes .................... 189

A2: Mechanical Properties of Unbruised Superior Potatoes ................. 190

A.3: Mechanical Properties of Bruised Snowden Potatoes .................. 191

A.4:

A.5:

B.l:

B.2:

C.1:

D.1:

D.2:

D.3

E.l:

B.2:

F.l:

F.2:

F.3:

Mechanical Properties of Unbruised Snowden Potatoes ..................... 194
Figure A.5-Yie1ded Core of a Fresh Superior potato tuber .................. 195
Figure A.6-Force-deformation Curve at yield of a

Superior potato ................................................................... 195

Calculated and Measured Quasi-Static Contact Pressure
for Superior potatoes ......................................................... 196

Calculated and Measured Quasi-static Contact Pressure for
Snowden Potatoes ............................................................... 198

Calculated and Measured Dynamic Contact Pressure
for Superior Potatoes .......................................................... 199

Table D.l Mineral Contents of Superior and Snowden
Potatoes ........................................................................... 201

Table D.2 t-test for the Differences of Mineral Contents of
Superior and Snowden Varieties (a=0.05) .................................. 208

Table D.3 t-test for the Differences of the Bruised and Unbruised
Tubers Impacted at Various Drop Heights (oz=0.05) ...................... 208

Figure E.1 Compressive Pressure Verification for Superior Potato
Core Specimen Using Pressure Sensitive Films During Uniaxial
Tests ................................................................................ 209

Figure E.2 Pressure-Deformation Trend on Whole Snowden Potato
Tubers (spherical indenter) ..................................................... 210

Figure F.1 Simulation of Force-Deformation for Snowden
Whole Potato Tubers ............................................................ 211

Figure F.2 The Deformed Model and its Deformation Contour
lines Due to 0.42 Inch Imposed Displacement (Snowden). The
Scale Shown on the Left is in Inch ......................................... 212

Figure E3 The Resulted Contour Stress Distribution Within the
Model due to 0.42 inch of imposed displacement (Snowden). The
scale on the left shows stress in psi ........................................ 213

F.4: Figure R4 The corresponding von Mises contour stress distribution in
the model due to 0.42 inch of imposed displacement (Snowden).
The scale on the left shows stress in psi ....................................... 214

F5: Figure ES The deformed model with the contour deformation due to

imposed displacement of 0.15 inch (Superior). The scale on the left
is deformation in inch ............................................................ 215

F6: Figure E6 The resulting contour stress distribution in the model
due to the imposed displacement of 0.15 inch (Superior). The scale
on the left shows stress in psi ................................................... 216

R7: Figure F.7 The corresponding von Mises contour stresses with bands

within the model due to 0.15 inch of imposed displacement. The scale
on the left shows stress in psi ..................................................... 217

Appendix G.1: Stepwise Analysis of Variance of BS
(Bruise susceptibility) .............................................. 218

Appendix G.2: Unwighted least square linear regression of BS
(Bruise susceptibility) .............................................. 219

xii

LIST OF TABLES

Tables Page
2.1 Distribution of several components in potato tubers

by zone (Reeve, 1969) ............................................................... 43
2.2 Concentration of components within the potato tubers (Reeve, 1969) ....... 43
4.1 Bruise susceptibility scale (Dean et a1, 1993) .................................... 81

4.2 Drop height and the corresponding percentage of bruised and
unbruised tubers ...................................................................... 88

4.3 Analysis of varaiance of dynamic bruising contact pressure
over variety ............................................................................ 91

4.4 Analysis of variance of dynamic bruising contact pressure
over dates of harvest ................................................................. 91

4.5 Analysis of variance of the event of bruising over variety ..................... 91

4.6 A step wise regression analysis of variance of dynamic contact

pressure for the bruised Superior potatoes ....................................... 92
4.7 Analysis of variance of yield strain over variety ................................ 99
4.8 Analysis of variance of strain energy over variety .............................. 99
4.9 Analysis of variance of yield stress over variety ................................ 99

4.10 Analysis of variance of modulus of elasticity

over variety ........................................................................ 100
4.11 Analysis of variance of yield strain over date of harvest ................... 100
4.12 Analysis of variance of yield stress over date of harvest ................... 100

xiii

4.13 Analysis of variance of modulus of elasticity over date of harvest ............... 101

4.14 Spearman correlation between mechanical properties of bruised
Superior potatoes ......................................................................... 102

4.15 Spearman correlations between mechanical properties of
bruised Snowden potatoes ................................................................ 102

4.16 A comparison of mechanical properties of bruised and unbruised
potatoes (a=5%) ......................................................................... 103

4.17 A comparison of mechanical properties of bruised Superior potatoes
with bruised Snowderr potatoes (oz=5%) ............................................ 104

4.18 Drop height, ppo activity, optical density of bruised,
unbruised and control potato tubers ................................................... 117

6.1 Simulation of uniaxial compression, and force at yield
deformation of cores of Superior and Snowden potatoes ............................ 164

6.2 Verification of the measured quasi-static contact pressure
on a whole potato with the pressure predicted by FEM ............................ 165

xiv

LIST OF FIGURES

Figure page
2.1 The formation of Melanin pigment resulting from oxidation of tyrosine

by phenolase (Fennema, 1976) ........................................................... 39
2.2 O-diphenol that serve as substrate

for phenolase (Fennema, 1976) ........................................................... 39
3.1 Configuration of two bodies in contact and intensity of pressure over the

surface of contact (Manor, 1978) ......................................................... 46
4.1 Two sheets type contact pressure sensitive film (Fuji Co.) ........................... 75

4.2 A sample of red color formation in the pressure sensitive film after being
impacted by a Snowderr potato from 15.24 mm (6 inch) drop height ............... 75

4.3 A sample of digitized dynamic contact pressure distribution over the
contact area of a potato tuber .............................................................. 76

4.4 A calibration curve for measuring contact pressure on the surface of
a potato tuber ................................................................................. 76

4.5 Light pink color developed in a bruised Superior potato tuber
dropped from 15 .24 cm (6 inch) drop height ............................................ 82

4.6 No pink color was developed in a fresh Superior potato tuber
dropped from 38.10 cm (15 inch) drop height ........................................... 82

4.7 Pink color in the bruised area of a Superior potato tuber dr0pped
from 38.10 cm (15 inch) height ........................................................... 83

4.8 A dense red colored circle of shatter bruise shown in a Superior potato
when dropped from 61 cm (24 inch) drop height ...................................... 83

XV

4.9 A blackspot occurred in a Snowden tuber at 5-6 mm below the skin when

4.10

4.11

4.12

4.13

4.14

4.15

4.16

4.17

4.18

4.19

4.20

4.21

4.22

4.23

dropped from 61 cm (24 inch) height .................................................... 84
Internal crack occurred in a fresh Snowden potato when dropped from

38.10 cm (15 inch) drop height .......................................................... 84
Color developed in a pressure sensitive film when a tuber dropped

from 38.10 cm (15 inch) height .......................................................... 86
Color developed in a pressure sensitive film when a potato tuber was

dropped from 61 cm (24 inch) height ................................................... 86
The effect of drop height on the percentage of. bruised

Superior potatoes ........................................................................... 87
The effect of drop height on the percentage of bruised

Snowden potatoes ........................................................................... 87
The dynamic contact pressure as affected by potential

energy of Superior potatoes ............................................................... 90
The dynamic contact pressure as affected by the potential

energy of Snowden potatoes .............................................................. 90
The dynamic contact pressure as affected by modulus of elasticity

of Superior potatoes ....................................................................... 94
The dynamic contact pressure as affected by the yield

stress of Superior potatoes ................................................................ 94
The dynamic contact pressure as affected by strain energy of

Superior potatoes ........................................................................... 95
The dynamic contact pressure as affected by yield strain of

Superior potatoes ........................................................................... 95
The dynamic contact pressure as affected by modulus of elasticity

of Snowden potatoes ....................................................................... 96
The dynamic contact pressure as affected by yield stress of

Snowden potatoes .......................................................................... 96
The dynamic contact pressure as affected by strain energy of

Snowderr potatoes ........................................................................... 97

xvi

4.24

4.25

4.26

4.27

4.28

4.29

4.30

4.31

4.32

4.33

4.34

4.35

4.36

4.37

4.38

The dynamic contact pressure as affected by yield strain of
Snowden potatoes ........................................................................... 97

Three minutes enzyme activity (Oxygen up take by catechol) of bruised
Superior potatoes drOpped from 38. 10 cm (15 inch) height ......................... 105

The resulting color of a supernatant of Superior potatoes after
3 min of reaction ......................................................................... 105

The deep yellow color of a supernatant of Snowderr potatoes
after 3 min of reaction ................................................................... 106

The effect of drop heights on polyphenol oxidase activity in Superior
potatoes ..................................................................................... 106

The effect of drop height on polyphenol oxidase activity in Snowden
potatoes ..................................................................................... 107

The light red color of homogenized extract of Superior potatoes before
open air oxidation ......................................................................... 111

The dense color of homogenized extract of Superior potatoes after 24 hrs
of open air oxidation ..................................................................... 111

A sample of homogenized extract of controls (left-Snowden,
right-Superior) for optical density measurement ..................................... 112

Homogenized extract of unbruised and bruised Superior potatoes
dropped from 15.24 cm (6 inch) for optical density measurement ................ 112

Homogenized extract (unbruised and bruised) of Superior potatoes
dropped from 38. 10 cm (15 inch) drop height for OD measurement .............. 113

Homogenized extract (unbruised and bruised) of Superior potatoes
dropped from 61 cm (24inch) drop height for OD measurement .................. 113

Homogenized extract of (unbruised and bruised) Snowden potatoes dropped
from 15.24 cm (6 inch) for OD measurement ....................................... 114

Homogenized extract (unbruised and bruised) of Snowden potatoes
dropped from 38. 10 cm ( 15 inch) drop height for OD measurement ............ 114

Homogenized extract of unbruised Snowden potatoes dropped
from 38.10 cm (15 inch) drop height ................................................. 115

xvii

4.39 Homogenized extract of bruised Snowden potatoes dropped from

5.1

5.2

5.3

5.4

5.5

5.6

5.7

5.8

6.1

6.2

6.3

6.4

6.5

6.6

38. 10 cm (15 inch) drop height for OD measurement ............................. 115
Bruising a potato at the stem end using a (7/ 8 inch dia) spherical
indenter on an Instron Testing machine .............................................. 124
The hook which indicates potato bruising during the compression
test using a spherical indenter .......................................................... 124
The deep red color formation in the pressure sensitive film due to
high contact pressure .................................................................... 125
Correlation between calculated and measured contact pressure for
Superior potato tuber .................................................................... 125
Load and deformation as observed in the bruised whole Superior
potatoes under loading by a spherical indenter ...................................... 127
Load-deformation as observed in bruised whole Snowden
potatoes under loading by a spherical indenter ...................................... 127
A nonlinear trend of measured contact pressures in whole
bruised Superior potatoes ............................................................... 128
Correlation between measured and calculated quasi-static contact pressures
in whole Snowden potatoes ............................................................ 128
A theothetical bruise initiation model for
soft material (Yew, 1956) ................................................................ 134
A FE model of a cylindrical core with 100 elements
mesh and force applied to it .......................................................... 149
The original and deformed Superior core model with a deformation of
7.62 mm (0.30 inch). The scale on the left shows deformation in inch ........ 150
The stress (02) in the core model due to the imposed deformation of
7.62 mm (0.30 inch). The scale on left shows stress in psi ...................... 151
The strain in the core model due to an imposed deformation of 7.62 mm
(0.30 inch).The scale on the left shows strain in in/in ............................. 152
The yield force of the core due to an imposed deformation of 7 .62 mm

(0.30 inch). The scale on left shows force in lb .................................... 153

xviii

6.7 The whole (Superior/Snowderr) tuber model with 136 elements mesh ............. 154

6.8 The model for Superior variety shows the displacement bands as a result of
10.16 mm (0.40 in) deformation. The scale shown on left is in inch ............ 156

6.9 The contour stress band (02) due to imposed deformation of 10.16 mm
(0.40 inch) in the Superior potato model. The scale on the
left shows stress in psi .................................................................... 157

6.10 The corresponding von Mises stress distribution with contour bands. The
scale shown on left is in psi ............................................................. 158

6.11 The resulted displacement of the Superior potato model due to imposed
5.08 mm (0.20 inch) deformation. The scale on the left shows
displacement in inch ...................................................................... 159

6.12 The stress (02) distribution with contour band due to imposed deformation
of 5.08 mm (0.20 inch). The scale shown on left is in psi ......................... 160

6.13 The rsulting von Mises stress distribution with contour bands due
to imposed deformation of 5 .08 mm (0.20 inch). The scale on the left
shows stress in psi ....................................................................... 161

6.14 A distinct blackspot bruise found approximately at 5.08 mm (0.20 inch)
below the surface of an affected potato tuber ....................................... 163

xix

1 INTRODUCTION

1.1 Economic Significance and Magnitude of the Problem

The potato is the fourth important crop in the world after rice, wheat and corn
providing an important source of vitamin C (Mary et al. 1939). About 356 million cwt
of potatoes were produced in 1988 in the USA. Out of this production, 50.3% were
processed, 33.5% were consumed as table stock and 8.5% used as seed (USDA, 1990).
The estimated value of the 1989-90 US potato crop was over $2.5 billion (MPIC, 1990).
The bruising in potatoes causes decrease in shelf life, consumer acceptance, low quality
and very high monetary loss. Approximately 8% of the total production is lost during
the harvesting and handling operations (USDA, 1990). According to National Potato
Council’s estimation, the potato industry lost approximately $150 million in 1985, in
other words it costs $12,000 to an average grower annually due to bruising. It also
costs over $11 millions to the potato processing industries. Washington State alone
produced 29 billion kg of potato crop worth $333 millions in 1990, which is 20% of
national production. One percentage of the crop loss due to bruise would cost the
growers about $3.33 million (Hyde et al. 1992). Therefore, bruising is of great
economic significance. The bruise is defined as damage to plant tissues by an
external force causing a change in texture, color (blackspot), and/or flavor (Mohsenin,
1980). The blackspot is an important indicator of mechanical bruising. The blackspot

bruise is usually identified as a small colored area 2—7 mm under the potato’s skin

1

(Sawyer, 1960; Scuder, 1950; Hughes, 1980; Chase, 1987). Several factors influence
the development of the blackspot. Impact during harvesting and handling causes bruising
and eventually the blackspot (Bishop, 1990). Soil condition, tuber temperature, variety,
chemical contents and the physical properties play a role in the development of
blackspots in susceptible potatoes.

According to Hardenburg (1938), 78% of potatoes affected by blackspots were
bruised during mechanical harvesting. Nylund (1955) stated that harvesting injury
contributed 26% of the observed damage. Soil conditions affect harvester operations.
For example, in clay soil, low forward speed of the harvester relative to chain results in
increased tuber movement on the chain which causes a higher incidence of tuber
bruising. On the other hand, greater harvester forward speed in sandy soil results in
direct contact of the tuber with the primary chain during harvester operation which
increases the chance of tuber bruising (Timm, 1989).

Davis (1952) reported that 43% of tubers were damaged by the time they, were
packed. A dry grading line resulted in 12% mechanical injury (Nylund, 1955). Low
tuber temperature may enhance impact damage. Seventy seven percent tubers were
splitted when dropped from 110 cm height at 4° C tuber temperature. Under the same
loading condition, 38% of tubers were bruised at 8° C tuber temperature (McRae, 1976).
Nylund (1955) reported that keeping the tubers at 3-5° C storage for 6 months increased
damage from 8% to 34%.

Impact during handling increases the incidence of blackspots and also stimulates

the bio-chemical synthesis in the tuber. Sowokinos (1987) found that sucrose

concentration exceeded 1% (fresh wt basis) in 10 days after mechanical handling. More
than 65% of the maximal sugar accumulation occurred within 5 days of handling. It has
been generally assumed that impact energy, contact pressure, minerals and polyphenol
oxidase activity (ppo) play a substantial role in the blackspot bruising phenomena. To
maintain a high quality of raw product it is necessary to know how potato tubers bruise
so that its remedy can be prescribed. Although bruising in potatoes is a major problem
due to mechanical harvesting and handling, the mechanism of internal bruising is not
fully understood.
1.2 Objectives

This dissertation is primarily concerned with the developing an understanding of
the mechanism of blackspot bruising. The specific objectives were:
1. to develop a bruise identification technique.
2. to study potato bruising during an impact.
3. to study potato bruising due to quasi-static loading.
4. to develop a measure of bruise susceptibility.
5. to study the relationships between bruise susceptibility and mechanical

properties, polyphenol oxidase activities, and mineral contents.

6. to develop a potato model using FEM, study stresses and apply failure criterion.

 

2 LITERATURE REVIEW

Potatoes are bruised during harvesting and handling operations. The severity of
damage depends upon the deformation during impact. The impact velocity, masses,
elastic or plastic characteristics determine the amount of deformation during an impact.
According to Schippers (1971), an uniform method of bruising is important for 3
reasons: 1) no blackspot will occur without damaging the internal cell of tuber, 2) the
intensity of discoloration of bruised tissues is dependent on the location of impact- stem
end or bud end, and 3) The severity of the blackspot is dependent on the force of
impact. In addition to these, 4) the blackspot is also dependent on contact pressure
which many authors have ignored.

2.1 Methods of Inflicting Bruise
2.1.1 Falling Mass

Kunkel et al. 1959; Kunkel et al. 1986 used a metal plug dropped through a
cylindrical tube to bruise potatoes. The plug was held by a magnet on the top of the tube
and allowed to free fall on to the tuber surface. Slots in the tube reduce air pressure
build up as the plug fell. The magnet holder was able to be moved vertically up and
down in the tube. The plug rested on a collar clamped to the bottom of the tube. The
height of the fall was adjustable to change the momentum of the striking body and the
bruising force. The potato tubers were grouped as hydrated and dehydrated. The tubers
were dehydrated for 24 hrs and tested at 21° C tuber temperature since blackspot

4

susceptibility was minimum (3 %) at this temperature. The tubers were scored on a scale
0-6. The results are discussed later in 2.2.1

To determine the effect of turgidity on blackspot bruises, Sawyer (1960) cut
potato discs from the vascular regions. The discs were dipped into mannitol solutions
followed by bruising by dropping 20 g mass from 10 cm height through a vertical
cylinder. The results are discussed in 2.2.1

Weaver (1966) bruised potatoes at the stem end by dropping a 100 g metal mass
4 times from 61 cm height. He defined bruise susceptibility by counting 3 blackspots
out of 4 impact in each tuber. The tubers were evaluated for intensity of black color in
a spot as recorded by a densitometer after 48 hrs of bruising. The results are discussed
in 2.2.2

Maas (1966) used an apparatus which resembled to Kunkel’s (1959) but he
modified the instrument. The new apparatus consisted of a 61 cm long and 2.22 cm dia
aluminum pipe clamped vertically to a ring stand. The bruising metal plug was 66 g
made of a round headed bolt. The drop height was 46 cm. The head of the bolt was 18
mm dia and 13 mm in radius that provided a convex striking surface. Tubers were
bruised immediately after removal from 5° C storage. Each potato was held firmly
against the bottom of the tube to avoid movement upon impact. Bruises were made in
a row along one side of the tuber near the bud, the middle and the stem end (no results
were reported).

Schippers (1971) used a procedure similar to Kunkel (1959) and Maas ( 1966).

Copper tubing 16 mm in dia of different lengths were closed at the bottom with a round

 

head bolt (16 mm dia). These tubes were dropped onto the potato tuber through a slotted
aluminum tube. Various sizes of lead cylinders were used to adjust the masses from 75
g to 225 g. The masses of the plugs were 75, 125, 175, and 225 g, and dropped from
8, 10, 12, 14, ..30 cm heights. The drop heights were adjusted by a side pin in the plug
which protruded through the slot in the guide tube. The bottom of the plug was inked
to show the point of impact. One bruise was inflicted about 1-2 cm from the stem end
of each tuber. Impacted tubers were left overnight at 15-20" C. The following day the
bruised spots were peeled until a maximum discoloration was found. The tuber's
temperatures varied from 1.7 to 128° C at the time of bruising. The size and the
intensity of color was measured on a scale 05. The results are discussed in 2.2.2

Howard et a1. 1961 also used a device similar to Kunkel et al. 1959. A bolt of
100 g was dropped once through a 60 cm tube onto the stem end. The bolt had a radius
of 2.5 cm. Only the larger tubers were bruised after harvest and bruised again after
being held for 3 days at 20° C in a ventilated storage. He looked into the effect of plant
age and post harvest response to bruise, effect of tuber temperature, humidity and storage
with modified atmosphere. He found that all fresh tubers harvested after 92 - 135 days
showed blackspots. Holding tubers for 3 days at 10-25° C decreased susceptibility. The
potatoes tubers kept under high and low humidity conditions had no difference in
blackspot appearance. A concentration of C02 between 0.5 and 5 % in storage caused
tubers to be highly favorable to blackspot bruises.

Fluck (1973) used an impacting device similar to that of Wright (1968). The

falling mass was acrylic plastic. Drop heights were 8, 10, l2, l4 and 16 cm. The

cylindrical specimen of potatoes were impacted with the falling mass. The falling mass
was adjusted from 365 g to 1650 g. The mass was held by a solenoid and dropped by
breaking the electric power. All cylindrical specimens failed at 16 cm drop height. A
piezoelectric accelerometer was mounted on the falling mass to monitor the deceleration
during the impact. He measured peak deceleration during impact. He found peak force
increased with height or mass. He stated that peak force, energy of impact, and resulting
internal stresses are the critical elements in the incidence of impact bruising. He also
recommended to study further to prove if bruise increased with increase in drop height
or mass or energy or force that causes bruise in agricultural products.

Massey et al. 1952 stored tubers at 8° C for 3 months and bruised tubers upon
removal from the storage. The tubers were returned to storage for 2 days then peeled
for blackspot. The bruised tubers were rated on the scale of 0-9. The results show that
there was a high correlation between specific gravity and blackspots index. The number
of blackspots gradually increased during the 3 months of storage and then declined
slightly. He reported that the effect of variety and geographical area significantly
affected the blackspot index.

Pavek (1985) used two methods of potato bruising: abrasive and dropping mass.
He used 30, 52 and 74 g metal masses to drop one by one from 16 cm heights onto the
tuber’s surface. Each potato tuber was impacted four times on the stem end resulting in
12 impact points on each potato. The impacted potato tubers were kept at l6-18° C for
24 hr and evaluated by hand peeling and counting the number of blackspot per tuber.

Enzymatic color development due to abrasive peeling was correlated with the amount of

blackspot due to mass impaction.

McRae (1978) conducted impact tests which showed that damage can occur at
24 cm drop height when potato tubers were dropped onto a steel web rod. Different
varieties sustained variable amount of damage.

M.Ito et al. (1994) investigated the effect of drop height on bruising of potatoes.
The sammes of potatoes were held at a preselected height by a vacuum cleaner. The
potatoes were dropped on to 5 different surface types by releasing the valve of the pump.
The impacted potatoes were evaluated using a damage index (D1). The damage index
was defined as the sum of the damage points from various types of damages inflicted
on the potato during impact. Damages were classified as skinning, cracking, and
bruising. He found that DI index increased linearly with an increase in drop height
beyond 40 cm but the DI was small at below 30 cm drop height.

Various researchers dropped varied amount of masses ranged from 30 g to 1650
g from different heights ranged from 10 cm to 60 cm and used abrasion to inflict bruises
to a stationary potato tuber. They incubated the bruised tubers in 5, 10, or 20° C for l
to 4 days before scoring the blackspots. Tubers were also left in high and low
humidity storage with various amount of C02 in it. The widely varying methods of
determination make it difficult to compare the results of different authors. However, in
real field, tubers are dropped on the hard surface during harvesting and handling
process. Therefore, bruising a tuber by falling mass onto it is not exactly desirable,
instead, bruising a potato by dropping it on to a hard a surface is more relaistic and

simulates harvesting and handling.

2.1.2 Pendulum

Parke (1963) used a pendulum constructed with a block of iron attached on one
end of a 261 cm long aluminum rod. The 7.6 kg rod was released from different
heights to give a range of velocities varying from 35 to 150 cm/s. The potatoes were
grouped by mass in 6 groups ranging from 55 g to 170 g. The potatoes being tested
were suspended by a pair of threads to the pendulum. Potatoes were struck at the stem
and at the bud end. He investigated the effect of the size of the striking bars, potato
mass and impact velocity upon energy absorption and tuber damage. He stated that the
absorbed impact energy by potatoes was correlated with the volume of bruised tissues.
He found a minimum energy value of 1.3x10° ergs that produced damage. Potato
structure also had influence upon the incidence of potato damage. The potato mass
interacted with impact velocity of the striking bar. Bar size influenced the amount of
damage at higher velocity above 200 cm/s. He concluded that a definite value of
minimum amount of energy absorption required to produce damage could not be
identified but bruise could occur at a very low level of energy absorption at 230 cm/s
velocity of a bar.

Noble (1985) also used a pendulum of 4 different masses with 4 different drop
angle to bruise potatoes. A hemispherical impact head of 12.5 mm radius was used.
The pendulum was fitted with a system for measuring the rebound height of the
pendulum and the deformation of a tuber upon impact. A piezoelectric accelerometer
was fitted to the pendulum head to measure deceleration. He plotted an acceleration-time

curve. The results are discussed in 2.2.2.

Slcrobacki (1989) used a pendulum that included an electronic readout which
displayed the rebound angle. An electromagnet held the 50 cm long impact arm in the
raised position until it was released. The mass of the arm varied from 76 g to 257 g and
the impact head radius varied from 5 mm to 10 mm. The impact energies varied from
0.08 J to 1.26 J and impact velocities varied from 1.4 m/s to 3.1 m/s. The tubers stored
at 5° C were impacted at four points around the circumference of tubers by the
pendulum. He left the impacted tubers at 20° C for 7 days to develop blackspots bruise.
The results are discussed in 2.2.2.

Hyde et al. (1993) used a pendulum of 4 m radius to bruise the tuber. The potato
was suspended with a thin wire so that the pendulum could hit the potato. A
piezoelectric force sensor and area sensor were used to measure the impact force and
contact area (no detail of how area measured was available). The contact pressure was
measured by dividing the force by area. He used constant height multiple impact
technique to determine bruise energy and dynamic contact pressure. Bruise energy =
Sum of [initial energy- rebound energy—equilibrium energy]. Several varieties were
tested. He dropped the tuber from a 17.5 cm 7 times until he found contact pressure
that remained almost constant. He found that the pressure decreased with each
successive impact. The average of the 5th, 6th and 7th impact was taken as the dynamic
yield pressure. The yield dynamic contact pressure ranged from 0.93 MP3 to 1.14 MPa.
The Russet Burbank was found to be the most resistant variety.

2.1.3 Penetrometer

A hand held penetrometer was used to demonstrate resistance to puncturing of

10

peeled and unpeeled tubers (Killick, 1972). He suggested that puncturing can not be
used as a means of selection for high resistant potatoes.
2.1.4 Simulated Handling

Ophuis (1958) ran potatoes once or sometimes twice over a grader and dropped
potatoes twice from one box to another. Potatoes were also bruised by dropping
through a 40 cm dia cylinder of 3.5 m length fitted with baffles to impact falling
potatoes.

Wiant (1951) bruised potatoes in a 56 cm dia spherical drum revolving at 20 hand
rpm. Tubers were steam peeled and examined for blackspots. He investigated the effect
of temperature on blackspots. He stated that 2 days exposure of potatoes to 18-24° C
before or after bruising, caused blackspot disappear significantly. Bruising following
an exposure to higher temperature resulted in a smaller blackspot index (% bruise x
score). Pavek (1985) peeled potatoes abrasively in a Herbert peeler. Each sample was
abraded for 303 with water flowing over the tubers. The tubers were kept at 16—18° C
for 24 hr after bruising. Scoring was on 0 (no color) to 5 (darkest color) scale.
Readings of discoloration were made with a photovolt model 67 reflectance meter. He
found that enzymatic color was correlated to abrasive force.

Skrobacki (1989) used an electric motor to drive an impact head by means of
speed reduction pulleys and a cam. The impact head was changeable to allow use of
several different radii and the arm lengths were adjustable to attain the desired impact
energy and impact velocity. Tubers were placed against a plastic port to receive impact

for determination of the shatter and the blackspot bruises. The striking masses were 2.9

11

kg and 1.7 kg. The height of impact was adjusted from 19.5 cm - 13.0 cm. The
impact energy applied were varied from 0.9 J - 0.20 J. The radius of the heads were
5, 7.5, and 10 mm. The findings are discussed in 2.2.2.

James (1945) reported that most of the blackspot found in commercial lots
developed soon after the potatoes were graded and sacked and it resulted from the
mechanical injuries sustained by pressure bruise during handling. He concluded that
temperature was found to have an effect on the development of blackspot. Blackspot
developed in greater amount at the lowest and least amount at the highest temperature.
2.1.5 Dropping Packaged Potatoes

Turczyn (1986) dropped packaged fresh and stored potatoes from various heights
onto a hard surface to determine the minimum impact shocks that will exhibit shatter
bruise. There were 5 replications of dropping in each of the following heights: 12.7,
25.4, 38.1, 50.8, 63.5 and 76.2 cm. The shatter bruise was measured by visual
examination. Both fresh and stored potatoes developed shatter bruise at lower
deceleration g levels than potato packed in boxes. Potatoes packed in baler bags
incurred bruising starting at 20 g while potatoes packed in boxes did not bruised at 30
g. He found that fresh potatoes packed in baler bags and fire board boxes exhibited
bruising at 457 cm/s (80 cm) and 605 cm/s (140 cm), respectively, but stored potatoes
damaged at 457 cm/s regardless of package type. He concluded that the drop heights and
impact acceleration that caused bruising were often found in the normal handling during
loading and unloading. He mentioned that temperature of tuber and type of handling

affected the severity of shatter bruise. He showed that fresh potatoes had a tendency

12

to shatter more than stored potatoes if tuber temperature was less than 10° C.
2.2 Bruise Identification and Measurement

External damage is readily identified visually. Methods developed by researchers
to identify blackspot bruises are as follows:
2.2.1 Visual Examination

Kunkel ( 1959) assigned bruised tubers a degree of discoloration on a scale of 0
to 6, 24 hrs after peeling:

0 = no black color

6 = intense black color

The results showed that as the force of bruising increased, the severity of black
spot increased. By hydrating, the tubers became resistant to blackspot. Rehydration in
brine solution over 49 hrs reduced bruise susceptibility. There was no correlation
between specific gravity and blackspot susceptibility. Tubers from low humidity were
more susceptible to blackspot. He reported that turgid tubers were more resistant to
blackspot susceptibility. Degree of discoloration of bruised tissues varied among tubers
of the same plant and with position on the tuber. He concluded that blackspot bruise
varies with impact force. He also found that susceptibility to blackspot varied
significantly among and within the tubers.

The results of Sawyer (1960) showed that a disc of fresh tuber developed deep
color upon treating with 0.8M mannitol solution. Long stored tubers when dipped in
to the distilled water showed no color after bruising. He concluded that the higher the

turgor, less the susceptibility to blackspots.

l3

2.2.2 Temperature and Time Treatment
Schippers (1971) incubated bruised potatoes left overnight at 15-20° C before
peeling. Size and intensity of color of the spots were measured on a scale of 0 to 5:
0 = no discoloration

1

very small spot and faintly colored
2 = gray or brownish color (3-5 mm dia)
3 = intense gray color (5-10 mm dia)

4

brownish black (10 mm dia)

5

intensity black color (10 mm dia)

The blackspot rating was measured by averaging a sample of 20 tubers. He found
that there was a highly significant interaction between variety and tuber temperature,
and between varieties and dates of test. The blackspot rating highly correlated with the
potential energy of the metal plug, varieties, and tuber temperatures. Skrobacki (1989)
held potato tubers for 7 days at 20° C after impact, then treated with tetrazolium chloride
solution, peeled and evaluated as reported by Schippers (1971). He found that the
shatter bruise index did not correlate with the tuber’s mass, but it correlated with impact
energy. The shatter index decreased considerably during storage at all impact levels.
The blackspots from lightly impacted tubers also decreased during storage. The
blackspot index did not correlate well with impact energy of pendulum or impact
velocity. Therefore, these do not appear to be useful parameters for predicting
sensitivity to impact damage.

Noble (1985) selected potato tubers of uniform shape and size. Two days after

14

harvest, they were cut in half longitudinally and impacted at 10° C tuber temperature on
the rounded side. After the impact, the tubers were exposed to 20° C for 10 days.
Width, length and depth of bruising were measured. He found a linear relationship
between the kinetic energy of an impact and the energy absorbed by a tuber. Impact
duration increased with increasing mass of the pendulum but decreased with increasing
drop angle. There was not any damage striking at 50°-60° angle with 445 g pendulum.
Some splitting occurred when a'pendulum of 577 g hit the tubers at 65° angle. He stated
that, for a given amount of energy absorbed, the type of bruise damage will depend on
the impact duration and impact velocity, i.e. long duration and low impact velocity
produced blackspot and short duration with high velocity produced shattering. He
mentioned that a large potato making a low impact will tend to sustain blackspots,
whereas small potatoes making fast impact will tend to shatter internally. He concluded
that shatter bruising was correlated with energy of absorption. He found a high
correlation between impact energy and bruise volume.

Weaver (1966) kept impacted potatoes in an incubator for three hr at 40° C at
intervals of 0, 3, 6, 12, 24 hr following bruising. The tubers were evaluated
immediately upon removal from the incubator (48 hrs) and after conditioning at 24° C.
He assessed the percentage of tubers with blackspots. He stated that a densitometer can
be used to measure discoloration of bruised tissues. He found that higher temperature
had significant effect on incidence of blackspot bruising. He also determined phenolase
activity. He found phenolase activity at the stem end was significant after 1 hr of

bruising.

15

Smittle (1974) dropped 100 g mass from 3, 6, 12 and 24 inch heights on to the
stem end of a potato once, twice or fourth to bruise it. He measured the number of
blackspot and shatter bruise. He stated that as the blackspot increased shatter bruise
decreased in all tuber temperature. Multiple impacts increased incidence and severity of
blackspot which affected shatter bruise. He also found that a tuber hydration level which
produced little damage when bruised at a tuber temperature ranged from l8-2l° C,
resulted in shatter bruise when subjected to the same force at 7—10° C. Conversely, a
hydration level which resulted in blackspot at 18—21° C resulted in a slight to moderate
bruise when subjected to the same impact at 10-13° C of tuber temperature.

2.2.3 Oxygen and Temperature Treatment

Weaver (1966) kept one set of potato tubers in 100% oxygen and another set in
20% oxygen for 8 hr at 24° C. Then the tissue temperature was raised to 40° C.
Potatoes were bruised at 0 and 6 hr after reaching tissue temperature of 40° C. Half of
each bruised group was kept in 100% oxygen and the other half in 20% oxygen for 12
hrs. All bruised tubers were kept at 24° C for 96 hrs, then peeled and the percentage of
tubers with blackspot was determined. The exposure time needed at 38° C or above was
between 15 to 24 hrs for blackspot to appear. The tubers subjected to 20% oxygen had
more blackspot than those exposed to 100% oxygen. He concluded that the tubers
subjected to 100% oxygen for 6 hr had deep blackspot when bruised after being exposed
to 40° C. Probably this is true because deep color formation is a result of polyphenolase

activity which depends on the amount of oxygen to react with substrate.

16

2.2.4 Reflectometer

Kunkel (1986) peeled tubers 24 hrs after bruising. The peeled area of a tuber was
placed against the orifice of the reflectometer centered on the black spot. The
reflectometer measured the intensity of reflected light. The reflectometer could not
differentiate between natural color and unusual black color.
2.2.5 Spectrophotometer

Birth (1960) used a spectrOphotometer to detect decay, greening, blackspot,
hollow heart and other discoloration. More colored area in the affected tuber absorbed
more energy than an unaffected potato tuber. It seems that the Spectrophotometer can
effectively be used to identify black color and thus optical density of impacted tubers.
2.2.6 Chemical Treatment

Aspinwall (1962) observed for color development after dipping potatoes in
solutions of paracresol and/or iodine. Impacted tubers were washed and dipped into
iodine solution (250 g iodine + 500 g potassium iodine + 5 gal of water) for 2-3
minutes, and/or into the paracresol solution (500 g paracresol + 100 g sodium hydroxide
+ 5 gal of water) for 3 minutes and allowed to stand for 10-15 minutes and were
examined for black and pinkish color, respectively. The treated potatoes were
categorized as undamaged if no bruise was found visually, and skinned, or noticeable
damage found by peeling were categorized as severely damaged.

Hudson (1977) immersed the tuber samples into a catechol solution (7.4g pyro
catechol/liter) for 10 minutes, and then removed, dried and scored them. The skinned

areas were scored against 2.4 cm2 equivalent areas (less than 2.4 cm2 was rated as 0).

l7

The depth of colored area due to catechol was measured on the basis of slices that were
removed. The depth of cut was measured by a potato peeler; one slice rated as
moderate, three as severe, and seven as cull.

Beaver (1985) used a triphenyl tetrazolium chloride solution (4 g in 1 gal of
water) at 45° C to 27° C to detect the bruised surface of potatoes. The affected potato
tuber was peeled and dipped into the solution for 45-60 minutes to develop pink color
in the affected tissues.

Sawyer (1960) used mannitol solutions (0.8Mole or 0.9Mole) to identify the
internal damage (blackspot) of bruised potatoes. Disks of potato tubers were cut from
the vascular region and dipped into the solution for 30 minutes followed by bruising with
a 20 g mass dropped from a height of 10 cm. Then potatoes were removed from the
solution and were examined immediately for black color formation. A number of other
discs were immersed into 0.8M mannitol solution, then in distilled water, and finally in
0.8M mannitol solution. After every dipping, discs were bruised and observed for
discoloration. The discs exposed to 0.8M or greater mannitol solutions developed color
after bruising.

Smittle ( 1974) found that in some cases the catechol could identify shatter bruise
but failed to detect internal blackspot. Lye peeling and abrasive peeling identified both
blackspot and severe shatter bruise. He suggested that this can be used to determine
shatter bruise.

Skrobacki (1989) immersed impacted tubers in a 2% catechol solution for 10

minutes. This showed all external damage. The samples were kept for 24 hrs at room

18

temperature, and then peeled to detect internal damage or blackspot appeared by color.
These results revealed that there is a great potential for using chemicals for
identification of internal bruise if such a method is developed.
2.3 Bruise Classification
Bruises of potato have been classified as external and internal damage. Bruise
occurs when tubers collide with moving or stationary parts of equipment, clods, stones,
other tubers and when they are dropped onto a hard surface, such as a floor (Hughes,
1980).
2.3.1 External Damage
Skinning: Some parts of immature tuber’s skin fall apart due to handling (Chase, 1987)
or due to insufficient skin set at the time of harvesting (Hesen, 1960). It can also be
caused by abrasion of tuber against a rough surface or against another tuber (de Haan,
1987; Witz, 1954).
Cuts and Scraps: When a piece of tuber is totally cut off during harvesting or handling
(Hesen, 1960) or when tubers strike or are forced against a sharp cutting object (Chase,
1987), the damage is called a cut. A flesh wound results when a piece of tuber is
knocked out during any operation (Hesen, 1960).
Cracks, Shatter or Splits: These occur due to impact. They may occur during
harvesting, transportation or handling (de Haan, 1987). Cracks or splits in the tuber
surface which penetrate the flesh may occur more often at low temperatures (below 10°
C) during harvesting and handling (Chase, 1987).

Pressure Bruise: This is a result of static pressure due to a high stack in storage which

19

is not maintained at a high humidity. Eventually a flattened, softened and indented area
develops in the tuber (Chase, 1987; Meijers, 1987).
2.3.2 Internal Damage

Internal damage is caused by impact of a mass onto a tuber. Internal damage may
take several forms depending on whether the cell walls (internal crushing and shattering)
or the cell contents (chemicals) have been damaged (Hughes, 1980). Thus, internal
damage may be evidenced by one or more of the following:
Internal Shattering: Short impact durations and high loading velocities will produce
internal shattering, that will look like a ring or star shaped damage (Noble, 1985).
Internal Crushing: Long impact durations and low loading velocities will produce
internal crushing. The resulting damage is brownish area with distinct edges with the
center of the bruise being dry leaving a hollow cavity. It occurs when a very large mass
impacts the potato tuber (Noble, 1985).
Blackspot: Blackspot is often a result of impacts with low loading velocities which
cause the disruption of the cell contents (Hughes, 1975). Noble (1985) defined black-
spot as a blue gray pigmentation. There are several definitions of blackspot which have
been used by various authors, they are as follows:

Hesen (1960): A tuber may be bruised internally without any visible surface
damage. The cell walls of the tuber tissues are broken down, the damaged tissues being
discolored due to an enzymatic process in the presence of oxygen. The intensity of color
increases with time after two to three days. The color is sometimes grey or brownish,

but usually intense blue.

20

Chase (1987): Blackspot is a dark spot in the tuber flesh beneath the skin as
a result of a series of biochemical reactions leading to the production of black pigment
(melanin) in the impacted tuber. This black spot usually develops 24 to 48 hrs following
an impact on a hard surface.

Hughes (1980): A diffused blue-black or brown zone found just under the skin
that develops in one to three days after an impact. The blue-black pigment (melanin) is
formed by enzymic oxidation of tyrosine by phenolase when cell membranes are
damaged.

Gray (1978): Typically a blue-grey (sometimes brown) spherical zone in the
region of the vascular tissue found one to three days after impact damage. The skin
often does not show any visible sign of damage. The pigments responsible for blackspot
are produced by oxidation of phenolic substrates (tyrosine and possibly chlorogenic acid)
by phenolase.

Meijers (1987): Blue discoloration found just below the skin of an affected
tuber, usually around the vascular bundle. The discoloration of the tissue is caused by
oxidation of certain phenols by phenol oxidase (enzyme). Not only tyrosine but
chlorogenic acid and caffeic acid are also involved in the discoloration phenomena.

Li (1985): Damage or injuries of tuber flesh under the skin which turns brown
or blackish over a period of time. The colored spot in the vascular region is not visible
unless the tuber is peeled. This colored spot is called blackspot and within 24 hrs it
reaches maximum blue-black color.

Sawyer (1960): A sub-surface discoloration appearing after handling, caused

21

by chemical reactions. It occurs most frequently at the stem end about 6-7 mm below
the skin. The skin of the tuber need not be damaged to the extent of a cut or break for
blackspot to occur. The color can vary from light grey or bluish grey to an intensive
black color. Discoloration is observed in susceptible potatoes 24 hrs after bruising.

Scudder (1950): A sub-epidermal defect of a potato tuber located 1-2 mm
under the periderm, which is not discernible until the external tissue has been peeled off.
It is induced by bruising forces great enough to rupture cells, and the color develops
within 24 hrs following bruising. The shape of the blackspot varies from spherical to an
oblate spheroid with the point of maximum diameter occurring below the periderm.

Kunkel (1986): A sub-epidermal blackening of tissue that results when bruising
forces, such as impact, rupture the cells of susceptible tubers.

Some authors explained that the gray to black pigment in pre-peeling blackening,
blackspot, pressure bruising and black heart of potato tubers result from the enzymatic
oxidation of tyrosine by polyphenol oxidase (tyrosinase)- a copper containing enzyme.
In the presence of oxygen the enzyme oxidizes tyrosine to 3-4 dihydroxyphenylalanine
(Dopa) which is then rapidly oxidized by the enzyme to dopaquinone. The dopaquinone
cyclizes to 5-6 hydroxyindole derivatives which are oxidized to the reddish-orange
dopachrome pigment. This is the pigment seen in the early stages of enzymatic
blackening. After formation of dopachrome, a series of non-enzymatic polymerization,
oxidations and reactions occurs with proteins to form brown to purple pigmentation, and
finally the black pigment called melanin is produced (Joslyn, 1951).

The extent of development of black tissue in a potato tuber depends significantly

22

on the force of impact with a colliding object. Thus, tubers with identical susceptibility
but when different masses dropped on to a solid object receive different levels of impact
and develop different degrees of blackspot (Peterson, 1975). High temperature after
damage accelerated the development of the blue coloration. However, not every instance
of damage resulted in blackspot (Wiant, 1951).

Several researchers have stated that synthetic melanin can be obtained by the in
vitro oxidation of benzenoid and phenolic amino acids. This oxidation may be
photochemical, chemical, auto-oxidative or even enzymatic (Mason, 1947—49). The
intensity of color of melanin is dependent on its chemical constituents and intercellular
oxygen content. Dense melanin granules appear black whereas sparse areas appear
brown or tan (Jacobson, 1934). Melanin is insoluble in water or organic reagents but
moderately soluble in alcohol and pyridine, and completely soluble in acid and alkali.
Any black, brown, reddish brown, tan or amber pigment is called melanin (Van
Middelem, 1953). Definitions of external damage are clear but there are some
ambiguities among the definitions of blackspot as described by many authors. However,
it is understood that a product of chemical reactions with certain substrates and oxygen
influenced by a certain enzyme which eventually forms a product known as melanin is
so-called the Blackspot.

2.4 Mechanical Properties and Measurement
2.4.1 Quasi-Static Loading
Huff (1971) determined the tensile strength, failure strain and failure modulus

of two varieties of potatoes. The specimens were collected from three locations of fresh

23

and stored tubers and tested at 21° C and 6° C under varying strain rates. The results
showed that tensile strength varied considerably with location of a tuber and with year
to year. Increasing strain rate, caused an increase in tensile strength and failure
modulus, but failure strain was decreased. The pith in the center was found to be stiffer
than the perimedullary zones surrounding it with increased strain rate. However, tensile
strength, failure strain and failure modulus (stiffness) were 0.69 MPa, 0.49 and 5.09
MPa, respectively.

Four months of storage caused tensile strength and strain at failure to increase
in the center and decrease at the skin. Failure modulus did not change significantly at
any location. Lowering tuber temperature to 6° C the specimen became stiffer but tensile
strength and strain at failure did not change. Tensile strength of the stored potatoes
decreased when stored at room temperature. The tensile strength and strain at failure
decreased near the center. There was a significant difference between varieties for any
property near the skin. There was an interaction between variety and storage
temperature. Mechanical properties of the skin itself were higher than the properties of
tissues directly under the skin (Huff, 1971). He concluded that the periderm consisting
of thicker cork cell walls was stronger than the tissues under it. The answer to the
question as to where and why blackspot bruise occurs under the skin, probably, can be
found by measuring contact pressure and by using a theory of failure.

Finney (1964) measured the stress relaxation properties of potatoes. Tubers
were removed from 5° C storage about 24 hrs before the testing and were kept at a room

temperature of 26° C. The whole tuber was loaded between parallel plates until the load

24

reached 17.5 kg at a loading rate of 2.54 cm/min. Rate of deformation had a greater
influence upon the relaxation process during the first few seconds after stopping the
loading cycle. He found that stress continued to decrease with time. He calculated the
time constant over a 4 hr period which was 1083. Finney (1967) postulated that since
only a relatively small proportion of the initially induced stress was dissipated during a
1-s interval, this indicated that the potato tubers were highly vulnerable to localized tissue
failure upon loading.

Finney (1967) determined Young’s modulus (E) of potatoes using uniaxial
compression of cylindrical specimens. Tubers stored at 5° C were tested at room
temperature. The stress-strain relationship for a cylindrical specimen of potato tissue was
linear during loading. The average degree of elasticity (elastic deformation/total
deformation) was 46%; i.e only 46% of total deformation was recovered during
unloading. Hence, the potato was considered as inelastic. Cyclic loads were also
applied to the potato tubers. Elastic hysteresis of the potatoes were found to vary from
72% - 90%, averaging 81.5% of the total energy expended during the loading process.
Tissues which had been loaded, unloaded and then reloaded, exhibited an increase in
modulus of elasticity (E) during subsequent loadings. During the initial loading, E was
3.50 MPa and due to subsequent loadings of 0.0 - to 0.7 MPa stress, E varied from 0.7-
MPa - 7.0 MPa. The E of the tissues taken from the central part of the tubers without
previous loading history, was found to vary from 3.2 MPa - 4.6 MPa. He also
determined volumetric modulus (K) by applying hydrostatic pressures on to a whole tuber

ranged from 5 psi - 50 psi. His study showed that volumetric strain was less than 1%

25

at all hydrostatic pressure levels. The average bulk modulus was 77.93 MPa (11,300
psi). This showed that the potato tuber became relatively incompressible under high
hydrostatic pressure (since 85% of a tuber is water). He showed that an average elastic
bulk modulus for the mature potato tubers varied from 68 MPa - 105 MPa. He
calculated Poisson’s ratio, p, to be 0.492 by using the following equation:

a = (3K-E) / 6K

where, E = 3.74 MPa 1: 0.30 MPa

K 77.93 MPa 1 11.59 MPa

Finney (1964) measured puncture force and surface pressure (force/area of dye)
to determine resistance of potato tubers to bruising. He used a metal solid cylindrical
dye of 0.05 in2 size to puncture the tuber. The resistance of potatoes to external forces
decreased with time before harvest and increased with time after harvest. It was
suggested that soil moisture might have interacted with time to cause a decrease in the
resistance of the potato to mechanical pressure during the pre-harvest season. He found
a significant difference between certain varieties in their response to applied pressure.
He concluded that after attaining maturity, the longer the tuber remains under the soil,
the less the resistance to bruise.

Irritani (1974) found that higher shear force was required for potatoes with
higher dry matter and higher specific gravity, and increased temperature from l-7° C.
Dal Fabbro et al. (1980) determined failure strain using uni-axial compression tests.
He used cylindrical specimens (1.27 cm in length and 1.27 cm in dia). He applied three

different stress rates. Dal Fabbro et al. (1980) concluded that the potato had a critical

26

failure strain which was 0.43 but it seems very high for cylindrical specimens.
2.4.2 Impact Loading

Parke (1963) studied the effect of impact force. The amount of energy absorbed
by the potato during an impact was dependent on the impact velocity. The amount of
energy absorbed by heavy potatoes was 2.7 times higher than light potatoes. The
amount of energy absorbed by the potatoes was correlated with the volume of bruised
tissue:

V = 0.0002928 * J - 381.9 (r = 0.786)

where, V = bruise volume, mm3

J = energy absorbed by the potato, Joule
r= regression coefficient

He found that bruising can occur at a very low level of impact. An energy of 0.136 I
did produce an internal bruise. He stated that one potato was bruised at an energy of
0.09 J while another absorbed 0.6 J without bruising. The lowest recorded energy level
to produce a split was 0.2 J for a 143 g potato impacted at 200 cm/s. He concluded that
the size and shape of potatoes were very important criteria to be considered for incidence
of damage. The possible reason for this anomaly might be due to higher modulus of
elasticity of the unbruised potatoes than those of bruised tubers. Besides, the impact
energy which developed contact pressure during impact probably did not overcome the
threshold pressure because of bigger sizes of tubers (higher contact area). This probably
explains as to why some tubers did not bruise at the same impact level.

Ghadge (1988) found that some tubers bruised at 0.4] and some at 0.7]. He

27

concluded that there was no minimum energy level that would surely cause a bruise in
a potato. Probably this is not true. Potatoes must have a threshold pressure value.
Noble (1985) measured the energy absorbed due to impact. Impact results showed that
the type of internal bruising depended upon the impact condition. For a given amount
of energy absorbed, the expected type of bruise depended on the impact duration and
the loading velocity, e.g., long impact duration with a low loading velocity produced the
most blackspot. For the same energy absorption a large potato making a slow impact
tended to sustain internal crushing whereas a small potato sustaining a high velocity
impact tended to shatter internally.

Hughes (1975) found that blackspot susceptible varieties deformed more for a
given kinetic energy. He stated that volume of damaged tissue was inversely related to
the amount of potassium fed to the plants.

Johnson (1969) found that an increase of 5° C in soil temperature would tend to
increase the impact force necessary to bruise by 2% (by dropping). For potatoes,
dropped from 30 cm onto metal rods, the proportion of receiving bruises would be
reduced from 11 to 4%. Also, it was indicated that a 5° C increase in soil temperature
at harvest reduced the damage by 10%.

Various researchers bruised potatoes by free fall impact of metal mass or
pendulum. They measured bruised volume, impact energy and percentage of bruise to
draw conclusions. They also tried to find out the minimum energy that bruised tubers,
but failed to conclude. None measured contact pressure which can be an important

parameter that is responsible for bruising. This dissertation includes a study on contact

28

pressure due to impact or quasi-static loadings and its effect on chemical changes.
2.5 Chemical Reactions and Measurement
2.5.1 Enzymes

Enzymes are organic compounds containing atoms of carbon, hydrogen, oxygen
and nitrogen and belong to a class called Protein. There are 20 different amino acids
with their characteristic side chains. They are strung together like beads on a string to
give a long polypeptide molecule, referred to as protein or enzyme. Precise estimation
of number of the different enzymes in each cell is not known yet but it could be around
3,000-50,000 depending on the cell of origin (Nicholas et al. 1989). Each has its own
particular pattern of amino acid residues in its molecules. Every molecule of a particular
protein has the same characteristic pattern of amino acids. Enzymes are known to have
a definite shapes and structure made up of parts of different helixes and chains held
together in a complicated arrangement. In proteins with biological activity, the structure
of the molecule must be intact if it is to function in a proper way. Proteins may have
one shape and behavior when they are in pure crystalline form and they may have
different shapes and behaviors when they are dissolved in a solvent. When proteins are
roughly handled, they lose their normal or native characteristics by breaking weak bonds
that hold molecules in their special shape and become denatured. Strong acids and bases
also cause protein to denature and so does agitation, eg. beating. Enzymes are especially
sensitive proteins and when subjected to denaturation they lose their biological and
catalytic activity, and in some cases they may be rendered biologically inactive long

before they show signs of protein denaturation. Since protein molecules are complex in

29

structure and have many different chemical groups, they are extremely sensitive to their
environment. Protein extraction should be performed in a cold room to prevent
denaturation. Enzymes are catalytic agents- they take molecules of one kind and change
them into molecules of another kind. Enzymes can speed up chemical reactions 10”
folds and catalyze reverse reactions. Enzymes are characteristically very selective.
Enzymes clearly influence chemical reactions and remain unaltered by reactions.
However, enzymes possess reaction specificity (it catalyzes only one kind of reaction),
substrate specificity (particular substrate) and stereo specificity, that is, if substrate can
exist as stereoisomer then only one will serve as a substrate for the enzyme (Ferdinand,
1976; David, 1968; Nicholas. 1989). Co-enzymes (non- proteins) can accelerate or
hinder enzyme activities. The co-enzyme subsequently restores the atoms or groups to
new substrates in new reactions catalyzed by other enzymes. Many proteins require a
non-protein (Cofactor-metal ion) component for its activity as an enzyme. For example,
Kinase, cytochrome c oxidase need Mg and Cu ions for reaction (Nicholas et al. 1989).
Thus, ions apparently alter the shape of the protein molecules in such a way that they are
better able to react with molecules of a substrate. Co-enzyme’s metal ions are enzyme
activators too and they help increase the activity of enzyme (David, 1968; Nicholas,
1989). Factors that lead to enzyme catalyzed reactions rate are proximity and
orientation effects, acid-base catalysis, covalent catalysis, distortion and changes in
environment.

Proximity and Orientation effgts: An enzyme can increase the rate of reaction involving

more than one substrate by binding the substrates at the adjacent sites and bringing them

30

into close proximity with each other. Orientation of reacting molecules with respect to
each other can influence the rate of reaction up to 108 fold.

Agid-base: Since enzymes contain a number of amino acids side chains they are capable
of acting as protein donors or acceptors. The acid-base can influence enzyme catalyzed
reaction.

Covalent gatalysis; Reaction can be speeded up by the formation of intermediator
provided that such intermediators are rapidly formed and rapidly broken down.
Distortion: If a substrate is distorted upon binding to an appropriate enzyme, this would
speed up the reaction if distortion lowered the free energy of activation.

Change of environment; The rate of many organic reactions are highly sensitive to the
nature of solvents in which they occur. Dipolar solvents are good for enhancing reaction
(Nicholas et al. 1989).

Active site; Amino acids make up the active site to create in the enzyme surface a sort
of "hole" in to which the substrate must fit. The "hole" in turn must have a certain
definite shape which will accommodate substrate and inhibitors but reject other kinds
of substrates and prevent them from coming in to active contact with the enzyme —
Fisher’s lock and key concept. This is called enzyme specificity and is important to
enzyme activity. Most enzymes have many residues of the same kind of amino acid but
only one of them may be involved in the active site (Nicholas et al. 1989).
W The rate of a particular reaction is dependent on the amount of enzyme
and the amount of substrate that are taking part in the reaction. There could be as many

as 3000-50,000 enzymes in a cell (Nicholas et al. 1989). Each reaction taking place is

31

cata
dep<
6an
com

3111

nth:

then

catalyzed by its own particular enzyme in a given cell. The rate of reaction also
depends on the concentration (amount/volume) of the enzyme and substrate. In most
enzyme reactions the concentrations of enzyme is quite small, compared to the
concentration of a substrate. If the concentration is doubled, the reaction will proceed
at twice the rate (David, 1968). At a very low substrate concentration, the rate of
reaction is proportional to the substrate concentration but further increase in the amount
of substrate present per unit volume does not cause the reaction to proceed at a faster
rate. The varying effect of substrate concentration on enzyme reaction rates is referred

to as Michaelis-Memen. The rate,

V. = Vm (So) / [Km+(So)l,

where, Vo = initial reaction rate
Vm '2 maximum reaction rate
Km = Michaelis Constant
S, = substrate concentration.

High concentration of a single substrate does not follow the Michaelis equation;
rather, the rate passes through a maximum as the substrate concentration is increased and
then falls. The products of some enzyme reactions are able to act as inhibitors of the
enzyme that produce them (Stephen, 1991). There are six major types of enzyme
catalyzes reactions which are as follows:

1. Oxidation reduction reaction catalyzes by oxidoreductase
2. Group transfer reactions catalyzes by transferase

3. Hydrolytic reactions catalyzes by hydrolase

32

4. l
5. l
6. l
(All
Vega!
Phem
asat
Muir
monol
(each
iscolc

Chan

4. Elimination reaction (double bond is formed) catalyzes by ligase

5. Isomerization reaction catalyzes by isomerase

6. Reactions in which two molecules are joined at the expense of an energy source
(ATP) catalyzed by ligase (Nicholas et al. 1989)

Phenolase refers to a group of enzyme that helps browning of injured
vegetables and fruits. This group includes Phenoloxidase, Cresolase, Potato oxidase,
Phenolase complex. Phenolase in this dissertation will be called as PPO. Phenolase acts
as a catalytic agent in two different reactions: 1) Oxidation of o-dihydroxyphenols to
o-quinone or oxidation of catechol to o-benzoquinone; and 2) Hydroxylation of certain
monohydroxyphenols to dihydroxyphenols. Phenolase has a copper content of 0.2%
(each enzyme molecule contains 4 molecules of copper). The pure form of phenolase
is colorless (David, 1968).

Characteristics of Enzyme

1. Enzyme has very strong affinity for a specific substrate and catalyzes only one
single reaction.

2. Extreme pH generally inactivates the enzyme because of protein denaturation.
Enzyme shows maximum activity between pH value of 4.5-8.0. According to Cash et
al. (1976), the optimum activity of crude enzyme in grapes was found at pH between
5.9 to 6.3 at 25° C to 30° C, after which reaction rate declined very rapidly with increase
in temperature due to inactivation of enzyme.

3. Enzyme acts slowly at subfreezing temperature and actively as temperature increases

up to 45° C, but activity is optimum between 30° 040° C (Fennema, 1976).

33

Oxidatlm

cuprous 1
1011’ t

4le ll

two ele
rapidly
enzyme
of a re
The ch

Hilly

Oxidation of Polyphenols

Activity of phenolase is based on the change of the copper from the cupric to
cuprous state (enzyme is isolated). The changes are as follows:
4Cu2+ ( enzyme) + 2 Catechol ------ > 4 Cu+ ( enzyme ) + 2 o-quinone + 4H“
4Cu* ( enzyme) + 4H+ + 02 ------- > 4 Cu2+ (enzyme) + 2HzO.

The substrate became oxidized by losing 2 electron and 2 protons. By taking
two electrons, copper of the enzyme changes to cuprous state. Two electrons are
rapidly transferred to 02. This immediately forms H20 and 2 protons are liberated. The
enzyme returns to cupric state and ready to repeat the catalytic cycle. Indirect oxidation
of a reducing agent (hydroquinone) occurs by phenolase with an o-dihydroxyphenol.
The changes are as follows.
o-dihydroxyphenol + 1/2 O; (Phenolase) ---------- > o-quinone + H20
o-quinone + RH2 ----- > o-dihydroxyphenol + R
(RH2 + 0.5 02 ------ > R + H20).

RH2 is the reducing agent (hydroquinone), R is the oxidized form, o-quinone (Frank,
1983). The mechanism of the action of phenolase on o-diphenolic compounds is very
complicated.

Oxidation of Monophenols

The hydroxylation of certain monophenols to
o—dihydroxyphenols, the second reaction catalyzed by phenolase is brought about in the
same enzyme molecules that produces the oxidation of o-dihydroxyphenols. The

induction period of this reaction is long and it increases with the amount of purification

34

Si

of the enzyme. However, phenolase oxidizes the o-dihydroxyphenols at a faster rate than
the mono hydroxyphenol (Frank, 1983).
o-quinone

o-quinone are catalytically formed by phenolase and are the precursors of the
brown color of certain fruits and vegetables. Colorless o-quinone are most reactive. The
formation of unstable hydroquinone results from the main reaction. These hydroquinones
easily polymerize and are subjeet to rapid and nonenzymic oxidation-the result is dark
brown slightly soluble polymer. o-quinone forms from o-dihydroxyphenols in the
presence of phenolase are reacted rapidly with cystine and glutathione, thus forming a
pigment (Frank, 1983).
2.5.2 Enzyme Discoloration of Bruised Tubers

The main ingredients of discoloration are Enzyme (phenolase) and substrate:
tyrosine, etc. The factors that affect substrate (tyrosine, catechol) content are: a)
climate, b) mineral, c) cellular damage and d) time and temperature in storage.
Discoloration of a substrate is related to the initial rate of reaction. Climate: high
rainfall caused production of high concentration of tyrosine and high browning potential.
Mineral: High amount of calcium can depress phenolase and tyrosine levels but it may
increase rate of browning (Mapson et al. 1963). Cell damage: Enzymic browning
normally can not occur unless cells are damaged. The greater the number of damaged
cells, the greater the discoloration. Temperature and storage: If potato tubers were
stored for 100-200 days at 5° C, the rate of browning increased and remained constant

at the end of 100 days. This change was related to the change of tyrosine content.

35

Depending upon variety, temperature tyrosine content may increase or decrease.
Variety, climate and cultural conditions influence susceptibility of tubers to enzymic
browning. Researchers have found that change of browning was positively correlated
with tyrosine content, but not with the phenolase content (Mapson et al. 1963).
Phenolase was found in all sub cellular fractions of Russet-burbank approximately in
proportion to the protein content of each fraction. Phenolic content is generally highest
in tubers of high specific gravity (Craft et al. 1966).. Ozeretskovskaya et al. (1965)
found an increased amount of phenols in physically damaged potatoes. Phenolic content
varies according to variety and maturity. Mature tubers are lower in phenolic content
(Walter et al. 1957; Mondy et al. 1960). The potato tubers, having relatively high
tyrosine content, are easily injured and tyrosinase can catalyze tyrosine and the o-
Dihydric phenols (Rastovski et al. 1981; Learner et al. 1950; Mulder. 1956). Melanin
(blackspot) formation: Due to the injury to the potato tubers, total phenolic and
orthodihydroxyphenolic content show small, but significant increase When cell ruptures,
certain phenols of the cell cytoplasm are freed (e. g. tyrosine, catechol) which are then
oxidized with the help of specific polyphenol oxidase (ppo) through a series of reactions
and the end product is called Melanin which is another name for blackspot. The steps
of the complex mechanism of melanin or blackspot formation is shown in figure 2.1.
The figure 2.2 shows diphenols as a substrate (Fennema, 1976). Bond (1961) found that
increased phenolic content caused increased discoloration in the potato tubers. In
general, polyphenol oxidase activity increases and then decreases as the tubers mature.

The activity is also directly related to the concentration of phenolic substances and

36

oxygen concentration (Walter et al. 1957; Baruah, 1964) in the potato tuber. Therefore,
polyphenol oxidase and amount of phenols are believed to be the major contributors of
blackspot formation in impacted tubers (Crafts et al. 1966). Mature tubers are lower in
phenolic content than immature tubers and the immature tubers may contain a higher
amount of phenolic content that leads to increased discoloration of post reacted product
(Mondy et al. 1959; Mondy et al. 1960). Phenol concentration decreases as enzyme
activity increases. Tubers having high specific gravity showed decreased polyphenol
oxidase activity (Mondy et al. 1966). In contrast Rastovski (1981) found that higher the
dry matter-greater the susceptibility to blackspot. Phenolase activity differs between
variety to variety (Heintz, 1962). Probably due to cell size and dry matter distribution
ie, larger the cell more susceptible to blackspot. Action and concentration of tyrosine
is higher in the central portions than in the peripheral portion of a susceptible tuber.
Concentration of tyrosine is considerably higher in the stem end, than the bud end (Reeve
et a1. 1969; Metlitsky et al. 1964 and Tsekhomskaya, 1964). Concentration of tyrosinase
and rate of enzymatic browning of bruised tubers were found to be closely correlated
(Mapson et al. 1963). The initial velocity increases with the increase in substrate
concentration up to a certain point, beyond this point it becomes independent of substrate
concentration. Thus, at low (<0.1) substrate concentration, the enzyme reaction is
approximately first order with respect to substrate concentration and the rate of reaction
is proportional to enzyme concentration. Ingraham (1957) found that the Michaelis
constant, KIln (mol/lit) for O2 is dependent on substrate structure and concentration.

Increased oxygen can enhance reaction rate for o-diphenol oxidase (polyphenol oxidase,

37

tyrosinase) (Kubowitz, 1938; Ludwig et al. 1939;1ngraham, 1955; Bendall et al. 1963).
Manitoba University (1969) observed a linear relationship between 02 uptake and
concentration of crude o-DPO. With potato o-DPO the Km value for catechol in air and
02 saturated reaction mixtures at pH 6 was 3mM. They found that the reaction velocity
was nearly linear during first minute of reaction and then it sharply decreased due to O2
depletion and inactivation of enzyme. Maximum reaction velocity was achieved by
supplying sufficient oxygen. They found that the rate of browning is linearly related
to tyrosine concentration. Polyphenolase: Reeve (1969a) studied the chemical
components in potatoes. Chlorogenic acid was more concentrated in the outer tissues
of the cortex than in the inner tissues and in the perimedullary zone. Tyrosine was
found to be distributed more (20—40%) in the stem end than in the bud end. Phenolase
also showed a characteristic distribution pattern. Peroxidase affects tissue differentiation
and specialization, and has a strong histochemical relationship with cell wall
specialization. Phenolase was associated with the distribution of phenolic substrates.
Mulder (1949) found that there was a high correlation between tyrosine influenced
blackening and potassium deficiency. Tyrosine was also found in the interior tissues
while O.dihydric phenols were concentrated in the exterior tissues. Enzyme activity
varied appreciably within a tuber for phenolase, peroxidase and catalase. However,

enzyme activity may change significantly during storage (Reeve, 1969a; Reeve, 1969b).
These results show that polyphenol oxidase and substrate are important contributors in
forming blackspot bruise. Therefore, this dissertation also studied the ppo activity in

bruised and unbruised potato tubers.

38

ENZYMES

 

 

A ’TYROSNASE‘ 8
+0 1” \‘s +0
O—gz euzme ’ "00'— ?‘2 ‘enzms °= 9‘2
HO VERY SLOW HO FAST :
N, A” N,choon o u’CHCOOH
H2 FASTF LATER H2 H2
rvnosme com DOPA oumone
5+zo FAST
0
H0 0: +0 HO
I + coz ‘ ——
Ho RELATNELY 0: com FAST HO COOH
N SL094 N \\ N
H H \ H
\
s, s-ormonomme HALLACHecue (new \\ LEUCO coupowo
FAST +0 \
\
+0 ‘
: ‘20 HO
: MELANIN - ----- m
OLE—J mm... o...
N SLOW HO N c
H H
INOOLE s, s-cumone s, o-omvonoxvmooue-

Z- CARBOXYLIC ACID

Ox 0 O:
o NICHZCOO" :gmcowfi HOO:EZO0H + °=(:UC°°H
N
H H

"2

DOPA QUINOIQ LEUCO CW DOPA HALLACHRWE

 

Figure 2.1 The formation of Melanin or blackspot pigments resulting from oxidation of
tyrosine by phenolase (Fennema, 1976).
Hc-cH-ccon

Q 0;. -

CAPFEIC mm °° ammg °"

AGO Am
emu: ACD

Figure 2.2 O-diphenols that serve as substrates for phenolase (Fennema, 1976).

39

2.5.3 Minerals

Copper, Calcium and Potassium contribute to phenolase activity, browning, and
bruise susceptibility in potato tubers. The mineral content of a potato varies with variety,
cultural practices and dates of harvest, as well as variability between potatoes grown
under identical conditions (Lampit et al. 1940). Minerals, dates of harvest, and variety
are three of many factors that affect processing quality of potato tubers.

Copper (Cu)

Copper deficient tubers were found to have higher tyrosine content but the
tyrosinase activity can be much lower than that of tubers supplied with copper. Potato
tubers having increased copper content showed more blackening than copper deficient
potatoes (Mulder, 1949). Copper also helps improve quality by increasing the starch
(1.2%) and the ascorbic acid (Vit C, 2.1 mg/100) contents ofa potato tuber (Paseka et
al. 1972; Khachatryan, 1972). Copper usually enhances phosphorylase, decreases
amylase activity and increases the dry matter and starch content of a tuber (Kostyushina
et al. 1974). Other researchers found a varied amount of copper contents in the potato
tubers from 1.43 to 6.89 mg/kg because of geographical and climatic effect (Glushek
et al. 1972). The copper is also an important part of oxidation of polyphenols, as
described earlier.

Calcium (Ca)

Calcium content is important in determining blackspot bruise susceptibility. The

potato tubers with low calcium content usually are not firm (Krausz et al. 1971).

Calcium level in the tubers can increase during the early stage of growth and decrease

before harvest (Bardyshev et al. 1970). Calcium can depress the concentration of copper
in tubers (Laughlin et al. 1974). Calcium can also depress phenolase and tyrosine levels
(Mapson et al.. 1963). Calcium content in a tuber is proportional to pH concentration
(Bardyshev et al. 1970). Some researchers found that a high level of calcium can
reduce the development of internal browning. A low level of calcium can increase the
occurrences of internal browning linearly. Deficiency in calcium nutrition has a
detrimental effect on cell wall characteristics that can lead to damage of tuber tissues and
develop internal browning (Combrink et al. 1972). Timm (1989) reported that high soil
temperature may result in ethylene promotion in root tissue, curtailing root hair growth
and lowering Ca2+ absorption. An insufficient supply of soluble Ca“+ can lead to
weakening of the vacuole membrane of the cells in the cortex tissue area of the potato.
The weakened vacuole membrane would then be more prone to rupture under bruising
impacts during mechanical harvesting.

Potassium (K)

Potassium (K) is an important factor in determining susceptibility to internal
bruising (blackspot) in potatoes. It increases the size of the root system, enabling the
roots to suck water from the soil (Kunkel, 1965). Potassium (K) level can increase
during growth and can rapidly decrease before harvest (Bardyshev et al. 1970).
Potassium helps in the water balance of a tuber, influencing the permeability of the cell
membrane to water (Hughes, 1975). Potassium is essential in the synthesis of reducing
sugars and starch (tuber formation) and in the translocation of carbohydrate (Buchner,

1951; Ward, 1959). Low concentration of reducing sugar signifies physiological

41

maturity and good quality (Welte et al. 1966; Muller, 1964). Potassium (K) deficient
tubers are more susceptible to blackspot bruise, but excess potassium does impair quality
of the tuber by lowering the starch content (Shumilin et a1. 1974). If the amount of
potassium increases, the darkness of the potato reduces, increases concentration of amino
acid, iron and polyphenol oxidase activity. Potassium tends to keep the reducing sugar
level down and some amino acids in tuber (Welte et al. 1966; Muller, 1964. ; Hughes,
1975). Usually, potassium (K) concentration is higher in bud—end than the stem-end,
thereby making the stem-end more susceptible to bruising (Reeve et al. 1969b; Johnston
et al. 1968). It was found that the potato tubers with less than 2% K in the dry matter
are highly susceptible to impact discoloration (Hughes, 1975). The potato tubers with
a dry matter potassium content above 2% are less susceptible to blackspot caused by
impact (Baukema et al. 1979). Potassium also keeps calcium content steady in the
potato tubers (Simson et al. 1973). Phenolic content of a potato tuber is also related to
potassium content. Researchers have found that potassium deficient potato tubers
contained 3-4 times higher amount of tyrosine and twice as much O-diphenols as tubers
with a normal supply of potassium (Mulder, 1956). Mulder ( 1949) showed that
potassium deficient tubers had a tendency to discolor easily. Mulder (1956) also showed
that K-level did not affect the polyphenol oxidase activity. Phenolic content of potatoes
and discoloration showed a positive correlation (0.83) as affected by potassium fertilizer
(Mondy et al. 1967). Robertson (1931) found that hydrogen ion concentration (pH)
increased during growth at the center and bud end but decreased slightly at the stem end.

Reeve (1969a, 1969b) reported that pH was lower at the stem end than at the center and

42

bud end (Table 2.1 and 2.2).

Table 2.1 Distribution of chemical components in potatoes by zone
(Reeve, 1969a).

 

 

 

 

 

 

 

 

I Component ll Bud End Middle Stem End I
Chlorogenic acid II + + + + +
Tyrosine + + + + + +
Phenolase + + + + + +
Peroxidase + + + + + +
Catalase + + + + + +
Iron + + + + + «1»
Potassium + + + + + +

 

 

 

 

 

+ = present, ++ = concentrated, +++ = more concentrated

Table 2.2 Concentration of components within the potato tubers (Reeve,

 

 

 

 

 

1969b).
I 1
Chlorogenic Tyrosine Phenolase Peroxidase Iron
Acid
Skin + + + + + +
Cortex + + + + + + + +
Perimedullary + + + + + + + + + +
zones
Pith + + + + +
" u

 

 

 

 

 

 

 

 

= present, + + = greater concentrations (lack of symbols does not indicate absence
of chemicals).

43

Reeves (1969a) noted that storage and environmental changes may alter the
distribution pattern of individual constituents while differential distribution may be more
pronounced in some varieties than in others. Muneta ( 1977) reported that different pH
levels (7.0, 6.3 and 5.0) resulted in rapid oxidation of tyrosine. Higher pH is associated
with greater oxidation. However, the combination of pH and bisulfite resulted in
decreasing tyrosine oxidation when decreasing pH from 7.0 to 6.3. At pH of 5.0 and
4.0, bisulfite is an effective enzyme inhibitor, and very low oxidation of tyrosine
occurred even after 4 days.

Gestur (1957) suggested that variety, storage temperature and the level of
potassium fertilization affect the polyphenol oxidase activities in potatoes. Some
varieties possessed higher activity at 5° C storage. The highest activity was shown in
tubers grown with the lowest potassium level and stored at 10° C. The bruised tubers
held at 24° C accumulated O-dihydricphenols (chlorogenic acid). The rate of
accumulation of total phenol was approximately 20 mg/100 g/day. Low temperature
affects the rate of respiration. However, it is necessary to provide energy for
synthesizing of phenolic substances. Therefore, any condition which causes a decrease
in respiration rate will also affect the rate of accumulation of phenolic substances.

It is revealed that polyphenol oxidase activity, amount of phenol and minerals
are also the contributors in forming blackspot bruises. However, there is not much
information available out there on the effect of impact on instantaneous ppo activity and
resulting blackspot bruise in the fresh potato tubers. It was postulated that not only post

biochemical activity but mechanical properties also affect the blackspot bruising process.

3 ESSENTIAL THEORETICAL BACKGROUND
3.1 Hertz Contact Stress Theory
Hertz (1881) assumed the followings to solve the contact problem.
1. The material of each body is homogeneous, isotropic and elastic according to Hook’s
law but the two bodies may be made of different materials
2. The equation for an ellipse can be used to approximate the distance between
corresponding points on any two spherical surfaces in contact
3. The boundary line of the area of contact is assumed to be an ellipse
4. Contacting stresses vanish at the opposite end of the body

5. The radius of curvature of contacting solid is very large compared to the radius of

the contact area
6. Surface of the contacting bodies are smooth such that no tangential forces exist.

The surfaces of the bodies near the point of contact 0 (fig. 3.1) have been represented

by homogeneous quadratic functions of x and y.

3.1
zl=A1x2+A2xy+A3y2 ( )

zez=le2+Bzxy+B3y2 (3.2)
The coordinates are orthogonal cartesian system and xy is the common tangent plane,

normal is z axis and A, B are constants. There is a common tangent plane to the

45

A View of Two Bodies
in Contact Before
Pressure.

 

 

Two Bodies in Contact
Under Pressure.

 

 

21 + 2” - o

Auxiliarv Section “ATE:
Contact Surface with
Elliptical Boundary.

 

 

 

 

 

 

L_s i ‘7] Front and Side Views of
‘F‘ H j__?_f__f_ z, E.q Semi-Ellipsoid Pressure
‘i O Discrabucrcn.

 

 

 

Figure 3.1 Configuration of two bodies in contact and intensity of pressure over the surface of
contact (Manor, 1978).

surfaces at the point of contact. An expression for the distances between two
corresponding points near the point of contact are needed. The corresponding points are
the points that lie on the surface of the contacting bodies and on a line perpendicular to
the common tangent plane. In order to determine the deformation of the two bodies near
the initial point of contact, the distance between two corresponding points are required.

The equation that approximates this distance is as follows,

(3.3)
2 =AX 2 +By 2

the curve representing this equation for a constant value of z is an ellipse. When a load,
P, is applied to these bodies, their surfaces deform elastically near the point so that a
small contact is formed. It was assumed that the points that come in contact with this
area are the points on the two surfaces were that originally at equal distances from the
tangent plane. Therefore, these equidistant points lie on an ellipse. Hence, the

boundary line of the contact area is assumed to be an ellipse, which is:

2 2
+1.4 (3.4)
b

nIH
N
N

where ’a’ and ’b’ are the semi—axes of ellipse. The distance between two corresponding

points on the surface of contact can be expressed as:

zzzl +21=Ax2+3y2 (35)
where A, B are the constants and depend on the magnitude of the principal curvature of

the surface in contact. A and B can be determined from equations 3.6 and 3.7

47

 

(3 -A) =

 

 

1(i+i]2+[—l--i]2+7{—l—-iI-1—-i)0082@
2 R1 R1 R2 R1 R1 R1 R2 R2

where R“ R,’ and R2, R2’ are the principal radii of curvature of lower and upper
spheres. d) = angle between the normal planes containing the curvature, l/Rl and l/R2
Hertz (1881) assumed that the distribution of contact pressure, q, over the

surface of contact is ellipsoid. Therefore, the total applied force, P, is:

P=ffqu=§nabqo (3.3)

from equation 3.8 he calculated the maximum pressure, qo, as:

q =2(_P_) (3.9)
° 2 nab
and it is at the center of the contact surface.
In the general case of two spheres, Timoshenko et al. (1970) showed that when
they are pressed together in the direction of normal to the plane tangent at O, a contact

surface with an elliptical boundary will be formed. The semi axes ’a’ and ’b’ of the

elliptical boundary of surface of contact can be calculated as,

3 k1+kz 1” (3.10)
:1: A+B

 

a=m

 

48

 

 

k+ "3 (3.11)
b= §nP(—‘ ’9
4 A+B
and (k,+k2) is as follows:
1~ 2 1- 2 (3.12)
(k1+ 2)= “1+ #2
1:5, 1:15,

 

 

where, E and u are the Young’s modulus of elasticity and Poisson’s ratio of the
homogeneous bodies, respectively. The constants A and B are defined as before and
constants m and n depend on cosB=(B-A)/(A+B). The values for m and n for various
values of 0 can be found in the text authored by Timoshenko et al. (1970). The values
for m and n for 90" is 1 when B—A=0. In other words, R1=R1’ and R2=R2 ’ represents
a particular case of bodies with spherical surfaces.

For a particular case where ordinates of contact radius are equal or a=b, Hertz
(1881) assumed that pressure distribution is the ordinates of a hemisphere and the shape
of contact area is a circle. The maximum pressure, qo is at the center of the contact
area. This was shown as:

p=3_q_°(na3) (3.13)
3 a

From the equation 3.13, qo can be calculated as follows:

=_3_[_P_] (3.14)

q, 2 no2

49

where, a is defined by equation 3.15:

1’3
%1:P(kl +k2)( R‘R1 )1 (3.15)

a:

 

R1 +R2

 

If R1 is on then k1=0. Therefore, contact radius, ’a’ and approach ’a’, can be calculated

from the equations 3.16 and 3.17 as shown below:

 

3 ”3

airman) (3.16)
9 “Wk:

.1 =_ (3.17)
16 R2

 

 

According to Timoshenko et al. (1970), the principal stresses at the center of

the elliptical contact surface can be calculated as follows:

0: 4qu -(1-2u)q.{i] (3-13).
a+b

a

0,= 44.11 -(1-2u)q.[—— (3J9)

a+b

 

(3.20)

where, x and y axes are represented by semi—axes a and b of the elliptical surface of

50

contact and u is an equivalent of Poisson’s ratio for both bodies of the same material.

The stresses at the end of the axes of the ellipse are:

050, (3.21)
ny=0 (3.22)
0 =0 (3.23)

It follows that there exist a pure shear, 7,0,, but its value is lower than the
calculated value obtained at a small distance below the surface at the origin.
3.2 Elastic Impact

Horsfield et al. (1972) derived an equation for maximum compressive pressure,

qo for a fruit that impacts on to a hard flat plane surface. The equation is as follows:

qo 20.899(Wh)1/SE‘/5(1/R)3/5 (3.24)

Where,
W = mass
h = drop height
R = radius of sphere
E = modulus of elasticity of a falling object
Hertz (1896) extended quasi-static solution to impact. He assumed the following
conditions:
1) The time of impact of elastic bodies is very large as compared to the time taken by

waves of elastic deformation in the body to transverse the distance x of the order of

51

magnitude of that part of their contact surface.
2) The time of impact is also large as compared to the time taken by the elastic waves
to transverse the impinging bodies from end to end. Based on the above criteria,
Timoshenko et a1. (1970) defined or as the distance between two centers of mass of
spheres approaching one another clue to local compression at a point. The velocity of
approach, or is:

(it =vl +v2 (3.25)
where,
v,= velocity of impacting spherel

v2= velocity of impacted sphere2

According to the impulse momentum law, the change of momentum is given as
A(mv) =1 026)

where,

A(mV) = ("Na-(mi)

I= impulse

m = mass of the object

v,= initial velocity at the beginning of impact

v,= final velocity at the end of impact
I=dez (3.27)

the impulse.

52

where,
F: force acting on the object due to its momentum change
t= time
According to Timoshenko et al. (1970), the above impulse momentum law

became:

mldvl =th (3°28)

(3.29)

mzdv2 =th

where, ml and m2 are masses of the spheres and F is the contacting force acting between

the sphere during impact. The acceleration of the impacting bodies can be written as:

 

 

 

3.30
a .p{’"1*"‘2 ( )
mrmz
In quasi-static case Timoshenko et al. (1970) calculated the approach, ’a’, as:
1 (3.31)
a =-2-(k1+lc,)qort2a
where, a = radius of contact which can be expressed as:
= gnp(k1+k2)(R1R2) "3 (3.32)
4 R1+R2

 

 

where, kl and k2 are material properties. He defined compression force, F as,

53

by subst

and fror

lien 11

Where,

V:a\

by SUbg-

(3.33)
F =na 3’2

by substituting equation 3.38 in equation 3.33, it gives:

F- 4 R1R2\ «3’2 (3.34)
3n (R,+R2)(k,+k,)

and from equation 3.30 the acceleration can be calculated as:

 

In
use (3.35)

 

 

.. [m1 +m2 16R1R2
a ::
mlm2 91:2(k1 +k1)2(R1 +R2)

 

When the velocity of approach, a=0 (a =vl+v2), the instant deformation is as:
2 215
a =§(V_] (3.36)

where, am: deformation at the instant of maximum compression

v = a velocity of approach of two spheres at the beginning of impact. nl and n are

"’1 ”"2

 

(3.37)

n1:

 

 

mrmz

.,,,
El 1 R119 (333)
9 «2 (kg/c2)2 R1+R2

 

 

 

by substituting m2= co in equation 3.37 and R2= oo in equation 3.38 we get:

54

n,=1/ml
n =(4Eri Rr)/3(1‘#12)
and oz=F/ml

Hoki (1973) calculated Fm from equations 3.36, 3.38 and 3.33 as:

5
Fm=i E, l/F‘ 36:11:12)” (3.39)
3 l-ui

 

Timoshenko et al. (1970) mentioned that if mass of one of the impacting sphere
is regarded as infinite, the time of impact is too small compared to the lowest mode of

vibration of that body. Therefore, he calculated ’oz’as:
0 2
a =—2-k2qon (3.40)

Where,
q°= maximum contact pressure at the center of the contact surface

a = radius of the contact surface. From equation 3.40, qo can be calculated as:

 

qo= 2“ (3.41)
rtzak.2
Hertz (1896) solved visco-elastic impact problem by considering:
2
F(t) z -mfl (3.42)
dt2

55

a(t)= [_ 9 (111th 2(0] “3

E 2R
(3.43)

where,

m = mass of a impacting sphere

E = modulus of elasticity of sphere

p = Poisson’s ratio of the impacting sphere

R = radius of the impacting sphere

Jar-Miin et al. (1989) verified the Hertz’s assumption of ellipsoidal pressure

distribution. He dropped an object from 4 cm drop height and he calculated am at

i=1...» am=0(tm)

.4
a = 2% (3.44)
max 8 ERO'S
where,
am = maximum deformation during impact

g = acceleration due to gravity

h = drop height

Fa...) .(2_9_50)°’ (m g'h)°‘[E—Lo :T (3.45)
l-u

By considering a(t) as follows:

3 (1-112)RF(t)]/3 (3.46)

a=(t) E

 

56

Jar-miin et al. (1989) calculated the maximum contact pressure, qo as shown below:

0. .8
qo=_l__( 60mgh) II E r (3.47)
rt R3 1-p2

He calculated visco-elastic contact pressure distribution at t=tm as follows:

 

 

2
a tmu

. .5
_ 2 + 2
(1(x.y,t,,,,,,)=qoam)l.1_(L_y_)ln (3.48)

3.3 Viscoelastic Impact
Pao (1955) used Hertz (1881) contact theory for solution of Viscoelastic body

under impact. The kl of impacting body was very high compared to k2 of the stationary

body. The theoretical contact force he obtained was as follows:

_ 16 RrRz ”2 33+ _ 3n 3 49
F(t)—3n[—RI+R2 G(t)[a [Mt x)ct dz] ( )

where F, R,, R2 and or are defined in the previous equations and tl/(t-x) denotes the
relaxation function of the material of the impacting sphere. For an elastic material the
relaxation time may be considered as infinite then the integral part vanishes. He

calculated impact force F, by substituting G=E/2(l +u) in equation 3.49,

E

F=§n
3 l+p

 

Rik: “an (3.50)
R1 +R2

Pao (1955) also assumed the pressure distribution as ellipsoidal as in elastic case. Pao’s

solution gave 21(1-u) times higher contact force than that obtained by Timoshenko et

57

al. (1970).

Yang (1966) developed a formula based on Hertz (1881) pressure distribution

but applied to a Viscoelastic material. He assumed a pressure distribution given as:

I
q(x,y,t)=Cpr(t-r)ditg(x,y,r)dr

where, C is a constant multiplier, and Rp(t) is:

1

Rt :—
0” nacho,

and g(x,y,r) is:
1 m
8(x.y.t)=[a2(t)-xz--A—2y’] H(t)
where H(t) is unit step function and a2(t) is:
a2(r)=ia(z)
MI!

where i/r is a coefficient and k can be defined as follows:

,._b<_t>
a(t)

(3.51)

(3.52)

(3.53)

(3.54)

(3.55)

where, a and b are the axes of elliptical indentation and ill, a and k are determined from

Yang’s (1966) nomograph.

Hamann (1970) expanded Yang’s (1966) formulation by using unit step function

as an impact force. The total force between two bodies in impact can be written as:

58

F: -m,& +19 (355)

where,

6: = acceleration of the center of mass

w falling body weight
m,= mass of a falling body
Researchers used the following methods to inflict bruise in a tuber.

l. Dropping potatoes on to a hard or soft surface
2. Dropping masses on to potatoes
3. Simple pendulum

Eagle (1976) concluded that neither impulse nor each of the parameters
measured during the impact by various researchers have been established as a bruise
indicator. Therefore, researchers are still looking for the most useful parameter that
explains the bruising mechanism.
3.4 Elasto—Plastic Impact

Tabor (1950) assumed that whenever the pressure during an impact reaches the
yield pressure, q0 plastic flow occurs and as long as the plastic flow continues the
pressure remains constant. The work done, W3 is the plastic energy which produces the
indentation:
w3=qovr

where, V,= permanent volume of indentation on the second material

59

4
V,="_“_ (3.57)
4r2

where,

a= radius of indentation in the second material, r2 = radius of the indenter after impact

and r1 = radius of indenter before the impact. The r2 is defined as: .

l 1 3 F (3.58)

 

1-ui l-ui
—+
E E,

 

 

1

W3 can also defined as:

Tabor (1950) defined W3 as folows: W3=W,-W2 where,
W,= energy of impact

W2: energy of rebound

W2: mgh2 (in case of elastic impact) and can be written as:

2 2
l-u1+l-p2
El E2

3F2

210a

(3.59)

 

 

 

 

He also defined W3 = (10". = qo(7r a‘/4r2)= Wl-W2 and by using equation 3.59

(3.60)

4 3 F2
3q°'q° (16)a

 

 

This can be written as:

W3=q°V,-(5/ 8) W2 and he calculated the yielding pressure, q, as shown below :

qo=mg[h,-%h,]-$- (3.61)

where, h1 = impact height and h2= rebound height, V,= volume of apparent indentation.
3.5 Plastic Impact
Siamak et al. (1986) considered fruit as a spherical plastic material. They

defined radius of contact, r in equation 3.62:

r2=R12-(Rl -X)2 =2R1X-X2 (3-62)
Where,
R,= radius of impacting sphere, X= distance of the center of the sphere depressed.
He assumed all the points on the surface are yielded, then, resistive force, F, and weight
of sphere W, are acting on the sphere. He calculated yielding force, F, as:
F=aA=arrr2, where, a: yielding stress and r= contact radius. By substituting r2, the
yield force became:

F=1ro(2R,X—X2). Summing all the forces Siamak (1986) found that:

E F: W—no(2RX—X2)=% (3.63)

Siamak et al. (1986) derived an equation for bruise diameter as given below:

 

neon”? 11" (3.64)

where,
H= drop height

W= weight of apple

61

3,2

acCOl

viii

 

D= apple diameter
F= magness tailor yield force
From the equation 3.64, once ’d’ is calculated, the maximum pressure, qo can be

calculated using Hertz (1881) equation as shown in equation 3.65.
=__ (3.65)

3.6 Peleg’s Model
Peleg (1984) developed a mathematical model assuming a perfectly elastic sphere

according to Timosenko et al. (1970) and Goldsmith (1960) and calculated force, F as:

3I2
59—“ (3.66)
3 np(6,+6,)

F =(
where,
a = approach of spheres in the contact area
or total deformation of both sphere in the contact area
B: geometry constant of deformation
5,, 62 = material properties constant which can be defined as:

2
I‘ll:
Bin

5‘: (3.67)

 

 

 

(i= 1,2)
where,
E = modulus of elasticity of sphere

p = Poisson’s ratio

62

 

 

for 1“

(heel.

Whlc); 2;}

 

For the configuration of a sphere and a flat hard solid plane, 6 is

B=— (3-68)

If the contact area of the deformed sphere is circular (Mohsenin, 1970; Holt et al.

1977) then the diameter of the contact area ’d’ for a given compressive force, F can be

calculated by:

(6,452)
p2

for two spheres in contact. If R2 = 00, 61:6 and 62:0, then, (1, can be calculated from

d3=6rtF (3.69)

 

the equation 3.70. R is the radius of the bottom sphere, F, E are defined earlier.

1’3

d: (3.70)

 

 

_ 2
51mm
E

The maximum contact pressure, qo, occurs at the center of the contact circle which can

be expressed as:

qo=£ (3.71)

pd’

From equations 3.66 and 3.69, a relation between contact diameter and approach is:

a = 9242 (3.72)
4

 

Which is a general case of two spheres pressed together.
Peleg (1984) also defined resistive force, F, in non-linear Viscoelastic material

as a non linear differential equation of the form (3.73):

63

Cx

Fits!

K0:

{’5‘

inst:

\i

u“ 11‘

F =K ax +rx3 +Cri +F/(sgni) (3-73)
or F=K1X+r1X3
where,
F = acting force on the test specimen
K.,x+rx3 = cubic elasticity force
02 = viscous damping force
F,(sgn x) = internal friction force
K° = elastic constant that quantifies linear elasticity
r = strain hardening or softening parameter that predicts non-linear behavior of material.
In case of static or dynamic loading, only external load F(t) was considered
instead of inertia of mass (M).
Peleg ( 1984) expressed elastic relaxation modulus, E(t), for a non linear

Viscoelastic solid as:

('4) ("-1) 3.74
E(t)=Eoexp 7' EJl-exp "1 ( )

where, Eo= K1+rl X,2

F
E_=Ki[(Kl+r1X3')(lc1+3r1+x:)+(—X—f)(Kl+3y-lx:)] (3.75)

f
Kr=(Kl+3rlX02)+(Kl+3rlxp2)‘
C
— (3.76)
f K,
X0: initial instantaneous deformation of the specimen under test

64

E0 = instantaneous non linear elasticity modulus at time t=0. E0, = residual relaxation
modulus after a time t> >Tr has been elapsed. Yang (1966) set r,=Ff=0, E(t) in

equation 3.74 and he found a relaxation modulus as expressed in equation 3.77:

-.£. '6‘:
r, ‘ -_‘
E(t) :K1____exp2k (1 -exp T') (3'77)
1

E(t) may be viewed as time and deformation dependent spring rate or as a time and strain
dependent stress-strain ratio. According to Pao (1955), Goldsmith (1960) and Yang
(1966) a good approximation of Hertz contact problem can be obtained for nonlinear
Viscoelastic bodies by using E(t) from equation 3.74 and by substituting in equation 3.67.
This is possible because geometry of the elastic and Viscoelastic cases are identical, but
the dimension of the indentation in the Viscoelastic case is time dependent. This indicates
a material property function 6(t) rather than material property constant 6 as in the elastic

case. Therefore, the equation 3.67 can be written as:

 

6(t)= 1'“: (3.7s)
rtE(t)

If the loading force Fw, pushing the two contacting bodies together is a constant, then
diameter, d(t) of contact circle and the approach a(t) will increase with time while
contact pressure, q(t) gradually decreases accordingly. If a Viscoelastic body is in contact
with a rigid plane then from equation 3.70 and 3.78, Peleg (1984) computed the contact

diameter as shown in equation 3.79,

65

 

 

 

_ 2 1’3
d(t): ———6F‘”(1 9’ (3-79’
E(t)
and from equations 3.71 and 3.79 the contact pressure, q°(t) was calculated as:
6F
400): W (3.80)
1td(t)2
Yang (1966) expressed 1/E(t) for visco—elastic spherical material as:
1 1 ’2 1 '2 1 1 1 "(I
_ T. _ T. - _ T. 3.81)
——-—ex +—1 ex -—+ — — e (
E(t) 5. p 5.1 p l E. (E0 5.)»

where E is the ratio of the asymptotic relaxation modulus Ea, to the instantaneous non
linear elasticity modulus, E0 and E = EglEo.

Therefore,

_t_:
Tr

5(,)=1_’_I*i (3.82)
‘R

 

a. 3-; ,p'
E_ £0 E_

Since E,, < E0 and 1/E,,>l/Eo, (1/Eo-l/E,)<0. Therefore, l/E(t) increases
exponentially from l/Eo at t=0 to 1/E,,, when ’t’ approaches co. From the equations

3.79—3.81, contact pressure in the Viscoelastic body, q°(t), at the center of the contact

area can be expressed as follows:

”3

61mm)2
1: 3R 2( l _ “2)2

(3.83)

 

 

 

66

The maximum contact pressure, qo(t) decreases from its initial value at t=0 when

E(t)=Eo to its relaxed value as ’t’ goes to infinity. When E(t)=E,., then

”3

613,53
1! 3R2(1_ I‘2)2

 

4,0.) =

 

 

is the contact pressure if Viscoelastic sphere is compressed against a plate.
Where,

R = radius of Viscoelastic sphere

Fw = loading force due to weight

E(o), E(t), E (a) = relaxation modulii as in equation 3.81

6(t) = material property function in equation 3.82

60 = material property constant in equation 3.67

67

(3.84)

 

3.7 Summary
The following equations have been developed to estimate the maximum pressure,
go, between a sphere and plate.

1. Hertz ( 1881) contact theory:

.0: 2 .5. a.»
2 n02
The radius of contact surface is
1/3
a=[3nrk,R,) (3.86)
4
2. In case of sphere on sphere
, .2 _P_ (3.37)
° 2 nab

where ’a’ and ’b’ can be computed by use of equations 3.10 and 3.11.

3. Elastic Impact (T imosenko et al. 1970):

 

2
q,= 2 a (3.88)
1: amk
where a is as,
5
a .2 K (3.89)
4 rml

 

According to Jar-miin et al. (1989):

68

 

 

0 .8
40:1[60m8hn E I] (3.90)
W R3 l-p2

4. Visco elastic impact. Pao (1955) calculated maximum pressure, qo, as

q .2(L) (3.91)
0 2 nab
and he expressed F(t) as,
R "2
F(t)=-1?T6KE% C(t)[a3’2+frlr(t-x)a3fldx] (3.92)

 

5. Peleg (1984) developed an expression for the maximum static pressure as:

1/3
617,153

R3RZ(1"|12)2

(3.93)

 

 

6. Elasto-Plastic impact. Tabor (1950) computed maximum pressure as shown below,

go ”"4”: ’% )1:- (3.94)

7. Plastic impact: Siamak et al. (1986) developed an expression for pressure as follows:

.2 4_F (3.95)
o 2 1rd2

The diameter of contact area was computed as follows:

69

d=5.63(m)1" (3.96)
F
8. Elastic impact (Horsfield et a1. 1972).

qo=0.899(Wh)“’5)(%)(3’5)E°" (3.97)

(Note: Horsefield et al. 1972 used equation 3.97 for agricultural materials. Therefore,

the equation was used for verification of the impact tests results in this dissertation).

70

4 POTATO BRUISING UNDER IMPACT LOADING

Impact between fruits, vegetables and a hard surface is a major cause of product
damage in harvesting and handling system. Such damage is evidenced by bruises, bursts,
blackspots depending on the magnitude of impact and product condition. Fruits and
vegetables are also subject to damage from static and slow loadings. According to Fluck
(1973) most damage that does not occur by static condition can occur by an impact
condition. Impact of non biological materials has been studied extensively (Gold smith,
1960). Deformation of fruits during impact has been measured by Fletcher, 1971;
Friedley et a1, 1966; Mohsenin et al. 1962, but none of them measured contact pressure
due to impact which may be an important parameter that causes blackspot.

Blackspot bruise detection has been a long standing problem in potato industry.
The methods for identifying bruise has been developed by researchers (0’ Leary, 1969;
Thronton, 1982; Irritani, 1985; Hammond, 1978). The need for a rapid test to identify
bruises is highly desirable (Stills, 1983). Chase (1980) used catechol and Irritani (1985)
used paracresol to detect surface bruise of potatoes. Tetrazolium chloride salt has been
used for colorometric determination of iodate and bromide (Hashmi et a1, 1964). Gary
(1985) developed a method using 2-3—5 triphenol tetrazolium chloride to detect surface
bruise of fresh potato tubers. Although these methods are effective, they require 6-48
hrs just to detect surface bruises and failed to detect internal bruise (blackspots). The
blackspot is related to mechanical and chemical properties. Therefore, the objective is

71

to study mechanical and chemical properties and characterize bruise susceptibility of
potato varieties.
4.1 Objectin

The specific objectives of this study were as follows:

1. To develop a method to identify blackspot bruise in a fresh potato tubers.

2. To determine threshold dynamic contact pressure for bruising.

3. To study the effect of mechanical properties on the dynamic contact pressure.
4. To determine polyphenol oxidase activities of bruised and unbruised potato

tubers under identical impact loading condition.
5. To characterize potatoes for bruise susceptibility based on the optical density

scale as developed by Dean et al. (1993).
4.2 Methodology

Three batches (20/batch) of freshly harvested potatoes (150-350 g) were
randomly selected. Each tuber in a batch was held at several preselected heights of
15 .24 cm (6 in), 38. 10 cm (15 in) or 60.96 cm (24in) by a vacuum pump (described in
4.2.2). The tuber was dropped freely once at the stem end on to a piece of pressure
sensitive film attached to the steel platform. The impacted area of each tuber was circled
with a marker pen. All impacted tubers were treated according to the bruise
identification technique as described in 4.2.1. Each normally air dried tuber was
enclosed in a clear bag and kept temporarily in a styrofoam container filled with ice chips
to deter polyphenol oxidase activity (ppo). The potato tubers were peeled, cut slice by

slice, and examined one by one for colored spots at the impacted area. The tubers were

72

categorized as unbruised (no color) and bruised (colored) groups after visual examination.
Two varieties, Superior and Snowden, were selected and hand harvested in September
and October, 1993 for this study.
4.2.] Blackspot Bruise Identification Technique

Fifteen tubers (3 tubers /group) were impacted at 10 cm drop height and were
held for 24 hrs at room temperature (25° C) to enhance polyphenol oxidase activities
(ppo). The chemical (2-3-5 tetrazolium chloride) solutions of five strengths were
prepared and assigned each group randomly for the bruise identification as follows:

Strength A: 1 gallon distilled water at 30° C + 1 gr chemical

Strength B: 1 gallon distilled water at 30° C + 2 gr chemical

Strength C: 1 gallon distilled water at 30° C + 3 gr chemical

Strength D: 1 gallon distilled water at 30° C + 3.5 gr chemical

Strength E: 1 gallon distilled water at 30° C + 4 gr chemical

The impacted tubers were dipped in the tetrazolium chloride solution for

color development in the impacted area under the skin of each tuber. The samples of the
tubers were removed from the solution after 1, 2, 4, 8 and 10 hrs. The skin at the
premarked area was removed slice by slice using a sharp knife until a distinct pink
colored spot was found. A digital vemier calliper was used to measure the bruise size
and depth. The strength with soaking time of 8 hrs developed the distinct pink color
in the affected potato and was selected for the entire bruise identification in this research.
4.2.2 Determination of Dynamic Contact Pressure

Pressure sensitive films were used to measure contact pressure. Four types of

73

film have been developed by Fuji Co. These are as follows:

1. Ultra super low. The pressure range is 2-6 kg/cm2

2. Super low. The pressure range is 5-25 kg/cm2

3. Low. The pressure range is 25 to 100 kg/cm2

4. Medium and high. The pressure range is 100 to 500 kg/cm2

Figure 4.1 illustrates two sheet type pressure sensitive films. The prescale
film is composed of an A-film, featuring a layer of microcapsulated color forming
material and a C-film layer of color developing material. The Fuji pressure sensitive
film consisted of thin and uniformly coated layers.

When a contact pressure is applied over the film against a surface, the micro
capsules on the A-film are broken and a color-forming material is released to be
absorbed by the color developing materials of the C-film. The C-film generates a red
color by reaction. The microcapsules of the color forming materials are adjusted to
break at different pressure levels. Therefore, it allows one to obtain a desired color
density depending on the magnitude of pressure applied to it.

The pressure sensitive film (type 2) as described above was used to measure
dynamic and static contact pressures. A pair of small pieces (5.08 cm X 5 .08 cm) of
pressure sensitive films (A and C) was tightly attached on the surface of a steel platform
using an adhesive paper tape before each drop and replaced by a new pair of films after
each drop since they cant be used more than once. A red colored area (fig 4.2) was
developed automatically in the C-film depending on the magnitude of the contact

pressure. The intensity of red color is proportional to the pressure generated due to

74

 

 

C-Iilm &

 

A/\/\/\A/\/\/\/\/\/\W—.— 38......

—v

1 layer
\r‘PET base

 

 

 

 

Figure 4.1 Two sheets type contact pressure sensitive films (Fuji Co.).

.5 3134K: 1

u‘ *3; ‘4:

'¢";{‘;{-‘TI
S M w o “9’24.“ 0' 3

6f?

Figure 4.2 A sample of red color formation in the pressure sensitive film after being
impacted by a Snowden potato from 15.24 mm (6 inch) drop height.

75

1“. “-‘I.l‘n ~ .-.~= ‘§i‘~ .~

 

_ 31> Luminance Sunace - , -

E 2 no one“

Rotation
X 45.0.0 '
Y 1 11.80
5mg.” ........
X 64
‘r .“
Vic-nun
2 211010

 

 

 

 

 

Figure 4.3 A sample of digitized dynamic contact pressure distribution over the contact
area of a potato tuber.

 

A

111

3

72‘

Fuji film Contact l’rossurng/an‘Z)
3' T1"

 

 

 

TOO 110 120 130 140 150 160 170 180 190 200 210 220 230 240 250
Gray value

Figure 4.4 A calibration curve for measuring contact pressure on a surface of a potato
tuber.

76

impact. The red color in the film was digitized (fig 4.3) using an image processing
software called OPTIMAS and its corresponding gray values were noted. A calibration
curve (fig 4.4) was constructed using known contact pressures and their corresponding
gray values. A curve fitted to these points. The fitted equation was as follows:

Y = 103.77 - 1.24389 X + 0.0056103 X2 + 0.0000093403X3
where, Y = measured contact pressure (kg/cm’); and X = gray values.
The correlation between gray values and the contact pressures was R2 = 0.96. The
calibrated curve was used to determine dynamic contact pressure as exerted by a tuber
during free fall impact. The mechanical properties, polyphenol oxidase activity and
optical density (as described in 4.2.6) of bruised and unbruised groups of potatoes were
determined.
4.2.3 Determination of Mechanical Properties of Core Specimen

Core tests were used to determine pertinent mechanical properties of potatoes.
These are yield stress, failure strain, failure strain energy and modulus of elasticity.
The relationships between the dynamic contact pressure and the measured mechanical
properties were investigated.

One cylindrical core specimen, 25.4 mm long and 15.54 mm diameter, was cut
out of each potato tuber using a metallic cylindrical borer. These cores were tested to
determine yield stress, yield strain, yield strain energy and modulus of elasticity. All
compression tests were performed on an Instron testing machine at 1.27 cm/min (0.5
inch/min) loading rate. The chart speed was 12.70 cm/min (5 in/min). The Instron

testing machine was calibrated before starting for force and displacement of the pen on

77

the chart using known loads. A pressure sensitive film (Type 2) was placed each time
on each cylindrical core and was compressed along with the core until failure. The film
developed color as soon as the crosshead came in contact with the core specimens. The
yield stress was calculated as: 00: F/A and the corresponding pressure was measured
from the digitized gray values in the calibrated curve. The yield stress and the pressure
was correlated. Analysis of variances were performed on these variables over the
varieties and dates of harvest. The compression loading continued until the core failed
which was seen as a point of inflection on the load-deformation curve. The crosshead
was immediately stopped and raised to avoid further compression. The yield strain,
yield strain energy, yield stress, modulus of elasticity were determined from the force-
deformation curve at yield of each potato core (Appendix A.5).
4.2.4 Determination of Dynamic Threshold Pressure
The measured dynamic contact pressures were plotted against the potential
energy of bruised and unbruised potatoes of both varieties to determine a threshold value.
4.2.5 Assay Preparation and Determination of Polyphenol Oxidase (ppo) Activity
Polyphenol oxidase activity is the rate of oxygen uptake by a substrate per min.
The polyphenol oxidase activity can be used to define bruise susceptibility. Higher the
activity higher the bruise susceptibility. To determine the PPO activity, a total amount
of 25 g of fresh tissue from each group of bruised tuber’s stem end was cut in equal
proportion after subjecting the tuber to impact by dropping on to a hard surface. The
25 g tissue sample was mixed with 50 ml cold (3° C) buffer (Trizma 0.1M, M=Mole,

pH of 6.5) and blended in a high speed electric blender for 15 seconds. The blended

78

mixture was filtered (Whatman it 4 filter paper). The filtrate was added to 100 ml
acetone of ~20° C to precipitate and hold ppo activity of the extracted enzyme. The
diluted solution was filtered again to collect ppt (enzyme). The collected ppt (enzyme)
was added to 25 m1 sodium acetate (0.1M, pH 6) and 5 ml cold CaCl, (0.1M) solution
of 3° C. The mixed solution of enzyme was homogenized by centrifugation at 10,000
rpm for 30 min at 5° C to obtain a clear supernatant of fresh enzyme. The cold and
clear supernatant (extracted enzyme) was poured into a 25 ml plastic vial and left in a
plastic bowl filled with ice chips until the ppo tests were completed in the
Spectrophotometer. The ppo activity tests of each bruised and unbruised sub groups of
potatoes were performed in the Perkin Elmer Spectrophotometer equilibrated at 30° C.
The selected wave length for the light absorption test was 420 nm. The standard reaction
mixture volume of each sample consisted of 0.40 ml catechol substrate (0.3M), 0.2 ml
supernatant (extracted enzyme) and 3.40 ml sodium acetate (pH 6). The enzyme kinetics
software was used to measure the change in absorbance (oxygen uptake) for 180 seconds
and for statistical analysis. The computer output included the enzyme activity, slope, and
standard deviation. The rate of change of color (absorbance, mole/lit/min) due to
oxidation of substrate was determined from the slope of the reaction curve. The same
procedure was followed for the controls and unbruised group of potatoes.
4.2.6 Determination of Optical Density

The optical density (OD) is an absolute value of light absorbance in a liquid.
This property can be used to define bruise susceptibility of potato tubers. Higher the

optical density, the higher the bruise susceptible. To determine the optical density a total

79

of 100 g tissue from impacted stem end and unimpacted bud end were cut from each
group of bruised potatoes in equal proportion. An amount of 100 ml potassium
phosphate buffer solution (pH 6.5) was added to stabilize any reaction. The tissues in
the buffer solution were blended immediately in a high speed electric blender for 15
seconds followed by filtering using a Whatman #4 filter paper. The filtrate was collected
in a glass beaker for open air oxidation at room temperature. The filtrate became black
after 24 hrs of oxidation. The oxidized filtrate was centrifuged for 30 min at 10,000
rpm. A quantity of 10 ml homogenized filtrate was diluted to 1:3 ratio. The optical
density of the diluted filtrate was measured in a Spectrophotometer at 475 nm wave
length. Five replications of reading were noted for each sample. The same procedure
was followed for the samples of unbruised potatoes and the control. The measured
optical density of the control, unbruised and bruised groups were compared with the
scale developed by Dean et al. (1993) to determine bruise susceptibility of each group
of unbruised and bruised potatoes.
4.2.7 Determination of Mineral Contents

A sample of 30 g tissue was collected from each impacted potato tuber and
oven dried at 75° C for 72 hrs. A total of 240 samples were prepared, dried and ground
to fine powder. The powder was sent to the Soil Testing Laboratory for the
determination of amount of calcium, copper and potassium contents in each sample.
4.2.8 Determination of Bruise Susceptibility

Blackspot bruise of potato depends on the tissue injury followed by the

biochemical reaction. Several methods have been used to determine blackspot bruise

80

susceptibility (described in 2.1). Homogenizing tuber tissue have been used by Dean et
al. (1993). He developed a method on the basis of optical density to determine blackspot
susceptibility variation among particular varieties. The method was described in 4.2.6.
The optical density was grouped in to 1-5 rating for susceptibility comparison. The table
4.1 as developed by Dean et al. (1993) was used to determine bruise susceptibility of
potatoes.

Table 4.1 Bruise susceptibility scale (Dean et a1, 1993).

 

 

 

 

 

 

Resistant Moderately Moderately Susceptible Very
resistant susceptible susceptible
Optical 0.0-0.2 0.21-0.40 0.41-0.60 0.61-0.80 > 0.80
Density

 

 

 

 

4.3 Results and Discussion

The impacted potatoes were treated by the tetrazolium chloride solution to
identify bruised area in the tubers. Figure 4.5 shows very light pink color in the treated
bruised potato dropped at 6 inch height. This means minor bruise has occurred. Figure
4.6 shows no color under the skin due to impact at 15 inch (38.1 cm) drop height. The
no color indicates no bruise. Figure 4.7 shows the dense pink color bruised area under
the skin than the one found in figure 4.5 which was lightly bruise area. From figure
4.8, a thick circle (about 3 cm dia) of dense pink color on the surface of the tuber was
seen. This was a case of severe shatter bruise when tuber dropped at 24 inch height.
The figure 4.9 clearly shows the severe blackspot at approximately at 6-7 mm below the
surface of the Snowden tuber due to impact from 24 inch drop height. This tuber did
not shatter but internally bruised severely. Internal bruise can also occur 6-8 mm below

81

 

Figure 4.5 Light pink color developed in a bruised fresh Superior potato tuber dropped
from 15.24 cm (6 inch) drop height.

 

Figure 4.6 No pink color was developed in a fresh Superior potato tuber dropped from
38.10 cm (15 inch) drop height.

82

 

Figure 4.7 Pink color in the bruised area of a Superior potato tuber dropped from 38.10
cm (15 inch height).

 

Figure 4.8 A dense red colored circle of shatter bruise shown in a Superior potatoe
when dropped from 61 cm (24 inch) drop height.

83

   

 
 

Bruised at 24 .. ‘
Snowden, Sept 7.4 93

Figure 4.9 A blackspot occurred in a Snowden tuber at 5-6 mm below the skin when
dropped from 61 cm (24 inch) drop height.

 

Figure 4.10 Internal crack occurred in a fresh Snowden potato when dropped from
38.10 cm (15 inch).

84

sh

lie

of

unb

the skin at a drOp height of 15 inch as shown in figure 4.10. This did not produce
blackspot. It can be said that tetrazolim chloride solution is capable of locating internal
and external bruises of a tuber. Similarly, under different impact loading condition, the
pink color developed in a pressure sensitive film was an indication of contact pressure.
The examples are shown in figures 4.11 and 4.12, respectively. Two different size and
masses of tubers were dropped from 15 inch (38.1 cm) and 24 inch (61 cm) on to a
pressure sensitive film. From figure 4.11 the dense pink color at the right sample
indicates higher contact pressure than the sample at the left side. This also shows that
at the same drop height contact area can be varied. If contact area varies then contact
pressure also varies. Figure 4.12 shows pink color formation in the pressure sensitive
films due to impact of two different potatoes at 24 inch (61 cm) drop height. The dense
pink color at the center of the right sample indicates higher contact pressure than the left
one. Therefore, it shows that sometimes at higher drop height contact pressure did not
increase at 61 cm (24 inch) drop height rather shatter bruise occurred. A higher
percentage of bruising occurred (shatter and blackspot) as they were dropped from
increasing heights. M.Ito et al. 1994 reported that damage index (summation of damage
points at a height) increased proportionately. It was found that the Snowden variety
showed 10% higher bruising than the Superior variety (table 4.2). Figures 4.13 and 4.14
also show the effect of drop height on percentage of bruise in both varieties. From these
figures a general trend can be observed; higher the drop height, the higher the percentage
of bruised tubers. The differences in the potential energy that caused bruised and

unbruised potatoes of the Snowden and Superior variety were found to be significant.

85

 

it“? «iii. 6
1 -;<'§.r.-“}"5'~':4‘7
éwoo' 013 522 ‘
érwao' 0012909?) // 0‘3
I
/ 3J//

Figure 4.11 Color developed in a pressure sensitive film when a tuber dropped from
38.10 cm (15 inch) height.

 

h .

. I. :
" £5. 3}; Fifi:
,; .2: v
‘." “UH Supt 17,993
9*: 61 CM

9‘94: 17,094 5 ‘1
9% t”

Figure 4.12 Color developed in a pressure sensitive film when a potato tuber was
dropped from 61 cm (24 inch) height.

86

Superior Variety
100 +

 

BRUISE (7:)
a)
o

 

 

 

   

 

o 10 20 30 40 60 60 70 80
DROP morn (CM)

+ cpl-l7 agar + apt-17 duller +3— aept-ao moot + cpl-00 Shatter

Figure 4.13 The effect of drop height on the percentage of bruised Superior potatoes.

Snowden variety

 

100
90‘
80‘
70‘
60-
50‘

404 \

30- '

20‘

10'

0 r . T . . . .
0 10 20 30 40 50 60 70 80
0120p HEIGHT (CM)

BRUISE (7o)

 

 

 

+ cpl-24 Bani. + cpl-24 shatter + act-22 Epot -'t— act-22 shatter

Figure 4.14 The effect of drop height on the percentage of bruised Snowden potatoes.

87

Table 4.2 Drop height and the corresponding percentage of bruised and
unbruised tubers.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Height Total bruise Black spot Shatter Unbruised
Variety (cm) (%) bruise (%) bruise (%) (%)
Superior 15 . 24 40 40 0 60
9-17-93
38.10 65 35 30 35
60.96 80 30 50 20
Superior 15.24 30 30 0 70
9-30-93
38. 10 90 10 80 10
60.96 100 0 100 0
Snowden 15 .24 50 45 5 50
9-24-93
38. 10 75 25 50 25
60.96 95 20 75 5
Snowden 15.24 50 44 ‘ 6 50
10-22-93
38. 10 75 20 55 25
60.96 98 8 90 2

 

 

 

 

 

Dynamic Contact Pressure

The pressure sensitive film color was digitized using computer vision and
OPTIMAS software. Their gray values were recorded as shown earlier in figure 4.3.
The contact pressures were measured from the calibration curve as described earlier.
The analysis of variances of contact pressures over variety and the date of harvests are

included in tables 4.3-4.4. The event of bruising between the varieties was significant.

88

 

The date of harvest and the variety had significant effect on the dynamic bruising contact
pressure. Dynamic contact pressure of bruised Superior potato tubers was significantly
higher than that of the Snowden potato tubers. It was seen that as the drop height
increased, the contact pressure increased to a certain extent, but after that, increased
impact energy resulted in tuber damage and no further increase of contact pressures was
noted. This indicated that there was a value of the contact pressure when bruising
occurs. It was seen that the potato tubers bruise when the contact pressure reached
within 800-1000 kPa depending on the variety. The bruising pressure remained nearly
constant with any further increase in the potential energy. From figure 4.15 it is seen
that there existed a threshold contact pressure which was 880.94 kPa (127.73 psi) for
Snowden. From figure 4.16 the threshold contact pressure for Superior was found to be
1027.11 kPa (148.86 psi). Hyde et al. (1993) determined dynamic yield pressure for
various varieties of potatoes which ranged from 0.93 to 1.14 Mpa. This is in close
agreement with the obtained dynamic yield contact pressure.

The contact pressures were measured from the calibrated curve. The contact
pressures were also calculated using equation 3.96 as developed by Horsfield (1972).
The data are shown in appendix C. The correlation coefficient between the measured
and the calculated dynamic contact pressure was 0.94. The relationship was linear.
This indicates a very low variability between the measured and the calculated values.
Therefore, the error was within acceptable range. The percentage of bruised tubers in
Superior and Snowden potatoes as affected by drop height are shown in figures 4.13 and

4.14, respectively. From the observation it can be stated that as drop height increased

89

Superior Variety

 

 

1500
’3
31300‘
A
E . .
11007 A” O 2 ‘
q; C] 1‘ Co: :1 AA “ ‘A
0. CE ‘ A “‘ ‘ ‘
910 953:5 c1 ‘
[P
E D 0 £5ch
Z 700'1
8
600 . . . .

 

 

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
901mm my (N—m)

Abru'nxl Dunbnumd

Figure 4.15 The dynamic contact pressure as affected by potential energy of Superior

 

 

 

 

potatoes.
Snowden Variety
1500
131300‘
0..
e
gliODr an ”I
3 “fl &
flytri‘x a” as Y T
QQOO'i 3“! fi‘;‘ ‘3.
K
E 700‘ £8: E: x”; "
a at:
O X
0 5007
300 '
0 05 1 15 2 25

am Xunbnnrd

Figure 4.16 The dynamic contact pressure as affected by the potential energy of Snowden
potatoes.

90

shatter bruise increased but the blackspot bruise decreased in Superior potatoes. The
similar trend was seen in Snowden potatoes as shown in figure 4.14.

Table 4.3 Analysis of variance of dynamic bruising contact pressure over

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

variety
DF SS M55 F-Value Prob
Between 1 53.517 53.517 13.886 0.0003
Within 178 685.98 3.85
Total 179 739.509
CV: 21.29%
Table 4.4 Analysis of variance of dynamic bruising contact pressure over date
of harvests.
DF SS M88 F-value Prob
Within 2 194.575 97.288 31.60 0.0000
Between 177 544.93 3.079
Total 179 739.506
CV == 19. 13

Table 4.5 Analysis of variance of event of the bruising over variety.

 

 

 

 

 

 

 

 

 

 

 

 

DF SS M38 F - Value Prob
Between 1 139.604 139.604 685.404 0.0000
Within 178 36. 866 0.204
Total 179 176.470
CV = 12 %

91

A step wise regression analysis was performed (table 4.6) on the dynamic
bruising contact pressure of bruised Superior potatoes to determine which parameters
were correlated with it. It can be seen from the table 4.6 that strain energy, elasticity,
and yield stress were correlated with the dynamic contact pressure. The adjusted R2
between potential energy and dynamic contact pressure was low. The reason for poor
correlation is that the contact pressure does not increase beyond the threshold value
regardless of an increase in potential energy.

Table 4.6 Stepwise regression analysis of variance of dynamic contac
pressure for the bruised Superior potatoes.

 

 

 

 

 

 

 

 

 

 

 

 

 

Source Individual Cumulative Cumulative Cumulative Adjusted
88 DF SS MS R2
Constant 4055. 1
1.02800 1 1.0280 1.0280 0.0382

Mass
Potential 0.009876 2 10379 0.51895 0.0106
Energy
Potato 0.32872 3 1.3666 0.45554 0.0033
Size
Yield 0.63013 4 1.9967 0.49919 0.0169
Strain
Strain 10.701 5 12.698 2.5396 0.7704
Energy
Elasticity 0.038875 6 12.737 2. 1228 0.7657
Yield 0.45031 7 13. 187 1.8839 0.7929
Stress
Residual 2.64050 36 15.828 0.43965

 

 

 

 

 

 

92

Cases included 37

Degrees of freedom 29

Overall F 20.69 P Value 0.0000
Adjusted R2 0.7929

R squared 0.8332

Mechanical Properties

The scatter plots of the dynamic contact pressures versus the core modulus of
elasticity, strain energy, yield strain, yield stress, of both varieties are shown in figures
4.17-4.24. The figure 4.17 shows that the dynamic contact pressure of the bruised
potatoes were higher and linearly related to modulus of elasticity. The dynamic contact
pressure of unbruised tubers were lower than that of bruised tubers. Figure 4.18 shows
that the dynamic contact pressures were linearly increasing as yield stress were increased.
While contact pressure on unbruised potatoes remains below the contact pressure of
bruised potatoes. Figure 4.19 indicated that a linear trend of dynamic contact pressure
as affected by the failure strain energy of bruised and unbruised potatoes. Figure 4.20
shown the dynamic contact pressure remains nearly constant at all strain levels in bruised
and unbruised tubers. This indicates that dynamic bruising contact pressure of bruised
tubers was a constant value at any strain. The figure 4.21-4.24 show the similar trend.
The measured mechanical properties, such as yield stress, strain, strain energy, modulus
of elasticity are also included in appendix A. The yield stress correlated with the
dynamic bruising contact pressure in both varieties. Analysis of variance of mechanical
properties over the varieties and the dates of harvest were performed. It was found that
the varieties and the dates of harvest had a significant effect on the mechanical properties

such as, yield stress, yield strain, strain energy and modulus of elasticity (tables 4.7-

93

Superior Variety

 

 

 

 

1500
1.?
3 1300 1
E a
3 1100-« .. 2 - ‘ g
g ‘ ‘ A AA“ AA: ¢ [£9
‘ a A ‘ A C] D
Q- ‘ ‘ B c: D
900 « 4 DB
E Egéaz ‘3
E 700«
8
600 ~ - -
2500 2700 2900 3100 3300 3500

1100ou5 or mncm (kPa)

Arlxuhul CD unhunui

Figure 4.17 The dynamic contact pressure as affected by modulus of elasticity of
Superior potatoes.

Superior Variety

 

 

 

 

1500
’3
Q.
51300~
g A
31100~ ‘ 2 . g 4
g ‘ a A A A : 2 :1

Q 1 ‘ . A A D (:3 g C]

”‘ 900~ . D a B a
:1 j D g :1 8 1:1
'5 700
8

500 - s .

700 000 900 1000 1100 1200 1300

YIHD m1k1’a)

Abru’lni Dunbrmnd

Figure 4.18 The dynamic contact pressure as affected by the yield stress of Superior
potatoes.

94

Superior Variety

 

 

 

 

1500
93 1300 -
E a

1100 « ‘ ‘ ‘A “e

‘ Q“ A A. D
A ‘ A‘ x “ Cpl D

0.. A A‘ D B D D

900 a A :1 B :1
E :3 Do :1 g D
g 700 ~
9

500 a . . . .

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

FAILURE STRAIN ENERGY (N-m)
A bur-:1 D unbnnnd

Figure 4.19 The dynamic contact pressure as affected by strain energy of Superior
potatoes.

Superior Variety

 

 

 

 

1500
£1300~
5
A
@1100« a
@ A‘:A‘Af§ C]
m . ‘ @190 C]
E900~ - 0 go
a

E 0308 1:1

700«
8

500

0.2 0.3 0.4 0.5
YIELD smut

Aunbnr'ndCanncd

Figure 4.20 Trend of dynamic contact pressure as affected by yield strain of Superior
potatoes.

95

Snowden Variety

 

 

 

 

1500
71300‘
2%

1100"1 as X

3""! l fl *
g 900~ Wis; '* ~ 3
.. “fir-n "" " x “ x
.‘ .‘ ”It ”8 x xx f

g 700‘ :l- I“ Kgxxx xxxxx
‘2 x X 3 xx
0 xx
0 500‘

300 , Y

2500 2700 2900 3100 3300 3500

IODUUS 0F £1115er (kPa)

i"bruins! Xunbnmnd

Figure 4.21 The dynamic contact pressure as affected by modulus of elasticity of

Snowden potatoes.

Snowden variety

 

 

 

 

 

1500
A 1300 ‘
a
3!
E “00 '1 n ‘
D _ 'fi"
2 x
g 900 .._-- x
.. x xxx x
x
g 700‘ x” 1:“ a xx x xx
E 32
8 500‘
300 T T r r 1
300 500 700 900 1100 1300 1500
YMD m (kPa)
"‘ bmiu X mum-ad

Figure 4.22 The dynamic contact pressure as affected by yield stress of Snowden

potatoes .

96

Snowden Variety

 

 

 

 

1500
$1300“
gnm~ .K
It 1""
t " x
900‘ "‘ "
CL ‘3“ q: 2. xxx! X
5700~ W .. 4 x ‘3 “
u! xx x i x
g xxilx f
500*
300 ' ‘ r r
0.1 0.3 05 07 0.9 1.1

mm: snum mam} (N—m)

*bm'ndxunbnnnd

Figure 4.23 The dynamic contact pressure as affected by strain energy of Snowden

 

 

 

 

potatoes.
Snowden Variety
1300
'3
a 1100 4 1‘ in
V ‘ ~ ‘
fl ' 2 § 3"
g 900 -1 u = u : x E ii §
*“. : gx
O- “ x ‘ ‘ g . if x 8 § X
7m‘ : ~ :83 xxx x
E x : x I x
55m‘
0
300 . .
0.15 0.25 0.35
YIELD SIRAIN

i"tannin-11 mennnd

0.45

Figure 4.24 The dynamic contact pressure as affected by yield Strain of Snowden

potatoes.

97

4.13).

The Spearman correlation coefficients of mechanical properties of bruised
Superior tubers are shown in table 4.14. From table 4.14 it is seen that the correlation
between contact pressure and yield stress was high. A similar trend was found in
bruised Snowden potatoes as shown in table 4.15. The differences in the elasticity, yield
strain, yield stress, and strain energy between bruised and unbruised groups in Superior
and Snowden potatoes were significant. The yield stress of bruised and unbruised
Snowden potatoes was significantly lower than that of the Superior variety in general.
It was also found that the modulus of elasticity and yield stress of bruised Snowden
potato tubers were significantly lower than that of the unbruised Superior and unbruised
Snowden potato tubers. The comparison of properties of bruised and unbruised tubers
is shown in table 4.16. Table 4.16 indicated that there existed a significant difference
of mechanical properties between bruised Superior and bruised Snowden potatoes. A
comparison of mechanical properties of bruised Superior and bruised Snowden tubers are
also shown in table 4.17. The difference in yield force, yield strain, strain energy, yield
stress, elasticity, and dynamic contact pressure are significant between the varieties.
Therefore, it can be said that the mechanical properties were significantly affected by the

potato variety.

98

Table 4.7 Analysis of variance of yield strain over variety

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

DF SS M83 F-Value Prob
Between 1 0.146 0.146 149.971 0.0000
Within 178 0.173 0.001
Total 179 0.319
CV = 11.08%
Table 4.8 Analysis of variance of strain energy over variety
DF SS M88 F- Value Prob
Between 1 261.167 261.167 214.463 0.0000
Within 178 216.763 1.218
Total 179 477.930
CV=21.77%
Table 4.9 Analysis of variance of yield stress over variety.
DF SS MS F-Value Prob
Between 1 231.310 231.310 203.73 0.0000
Within 178 202.09 1.135
Total 179 439.346
i=— 1
CV=12.54%

99

 

 

 

Table 4.10 Analysis of variance of elasticity over variety. -

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

DF SS MS F-Value Prob
Between 1 119.115 119.115 9.03 0.0030
Within 178 2347.99 13.19
Total 179 2467. 106
CV = 12.12%
Table 4.11 Analysis of variance of yield strain over dates of harvest
DF SS M88 F- Value Prob
Between 2 0.146 0.073 74.171 0.0000
Within 177 0.173 0.001
Total 179 0.322
CV = 1 1. 10%
Table 4.12. Analysis of variance of yield stress over date of harvest.
E
DF SS M88 F-value Prob
Between 2 236.80 118.401 106.595 0.0000
Within 177 196.60 1.111
Total 179 433.407
CV = 10. 12 %

100

 

 

 

Table 4.13 Analysis of variance of elasticity over date of harvest.

 

 

 

 

 

 

 

 

 

 

 

DF SS M88 F-Value Prob
Between 2 177.434 88.717 6.858 0.0010
Within 177 2289.671 12.93
Total 179
CV =11.99 %

101

 

Table 4.14 Spearman correlations between mechanical properties of bruised

 

 

 

 

 

 

 

 

Superior potatoes.
Potential Yield Strain Elasticity Yield Contact
Energy Strain Energy Stress Pressure
Potential 1.0
Energy
Yield 0.10 1.0
Strain
Strain 0.09 0.85 1.0
Energy
Elasticity 0. 15 -0.29 0.08 1.0
Yield 0.075 0.51 0.88 0.49 1.0
Stress
Contact 0.25 0.08 0.53 0.55 0.81 1.0
Pressure

 

 

 

 

 

 

 

 

Table 4.15 Spearman correlations between mechanical properties of bruised
Snowden potatoes.

 

 

 

 

 

 

 

 

Potential Yield Strain Elasticity Yield Contact
Energy Strain Energy Stress Pressure

Potential 1.0

Energy

Yield 0.11 1.0

Strain

Strain 0.15 0.98 1.0

Energy

Elasticity 0.24 -0. 10 0.07 1.0

Yield 0.19 0.92 0.97 0.19 1.0

Stress

Contact 0.56 0.55 0.62 0.40 0.70 1.0

Pressure

 

 

 

 

 

 

 

 

102

Table 4.16 A comparison of mechanical properties of bruised and
unbruised potatoes (or: 5%).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Variety Mechanical t-value SD of mean Significance
Properties difference level
Superior Yield force -2.90 1.0500 *
Superior Yield Strain -1.18 0.0067 *
Superior Strain energy -2.71 0.2953 *
Superior Yield stress -3.14 0.2461 *
Superior Elasticity -2.64 0.9207 *
Superior Potential 4.07 0.0824 *
Energy
Superior Dynamic 7. 18 0. 1903 *
Contact
pressure
Snowden Yield force -9.03 0.5772 *
Snowden Yield strain -5.00 0.0065 "'
Snowden Strain energy -7.55 0.1963 "‘
Snowden Yield stress -9.028 0.1823 *
Snowden Elasticity -5 .79 0.4425 *
Snowden Potential 7.07 0.0608 *
Energy
Snowden Dynamic 21.90 0.2121 "'
Contact
pressure

 

 

 

 

 

* significant

103

 

Table 4.17 A comparison of mechanical properties of bruised Superior
potatoes with bruised Snowden potatoes (a= 5%).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Mechanical t - value SD of mean Significance

Properties difference

Yield Force 15.56 0.6760 *

Yield strain 11.11 0.0058 *

Strain Energy 15.31 0.1690 *

Yield stress 15.56 0.1596 *

Elasticity 5.47 0.9274 *

Potential Energy 0.39 0.0779 ns

Contact pressure 23.67 0.1723 "‘
mean:

Polyphenol Oxidase (ppo) Activities:

A sample of 3 min ppo (oxygen uptake) activity of Superior potatoes is shown
in figure 4.25. The slope of these curve indicated the uniformity of the sample. The
colors of the supernatant after 3 min of reaction in fresh bruised Superior and Snowden
potatoes are shown in figures 4.26 and 4.27, respectively. The faint and the deep color
indicated the results of lower and higher ppo activity, respectively, within a 3 min period
of reaction. The effect of drop heights on ppo activity of Superior and Snowden
potatoes are plotted in figure 4.28 and 4.29, respectively. There existed a high positive
correlation (R2=0.90) between the drop height and ppo activity in bruised potatoes and
unbruised groups of potatoes. The trend of ppo activity of bruised and unbruised
Superior potatoes of September 17, 1993, plotted against drop height are shown in

figure 4.29. The differences in ppo activity between bruised and unbruised groups of

104

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Figure 4.25 Three minutes enzyme activity (Oxygen up take by catechol) of bruised
Superior potatoes dropped from 38. 10 cm (15 inch) height.

 

Figure 4.26 The resulting color of a supernatant of Superior potatoes after 3 min of
reaction.

105

 

Figure 4.27 The deep yellow color of a supernatant of Snowden potatoes after 3 min of
reaction.

Superior Variety

 

ii ./

PPOXB (MOLE/U MIN)

 

 

 

30 4'0 50 0'0 7'0 110
01111? 11110111 (cu)

* apt-17 (W) * apt-17 (W) '8' apt-60 (“in“) 'I‘ 111-30 (NM

Figure 4.28 The effect of drop heights on polyphenol oxidase activity in Superior
potatoes.

106

Snowden Variety

 

 

Ppoxea (MOLE/UMIN)

 

 

 

0 10 20 30 40 50 60 70 00
DROP HEIGHT (CM)

-fi- apt-24 (unbnl'sed) ‘9‘" apt-24 (bun!) '8' 001-22 (unbriul) '.' act-22 (bush!)

Figure 4.29 The effect of drop height on polyphenol oxidase activity in Snowden
potatoes.

107

 

the Superior variety of the two harvests at all drop heights were significant. There
existed a high positive correlation (r2=0.90) between the drop height and ppo activities
in bruised potatoes and between drop height and polyphenol oxidase activity of
unbruised potatoes. This result indicated that if a tuber was impacted at a given height,
certainly ppo activities would increase. The difference in polyphenol oxidase activity
between the control and impacted potatoes was significant which was expected.

The unbruised group of Superior potatoes harvested on September 30, 1993,
had significantly higher ppo activity compared to that of the control. This indicated
that a greater potential to show blackspot in some unbruised potatoes eventually. The
answer to the question as to why the ppo activity was higher in this case inspite of
100% shatter bruise at 61 cm (24 inch) drop height in this particular harvest is not
known. The difference in the ppo activity among control of Snowden potatoes of
September 24, 1993, was insignificant. The trend of ppo activity in bruised and
unbruised Snowden potato tubers is shown earlier in figure 4.29. It was observed that
once a potato tuber was dropped from any height ppo activity began immediately. It is
to be noted that the ppo activity at this particular case of 61 cm or 24 inch drop height
24 inch was lesser than 31 inch and 15 inch drop heights. This was expected for two
reasons: 1) some of enzymes (polyphenolase) might have been denatured due to
shattering; and 2) the substrate in the shattered area had started oxidation due to open
air before the sample was collected which resulted in lower ppo activities. The ppo
activity of potatoes harvested on September 30’193 was significantly higher than that of

the Superior potato tubers harvested on September 17, 1993.

108

The trend of the ppo activity of Snowden potato tubers of October 22, 1993,
is also shown earlier in figure 4.29. The ppo activity was significantly higher as
compared to that of Superior potatoes of September 17, 1993 (90% tubers shattered at
61 cm or 24 inch drop height). Similarly, the difference in the ppo activity of the
unbruised and bruised Snowden potato groups were significant. The results showed that
Snowden potato had significantly higher ppo activities than the Superior potatoes.
Therefore, the Snowden potatoes can be classified as more susceptible than the Superior
potatoes.

It should be noted that the severity of the shattering in Snowden potatoes was
considerable as compared to that of the Superior potato tubers. The mechanical
properties of Snowden tubers showed significantly lower values while its ppo activity
was significantly higher than that of the Superior potato tubers. These properties
strongly suggested that Snowden tubers were susceptible to tissue failure causing
blackspots compared to the Superior potatoes. The Snowden potatoes exhibited higher
number of blackspots and shatter bruises. The shattered Superior potatoes that were
impacted at 61 cm (24 inch) drop height showed higher polyphenol oxidase activities as
compared to that at lower levels of impact.

In general, polyphenol oxidase activities were significantly higher in bruised
Snowden potato tubers than that for bruised and unbruised Superior and unbruised
Snowden tubers. These results clearly indicated that the Snowden variety was more
susceptible to bruising than the Superior variety. Therefore, it can be generalized that

higher the contact pressure, higher the enzymatic activity and blackspot bruising.

109

Optical Density (0D)

Figures 4.30 and 4.31 show before and after 24 hrs of open air oxidation of
homogenized extracts of unbruised and bruised Superior tubers, respectively. From
figures 4.30 and 4.31 it was seen that before oxidation, the color of the extracted juice
had faint red color. After 24 hrs of open air oxidation the color became light black
(unbruised) or deep black (bruised). Therefore, it supports that blackspot bruise is a
result of oxidation process. In order to distinguish the differences of color properly,
after complete oxidation, the extracts of bruised adn unbruised potatoes were
homogenized after complete oxidation. The homogenized extracts were measured for its
optical density by using a Spectrophotometer. The difference in the optical density
among and between the controls of Superior and Snowden potatoes were insignificant.
This indicated that the selected controls had low and uniform activity which was expected
since no impact was made to these potatoes (fig 4.32). According to susceptibility table
4.1 the controls of Superior and Snowden potatoes were ranked as moderately resistant
and moderately susceptible, respectively. The effect of the drop height on the optical
density of both varieties, as determined in the bruised and the unbruised potato tubers,
are shown in figures 4.33 - 4.39. From figures 4.33, 4.34 and 4.35 it was seen that
there were differences in OD (Superior) between unbruised and bruised extracts at 6
inch, 15 inch and 24 inch drop heights. The OD of unbruised extracts were insignificant
but OD of bruised extracts were darker with some variation. From figures 4.36-4.39,
in all cases, it was seen that OD of the bruised and unbruised extracts were light to dense

regardless of drop heights. This implies that, even a tuber did not show bruise at the

110

 

Figure 4.30 The light red color of homogenized extract of Superior potatoes before open
air oxidation.

 

Figure 4.31 The dense color of homogenized extract of Superior potatoes after 24 hrs
of open air oxidation.

111

 

Figure 4.32 A sample of homogenized extract of controls (left-Snowden, right-Superior
for optical density measurement.

 

Figure 4.33 Homogenized extract of unbruised and bruised fresh Superior potatoes at
15.24 cm (6 inch) drop height for OD measurement.

112

 

Figure 4.34 Homogenized extract (unbruised and bruised) fresh Superior tubers dropped
from 38. 10 cm (15 inch) drop height for OD measurement.

 

Figure 4.35 Homogenized extract (unbruised and bruised) fresh Superior tubers dropped
from 61 cm (24 inch) drop height for OD measurement.

113

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WM” 5mm 24'93 .-
”HM I! finch Bmised at 6 inch '

l .
L..-

 

‘.2_.c...-.... ~._.a.-__..~_ . ..__ z..._ .._. #_- - '

Figure 4.36 Homogenized extract of (unbruised and bruised) Snowden tubers dropped
from 15.24 cm (6 inch) height for OD measurement.

P O

Unbruised at 24" Bruised at 24“ '
Snowden, Sept 24'93 Snowden, Sept 24 93
/

 

Figure 4.37 Homogenized extract (unbruised and bruised) fresh Snowden tubers dropped
from 38. 10 cm (15 inch) drop height for OD measurement.

114

    

 

J .' , A
- f” ‘ snowman
- 033mm A'rw

Figure 4.38 Homogenized extract of unbruised fresh Snowden tubers dropped from
38.10 cm (15 inch) drop height.

   

SNOWDEN'IQ”. SNOWDEN' '
BRUISED AT 15" BRUISED Am;
Grl Gr2

 

*—

Figure 4.39 Homogenized extract of bruised fresh Snowden tubers dropped from 38. 10
cm (15 inch) drop height for OD measurement.

115

same drop height but bio-chemical reaction has started due to impact and eventually
unbruised potatoes would show blackspot. The differences of the optical densities
between the extracts of bruised and unbruised groups of potatoes at all heights were
significant in both varieties. The optical density of the extracts of unbruised Superior
potatoes remained nearly unchanged, as expected, since the potato tubers did not bruise
but unbruised Snowden potatoes did show activities. The overall results of the tests
demonstrated that the optical density of the Snowden potatoes was significantly higher
than that of the Superior variety. This was expected since ppo activity in bruised
Snowden potatoes were significantly higher, which resulted in higher optical density for
Snowden potatoes. According to the defined susceptibility scale developed by Dean et
a1. (1992), unbruised or bruised groups of potatoes in the Superior variety were ranked
as moderately resistant. The unbruised groups of Snowden potatoes were ranked as
moderately susceptible and bruised as susceptible, respectively. The summary of the
ppo, optical density is shown in table 4.18.
Hypothesis: A combination of physical and chemical properties can be used to
determine the bruise susceptibility of potatoes in a lot. The equation developed was:
BS= - 0.12- 2.79'“ (CP) - 1.28“ (E) + 5.03 (OD) + 1.09"1 (PPO), Adi R2=0.9l
Where, BS= blackspot susceptibility (table 4.1)

CP= contact pressure (kPa)

E = modulus of elasticity (kPa)

OD = optical density

PPO= polyphenol oxidase activity (mole/lit/ min)

116

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117

Determination of Mineral Contents

The Calcium, Copper and Potassium contents in each tuber were determined in
the Soil Testing Laboratory (Appendix D ). In order to compare the mineral properties
of bruised and unbruised potatoes in both varieties, t-tests were performed. The results
show that there was a significant difference in the amount of Copper, Calcium and
Potassium contents between the Snowden and the Superior varieties in general. The
mineral contents of bruised and unbruised Snowden and Superior potatoes were not
significant. It can be said that the mineral contents probably contribute to the bruising
phenomena in fresh potato tubers. It needs to be studied further before one can draw
a concrete inference about their roles in blackspot bruising in potatoes.
4.4 Conclusions

The bruised Snowden potatoes had significantly higher ppo activity than

that of Superior potatoes. The optical density of the bruised tubers was significantly
higher than that of the unbruised potato tubers in both varieties. This was expected
since higher ppo activity caused higher optical density. The control of the Superior and
the Snowden potato tubers were ranked as moderately resistant and moderately
susceptible, respectively. The optical density of the estracts of bruised Snowden tubers
was significantly higher than that of the extracts of bruised and unbruised Superior and
unbruised Snowden varieties. The bruised potatoes in Superior and Snowden were ranked
as moderately susceptible and susceptible, respectively.

The cores of Superior and Snowden potatoes were tested to determine

mechanical properties, such as yield strength, yield force, total deformation at yield,

118

yield energy and modulus of elasticity (Appendix A). The yield stress of the Snowden
variety is significantly lower than the Superior variety. From the analysis of variance
it can be stated that the modulus of elasticity of bruised Superior tubers was significantly
higher than that of Snowden tubers. It was found that the yield stress and the modulus
of elasticity of bruised Snowden tubers were significantly lower than unbruised Superior
and Snowden tubers. The analysis showed that the varieties and dates of harvest had a
significant effect on yield strength, total deformation at yield, yield energy, yield force,
yield strain and the modulus of elasticity in both varieties. The differences in elasticity,
strain, stress and the strain energy of bruised and unbruised groups were also significant
in both varieties. It was found that the properties of unbruised and bruised potatoes
were significantly different in both varieties. It was found that the mechanical properties
of bruised Superior potatoes were highly significant compared to that of the Snowden
potatoes.

It was found that the correlation between strain energy and yield strain, yield
stress and strain energy, dynamic contact pressure and yield stress are positive and very
high. There existed high correlation 0.90 to 0.97 between the drop heights and the
percentage of bruised potato tubers. The step wise regression shows that the dynamic
contact pressure of bruised Superior potatoes was highly correlated with strain energy,
modulus of elasticity and yield stress. The adjusted R2 in this analysis was 0.79. These
tests clearly revealed that dynamic yielding contact pressure can be an important
parameter that explains blackspot bruising phenomena in potato tubers.

There existed a threshold value of the contact pressure at which a particular

119

variety would bruise when subjected to an impact loading. The threshold pressures
values were 880.94 kPa (127.73 psi) and 1027.11 kPa (148.86 psi) for the Snowden and
for the Superior potatoes, respectively, indicating that the Snowden variety was more
susceptible to bruising than the Superior potatoes. Therefore, higher percentage of
bruised tuber can be expected in Snowden potatoes than the Superior potatoes. There
was a positive correlation between the drop height, ppo activity and the optical density.
The ppo activity of bruised Snowden tubers was significantly higher than the unbruised
Snowden or Superior potatoes. The optical density of bruised Snowden potatoes was
significantly higher than the Superior potatoes. The percentage of bruised Snowden
potatoes was significantly higher than that of the Superior potatoes which further
supports that the Snowden variety was more susceptible than the Superior variety.
The threshold values of the dynamic contact pressures may be used to
determine the bruise susceptibility of potato tubers. Potato bruising caused a significant
change in the polyphenol oxidase activity and the resulting optical density. Bruise
susceptible potatoes exhibited higher polyphenol oxidase activity and resulted in higher
optical density. There was no strong statistical support to the fact that minerals play a
significant role in blackspot bruising phenomena in the fresh potatoes. The mechanical
and chemical tests show that the Snowden variety is more susceptible than the Superior
variety. Significant differences existed between the mechanical properties of Superior
and Snowden potatoes. The varieties and the dates of harvests had significant influence
over the mechanical properties of the potatoes. The yield stress of Snowden potatoes was

significantly lower than the Superior variety causing the Snowden potatoes to be more

120

susceptible to bruising than the Superior variety.

Pressure sensitive film is a viable method of measuring dynamic contact
pressure. Susceptibility to bruising is an inherent property that can be determined by
studying mechanical properties. The yield stress, modulus of elasticity and strain energy
of the Snowden potatoes were significantly lower than that of the Superior potatoes. The
varieties and the dates of harvests had significant influence over the mechanical
properties of potatoes. Dynamic property, mechanical properties and chemical property

revealed that Snowden variety is more susceptible than that of Superior variety.

121

5 POTATO BRUISING BY QUASI-STATIC LOADING

Blackspot bruising is caused by internal tissue failure. The internal tissue failure
is an indication of resistance to bruise. The method widely used in determining
resistance of a material are static indentation, using spherical, pyramid and flat indenter
(Tabor, 1950; Davis, 1949). According to Hadfield (1976) a yield point is the measure
of resistance to failure of a material. Finney (1963) used spherical indenter to measure
strength of a potato when tuber severely failed locally as shown by a hook on the force-
deformation curve. He defined this point as rupture point. He stated that there was no
yield point found in the load-deformation curve. This section will verify his statement
with explanation.

The objective was to determine the potato behavior under quasi-static loading.
5.1 Methodology
5.1.1 Determination of Quasi-static Contact Pressure

A separate group consisting of 60 randomly selected freshly harvested tubers
were used to perform compression tests. A steel spherical indenter 22 mm (7/11") dia was
fitted on the crosshead of the Instron testing machine. A pair (5.08 cm X 5.08 cm) of
Fuji pressure sensitive film (A and C, type 2) were placed together on the top of the
whole before the load was applied. A new film was used for each tuber. The indenter
compressed the whole tubers with the pressure film at a 1.77 cm/min (0.5 in/min)
loading rate. The film started developing color as soon as the indenter touched the
pressure sensitive film. The compression was continued until the tuber began to fail

which was seen as a hook on the chart. The crosshead was stopped and raised

122

immediately to deter further deformation. The pressure sensitive film developed a red
color depending on the magnitude of the contact pressure. The chart recorded the
corresponding force and deformation. The color of the pressure films was digitized and
the corresponding gray values were recorded. The same calibration curve was used to
measure the quasi-static bruising contact pressure.
5.2 Results and Discussion

The figures 5 .1 shows instron testing machine fitted with a spherical indenter
and potato bruising aarangement and 5.2 shows the hook that indicates bruising of a
potato under spherical indenter test. It can be seen that there was no yield point on the
curve except a hook at the tip of the curve but yielding must have occurred well before
the localized tissue failure. The book correspond to a rupture force or maximum strength
of the tuber. The reason why the yield point was not shown on the curve can be
explained as follows: as the indenter starts loading the potato, the contact area increases
continuously causing a steady increase in force to deform the tuber. There is no sudden
change of area occurs until it fails severely by skin separation. This is the point when
hook appears due to sudden change of contact area and skin separation. That is contact
pressure excwds the ultimate strength of the tuber, causing localized major tissue failure.
This is the only failure indicated by the hook in the force-deformation curve as shown
later in figure 5 .2. The resulted intensity of color of the pressure sensitive film is
shown in figure 5 .3. The dense pink color was digitized as described earlier and
maximum pressure for each tuber was measured from the calibration curve. The

bruising contact pressure for Superior potatoes ranged from 2197.44 kPa (318.69 psi) to

123

 

 

 

Figure 5.1 Bruising a potato at the stem end using a (7/8 inch dia) spherical indenter
on an Instron Testing machine.

 

17 _. . .141.

Figure 5.2 The hook which indicates potato bruising during the compression test using
a spherical indenter.

124

   

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s.» out awn page, 514 27"}3
4p,

gait

Figure 5.3 The deep red color formation in the pressure sensitive film due to high

Measured Contact Pressure (kg/cm"2)

contact created by the spherical indenter.

 

R2=0.96

 

“202'12’22Esz‘425isZ7zsz‘9
Calculated Contact Presurefkg/crn‘Z)

 

 

(A1
5

A pressure sensitive film

Figure 5.4 Correlation between calculated and measured contact pressure for Superior
potato tubers.

125

2618.29 kPa (379.73 psi) having an average of 2498.61 KPa, SD 1.40 (362.37 psi, SD
19.93). Similarly, the bruising contact pressure for the Snowden potatoes varied from
2065 kPa to (360.72 psi) 2487.82 kPa (272.50 psi, SD 21.05) having an average value
of 2340.67 kPa, SD 1.89 (339.52 psi, SD 26.89). The calculated maximum contact
pressure in each case, qo was calculated using the equation given by Timoshenko et al.
1970 (page 412) as follows:
q0 = 1.5 (F/rr a2),
where, F: yield force
k1 and k2 are property constants of the materials.
R1 and R2 are the radius of potatoes and sphere, respectively.
’a’ the radius of contact can be calculated from the following relation:
a3={3 1r F (k,+k2) Rl R,}/[4(R,+R,)].
The data for the Superior and Snowden potatoes are included in appendix B .

The correlation between the calculated and measured yield pressures was 0.96
for whole Superior potatoes as shown in figure 5.4. From the load-deformation curves
of Superior and Snowden potatoes, figures 5.5 and 5.6, respectively, it can be seen that
for a deformation of 10.16 mm (0.40 inch) the force was almost 50 kg (110 lb) for
Superior potatoes whereas it was about 36.39 kg (80 lbs) for Snowden potatoes. The
average deformation was 10.16 mm (0.40 inch) for Superior potatoes and 10.66 mm
(0.42 inch) for Snowden potatoes. It appeared that Snowden tubers can exhibit a wide
range of deformation from 7.87 mm (0.31 inch) to 13.97 mm (0.55 inch).

The figure 5.7 shows a non linear trend of contact pressure as deformation

126

 

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1304

D.

110+ .
100* ,,

Lowllllrl

e$$¥é§§$§

 

 

 

a 0.35 014 0.35 0.5
Dcformationt in)

A Whole Potatoes

Figure 5.5 Load-deformation as observed in bruised whole Superior potatoes under
loading by a spherical indenter.

 

110
1W

901 ' .

5

D
D

D

Load (lb)

 

 

 

c8

.3 055 0.4 0.115 05 055 0.6
Dcforrnationtin)

A Snowden Potatoes

Figure 5.6 Load-deformation as observed in bruised whole Snowden potatoes under
loading by a spherical indenter.

127

 

2.75

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as
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I v.»
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so

A

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Contact Pressure (MPa)

2.15“

 

 

 

0.7 0.75 0.8 0785 019 0'95 1 1.05 1.1 1.15 1.2 1.25
Deformation (cm)

A Calculated X Measured

Figure 5.7-A nonlinear trend of measured contacr pressure in whole bru1scd Superior poratocs.

 

 

 

 

a 32
‘
E
E» R-=o.94
o 27*
I-
? .. A
. a?

:1: :‘
‘6 22‘ ‘
8 A
S
U A
g 17~ A
2

1- .

12 17 22 2'7 32

Calculated Contact Prusurctkgrun‘ll
A whole potato

Figure 5.8-Correlation between the measured and calculated contact pressure in whole bruised
Snowden poratoes.

128

continued in Superior potatoes. The difference between the measured and the calculated
contact pressure was within 5% of accuracy. The contact pressure of the bruised
Superior potatoes remained nearly constant beyond the deformation of 10.16 mm (0.40
inch). A similar trend of contact pressure and deformation as affected by the bruised
Snowden potatoes was shown in appendix E.2. It indicates that all tubers have been
undergone a permanent deformation above 2503 kPa (363 psi) when major tissue failure
occurred. This was because of considering hook as an indicator of failure where as the
hook was a major localized failure point not an on set of permanent deformation. The
correlation between the measured and calculated quasi-static contact pressures for
Snowden potatoes was 0.94 shown in figure 5.8. Therefore, the measured contact
pressure can be used for analysis with confidence. From the observation it was evident
that as the dates of harvests passed by, the contact pressure showed a decreasing trend
after certain time. It indicated that potato must have an optimum time for harvesting.
The potatoes keeping longer under the soil would not increase resistance of bruising.
The difference in quasi-static bruising contact pressures between the Superior and
Snowden potatoes was found to be significant.

It can be stated that potato should not be subjected to pressure equal or more
than yield stress. Finney (1963) stated that there was no yield point of potato found
on the force-deformation curve, this observation was confirmed with explanation.
Therefore, yield point of a potato may not be observed on th force -deformation curve
by using spherical indenter. The FEM models were used to verify the induced contact

stress distributiom, particularly, maximum stress and its area of location.

129

5.3 Conclusions

The pressure sensitive film can be used to measure quasi-static bruising contact
pressure. Spherical indenter pressure test is not a good method to determine on set
of bruising in potatoes. The evidence clearly showed that by the time a hook is seen on
the force-deformation curve, actually major localized bruise has already occurred.
Therefore, the hook in the force-deformation curve is the indicator of severe localized
bruise, not an indicator of onset of bruising. The measured contact pressure was much
beyond the yield strength of the potato which resulted in a major localized tissue failure.
Therefore, yield point may not be seen on the force-deformation curve as generated by
spherical indenter pressure test. Particular potato variety must have an appropriate time
for harvest. Keeping potatoes longer under the soil would not increase resistance to

bruising.

130

6 MODELING BASED ON FINITE ELENIENT IVIETHOD

The mechanisms of potato bruising are complicated due to combined stresses and
strains. Slow compression causes a bruise to occur internally around the center region
of the potato. Bruising also occurs close to the periphery under impact loading (Serif
and Segerlind, 1976; Holt and Schoorl, 1983). Blackspots bruises are often found 2-7
mm below the skin of potato (Sawyer et al, 1960). The potato tissue failure is related
to the induced stress in the material resulting from an applied load. It is important to
know the intensity and the distribution of stresses under a given load in a tuber so that
an inference about the tissue failure can be made (Chen et a1, 1984).

Although biological materials fail abruptly, the strains develop before failure are
larger than those in brittle materials. The deformation properties of potato tubers
indicate that they can be considered as elastic, elasto-plastic or Viscoelastic depending on
the level of load, duration of loadings and the condition of the potatoes. Therefore, any
mathematical constitutive relationship for biological material should be sufficient to
explain elastic, Viscoelastic or elasto-plastic behavior that occurs after a critical amount
of strain has developed. Many researchers have mentioned that an analytical solution by
idealization does not lead to meaningful results.

Many techniques have been used to study the response of fruits and vegetables
to applied loads (Hamann, 1967; Miles et a1, 1971; Horsfield, 1972). Instrumented
sphere techniques (Siyami et a1, 1988), photographic technique (Anazodo et al. 1983),

131

finite element modeling (Sherif and Segarlind, 1976; De Baerdaemaker, 1975; Rumsay
et al, 1974; Apaclla, 1973) and electron microscopy (O’Brien et a1, 1984) have been used
to study bruising of fruit and vegetables. Contact stresses occur during storage,
handling and harvesting. Numerous research studies in the past indicated that potato
bruising resulted from impact during harvesting and handling, but the exact mechanism
of the potato tissue failure yet to be identified (Chen et a1, 1984). Mathematical
modeling and computer simulation seem capable of providing information on the failure
mechanism of biological materials (Sherif and Segerlind, 1976). Researchers concluded
that the finite element modeling can provide a better understanding of the bruising
mechanism and may effectively be used to solve the contact problems (Fayu et a1, 1989).
The objective of this section was to investigate contact pressure due to quasi-

static loading that may cause tissue failure in fresh potatoes using finite element method.
6.1 Theoretical Considerations
6.1.1 Stress-Strain Relationship in Elasto-Plastic Domain

When a specimen is subjected to a loading that exceeds the yield point, a
permanent plastic deformation occurs. The general strain increment equation can be
written as deij=deij° +deij". where the superscripts e and p stand for plastic. The elastic
part is related to general Hooke’s law o,j=>\eu 5fi+2 peij while the plastic part of the
strain is related to the yield criteria dis,j =Sijd>x or deviatoric stress, which is the difference
between the actual normal stresses and hydrostatic pressure, S,,-=o,,--S 6%, where,
S is the hydrostatic stress which equals 1/30,,. This is often called mean stress. The

corresponding deviatoric strain is eij=e,j-et5,j. where e = (‘/a)e,,, deb-P = Sfidh and dx=(3/2)

132

dE/E. When a specimen reaches its yield stress at the inner boundary, a permanent
plastic deformation occurs. Then the total deformation is the sum of elastic deformation
and plastic deformation. As the load increases, the plastic region spreads outward until
the specimen is completely in the plastic range plastic reaching an equilibrium with the
outside loads.
The elastic strain increments are given by Hookes’s law as follows:

de,°=(1/E)[do,-pd(oa+o,)]

d59°=(1/E)[doo-ud(o,+oL)]

de,‘=(l/E)[do,-ud(o,+a,)]
The stress-strain relation, known as the Reuss equation is:

d6, = E"[da,-pd(o,+ 0)] +‘/ad>\(20,-ao-o,)

(150 = E'1 [duo-ud(a,+ 0)] + VadMZoo-apoL)

dc, = E" [dot—ud(o,+ 00)] +‘/ad>\(20,-o,-a,)
where, E and u are the modulus of elasticity and Poisson’s ratio, respectively, and

dA = def/06 = ‘Ad'yijp/Tij

or (de,"-de,")/(o,-o,)

or (3/2)(deP/or) where,

deP=t/(2/3) at,» (ie,-P

Yew (1956) defined strain from the geometry (fig. 6.1) as follows:

133

 

 

 

 

 

Figure 6.1 A theoretical bruise initiation model for soft material (Yew, 1956).

134

1-2_’_
6 =2 R2
rr R 2 (6.1)
r
1‘1?
_D r2 (6.2)
cor— 1-——
R R2
3r2
0‘2“?)
. =-______ (6.3)
a R
1-[1
R2

D= displacement
R= radius of the potato or a sphere
r= contact radius.
From the deviatoric strain, cu’ can be written as:
5,, ' = Eu'(1/3)€n
or
en ' = (%)521
Due to the change in the bruise geometry in the plastic region, the total strain, enp
can be written as:

eu" = eu-Vaeu

135

2-_

p__ l D R 2
ea- 3? 2 (6.4)

r
1'?
From Levi-von Mises criteria for permanent deformation in plastic region
e£=glé153 (6.5)
2 0

Sn: deviatoric stress

1? = effective strain

3 = effective stress

enp = total strain in the plastic region

Combining (6.4) and (6.5), the deviatoric stress component, 8,, can be expressed as:

 

231.2

s =-123§ R2
‘1 3 R 3 g (6.6)

r2

1-_

R2

From the definition of effective strain Yew, 1956 defined it as follows:

E=§J [-:-(€rr'€a)2+(€,,‘€w)2+(ew-ea)2+%7:] wheregyfffi (6-7)

After derivation of the expressions within the parentheses in equation 6.7, the following

equations were obtained:

136

_90r2 + 8lr‘)
R2 R4

 

 

rr .2 R2 r2
(XI-F)

 

Substituting (6.8), (6.9) and (6.10) in to

'/2[(en-en)2+(en-690)2+(eu-699)2] and simplifying it became:

 

 

2
25_9Ol’ + 81rd +9r4+1—.2L2+fl:
R2 R4 R2 R4

 

18R2(1—’_2)
R2

By substituting the (6.11) in (6.7) becomes:

 

 

 

2 4 2
20.11-81.77; +7555; +8r4+ 3R
'E'=_2 1 R2 R4 r
m r,
(1 ’32-)

 

V

Substituting (6.12) in to (6.6) yields:

137

(6.8)

(6.9)

(6.10)

(6.11)

(6.12)

3r2

2-_

S __4— ( R2)
a- .30 _ (6.13)

2 4 2
J (20.11-81.77_’_+75.55’_+8r4+3R_)
R2 R4 r2

 

 

and by multiplying and dividing the above (6.13) by r the final results of:

(2 3’2)
_ 4- F

4 6
J 20.11r2-81.77r_ +7555:— +8r6‘I-3R2
R2 R4

 

 

obtained. The yield criteria for plastic deformation is, ou=Su + S, where S is
hydrostatic stress. If, it is assumed hydrostatic stress has no effect on yielding
(according to von Mises), then, (6.14) is the maximum stress that cause yielding.
6.2 Finite Element Formulation

The region under consideration can be divided in to small segments called
elements that are connected at the node points along the boundaries. The two unknown
displacements u, v are approximated over each area or element by polynomials using
parameters. The polynomials are as follows:
u = 71+72r+73z+74r2+75rz+nn
v =o,+o,r+o,z+o,r2+o,rz+....
where, 7 = alq (a/2)(kl+k2)

k1 = (1-#2)/Ei

k2 = (1'#22)/Ez

138

a) = (1/2R,+1/2R2) and
q = pressure distribution over the contact area

E = modulus of elasticity

2"
11

radius of a tuber

R2 = radius of the spherical indenter

u = horizontal displacement

v = vertical displacement

a = contact diameter

Plane strain:

The displacement in each linear element can be expressed in terms of shape function as
follows:

U=[N][U]

Where,

[N]= shape function relating the element’s displacement u and v to the nodal
displacement.

u = N,u2,.,+N,-u2j-1+ Nku21H

v=Niv2i+va2j+Nkv2k

where, the shape functions are as shown below:

N,=(a,+bix+c,y)/2Ao

Nj=(aj+bjx+cjy)/2Ao

Nk=(a*+bkx+cky)/2Ao

ai = XiYk'kai

139

b,=Yj-Yk
c,=Xk-Xj
Ao= Original area of an element.

According to Sherif et al. 1976 hydrostatic pressure can be defined as
H=(3/2) a/G(1 +11) where a, p and G are normal stress, poisson ratio and shear modulus
respectively. The mean hydrostatic pressure can be written in terms of each nodal value
(shape function).

h=[N]{H} where HT={Hi H]. Hk}T and [N]=[Ni NJ. NJ
Axisymmetric strain can be written as,
em, = (1/2)[(6u,,/8xm)+(aum/axn)+(au,/axn)(au/axm)] where n, m, 1 takes the values 1,
2.. and 613:0 and e33=(1/2)()\2-1) and A: 1+(U/r). U = length of deformed

circumference and r= original length.

Now,
6,, = (Bu/6r)+(l/2)[(du/dr)2+(dw/dr)2]
e” = (u/r)+1/2(u/r)2.
6,1 = (8w/82)+[(du/dz)2+(dw/dz)2]
Shear strain:

7,, = (do/dz)+(aw/6r)+(Bu/8r)(6u/az)+(8w/8r)(6w/dz).
Sherif (1976) cited the elastic field equation given by Herman and Toms (1964)
Tij=>teu6fi+ZGefi(3>\+ZG)
where,

k, G are the Lame’s constant

140

p = Poisson’s ratio
1,,- = total stress
The mean effective pressure
cr=‘/31',‘k
or o=‘/3(3>\+2G)eu, If k: ZGp/(l-Zu), then
a=%G(l+u)eu/(l-2u). By substituting G in the above equation, finally 1,],
becomes:
131-=(3 Up. <50)/(1+p)+ 2G5“.
The relation between mean effective pressure and hydrostatic pressure, H
H=ru/E= 30/[2Ci(1+p)]
Total stress, ru- = G[(2£,j-eu5,j) + (SI-16,3]
or rij=ZGeij+2pGHdij
For an incompressible material, stress-strain relation is
rij = 20 511 + 0,]. This can be written in matrix form as follows:
[r]=[Dl{E}-
Strain and elasticity can be written in matrix form as :
[6] = [3011(1}+[BG]{C1} and
[E]=[B]{q}. Strain component can be expressed in terms of displacement as
euz+eyy2+‘I‘zvs={q}T[I][B]{q}
and eu+en={q}T[B]T(J)
The governing equation for an element is:

[k]{¢}={p} where, d>=[q/H] and [p]={q/O}, ¢= nodal values. q=unknown

141

displacement.

[K]: Global stiffness matrix

[kill-:26 1 lelTllllBldV

[kn] =2rtG l lelTU } [nldv

[kzzl=-2#G(1-2u) l [NlTlNldV

Force matrix, Q: l VlNl{F}dV + l 1'1N111T1d51

Element stresses are [1'] = 26mm + 21161111}, where h=[N][I-I] and JT=[1 0 0]

[o]=ZG[I][B]{q} + 2pG[N]{H}{J}. For axisymmetric cases, element stresses can be

expressed in terms of matrix form as follows:

o= awn/(Homo r_1 u/(l-u) u/(l-u) 0 .,—l
ML») 1 rt/(l-u) 0 6.
1 0 e,

i... 1 -2,1/2(1+,1_):_v

 

 

Plastic Stress-Strain matrix.

In Prandtl-Reuss stress—strain relation, the strain increment, dc“, is related to the
stress-increment, do“. The relation together with the differential form of the von Mises
yield criteria can be represented in matrix form by

{d6} =[C'1{d0}

where {dc} and {do} are the column matrices of deg and da,,- respectively.
Similarly, from the above equation, stress increment can be written as
{do}=[D"]{de} where [D’]=[C']".

’p’ stands for plastic.

142

Matrix formulation:
For the isotropic, elastic material, stress by Hooks law
{0}= ElD‘lle}
= 2(1+#)G[D°l{6}
where {a} and {e} are the column matrices of stress, ‘70 and strain, 5,-

1

respectively, and [D‘] represents the 6X6 symmetric matrix which is:

 

lT-tt/r-zp u/l-Zu arr-2,1 o o 0—
u/l-Zu 1-0/1-211 u/l-Zp. o o o
[D=]=1/(1+,t) ,1/1-211 ,1/1-211 141-25 0 0 o
0 o o 1A o 0
o o 0 0 vs 0
l_0 0 o o 0 ‘A

 

E, G and u are the Young’s modulus, torsion modulus and Poisson’s ratio respectively.

Plane stress this can be written as:

a, e,
a, = E[D‘] e,
7,, 1,,
r__l u 0 —1

and [D‘]=(1/1-u2) u l 0

 

 

0 0 1- /2
L. ( I1);
An expression for plastic stress-strain was developed by Yamada(1967) as described

below. His assumption was that the plastic deformations are incremental.

143

The Prandtl-Reuss equations for the deviatoric strain-increment deu- ’ during loading is

d6 :0“ ’dk+(d0;j ’/ZG)

0'
where, d>t=(3/2) dEP/E = (3/2)(d5/3 H ’), c7 and d? are the equivalent
stress and plastic strain increment, respectively. These can be written as follows
E=J(3/2 aij ’o, ') and dEP=) (%deupdefip)
H ’= (13 M?" is the slope of the equivalent stress to plastic strain, ( j d?)
The von Mises yield criterion and its differential form is given below as:
o,,-' 00' ' =2/3b7
0,,- do” ’ =2/3 Ed?
=4/9 3" H’ dh. By eliminating dog“ from the above
equation it becomes as follows,
260,,- ‘(deij 30,]- ’drt)=4/9 ? H 'd>t from which
d>t=o,J-’ def/S
=01,- 'deij/S
where, S=2/3;2 (1+H’/3G).

de

a —o-- ’=ou- ’deij. Since, or-n’=a,’+o). ’+a,’= identically zero and by putting d>t

ij ij — 1j
and dcij ’, the deviatoric strain increment can be written as

deij =deij-5ijdEu/3

defi=dex+dey+der

and deviatoric stress increment, 0,1. ’ can be written as follows:

daij I =261d51j"01j ’ (Juden)/S].

=26[deij-(5ijdéii)/3-(Oij ’0'“ ’dekD/S].

144

The identity oij’deu- sou’ dc“. The total stress increment do”- by definition

dou- =doij ’ +E/{3(l-2p)}5,jde,j.

dou- =do,,- ’ +%/a(l +u)G/(l-2p)6,jde,,-.
After manipulation, Yamada (1967) found the folowing equation,

dou- =ZG(deU+p/{ l-Zp} dijdeii-ou ' on 'deu/S)
This equation can be written in matrix form as:

{do} =E[D"]{de}

or {do} =2(1 +p)G[D"]{de}. This is equivalent to the first equation

in page 142. Yamada (1967) called this as elasto plastic analysis where Dc was replaced

by Dp for yielded elements. The plastic Sress-strain matrix [D9] is symmetric.

145

6.3 Model Development

A commercial finite element program called MARC was used to create the
model and to determine the contact stresses. The tuber size, shape and composition
influence bruise susceptibility of potato tubers (Hugh, 1980). Jasan et al. (1988) stated
that the size of bruise is not important to predict tuber quality. Therefore, size of bruise
was not considered in the models. The potato properties used in the model were
modulus of elasticity, Poisson’s ratio, mass density and potato size. The selection of
proper number of elements in a model was made by trial and error. Several mesh
generations were accomplished. The number of elements used to construct the model
were 90, 100, 120 and 136. The models were run to ascertain that the output values of
contact stress were within the acceptable error limit (6%) compared to the analytical
solution as described by Timoshenko et el. (1970). The preliminary simulation results
show that the whole tuber model consisting of 136 elements was found within the
acceptable accuracy of output compared to the analytical and experimental values. Two
varieties namely, Superior and Snowden potatoes, were considered for simulation.

The Poisson’s ratio was taken as 0.49 (Finney et a1, 1967). The Young’s
modulus and deformations were predetermined by core tests on the Instron testing
machine in the laboratory. The modulus of elasticity taken were (426 psi) 2.94 MPa and
(410 psi) 2.82 MPa for Superior and Snowden potatoes, respectively. The mass density
of potatoes was taken 1.01 g/cm’. These data were entered in the core model to test its
response. The average displacement of the core specimen of Superior potatoes at yield

was 7.6 mm (0.30 inch, SD 0.02) and for Snowden potatoes was 6.3 cm (0.25 inch, SD

146

0.03), respectively. The average displacement of a whole Superior potato was 10.16
mm (0.40 inch, SD 0.03) and 10.67 mm (0.42 inch, SD 0.05) for Snowden,
respectively, as obtained by the compression tests. These data were used in the whole
models to test its response:
6.3.1 Core Model

To verify finite element model, a finite element model of a cylindrical potato
core was formulated. The core model was constructed with 100 quadrilateral elements
The fixed boundary conditions were applied. The nodes on the y-axis has no
displacement in x-direction and nodes on the x-axis has no displacement on y-direction.
The core model was subjected two predetermined deformation of 7.6 mm (0.30 inch)for
Superior) and 6.35 mm (0.25 inch) for Snowden, respectively to verify stress, strain,
and force at yielding. The solutions were compared with the calculated and the values
obtained from the laboratory experiments.
6.3.2 Whole Tuber Model

The tuber was considered an axisymmetric sphere, therefore, only one quarter
of the structure was needed to model and analyze quasi-static contact stresses. The
radius of each model was 40.6 mm (1.6 inch) with skin. It was assumed skin had no
effect. The model loading was compression by a spherical indentor of 22.22 mm (7/8
inch) diameter. A deformation of 10. 16 mm (0.40 inch, for Superior) and 10.67 mm
(0.42 inch, for Snowden), were imposed to the model for analysis. These deformation
values were obtained experimentally and correspond to tissue failure.

A deformation equal to (0.20 inch) 5.08 mm was applied to the model to predict

147

corresponding, stresses, deformation, and von Mises stress in Superior potato model.
This model represents Snowden whole potatoes.
6.4 Results and Discussion
6.4.1 Core Model

The original shape of core model with mesh and elements is shown in figure
6.2. The corresponding deformation, yield stress, strain and yield force due to imposed
deformation of 7.62 mm (0.30 inch, Superior) are shown in figures 6.3, 6.4, 6.5 and
6.6, respectively. The predicted FEM stress, strain and yield force were 0.96 MPa
(138.80 psi), 7.62 mm (0.30 inch) and 18.73 kg, (41.22 1b), respectively. The
calculated stress, strain and yield force due to a displacement of 7.62 mm (0.30 inch)
were 0.96 MPa (139.93 psi), 7.87 mm (0.31 inch) and 18.40 kg (40.50 lb),
respectively. The error among FEM and calculated and measured values were within 1%
(table 6.1). These results verify the model.

Similarly, for an imposed deformation of 0.63 cm (0.25 inch, Snowden) which
the model predicted stress, strain and force as follows: 0.73 MPa (105.60 psi), 6.3 mm
(0.25 inch) and 14.25 kg (31.36 lb), respectively. The calculated stress, strain and yield
forces were 0.73 MPa (105.67 psi), 6.3 mm (0.25 inch) and 14.10 kg (31 lbs),
respectively. The results are shown in table 6.1. The error is within 1%. Therefore,
these findings were verified by the finite element results.
6.4.2 Whole Tuber Model

The whole tuber model of the Superior variety with 136 elements and mesh is

shown in figure 6.7. The model was tested for a predetermined displacement of 10.16

148

Force

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 6.2 A FE model of a cylindrical core of potato with 100 elements
mesh and force applied to it.

149

‘i‘f -1.510o—01
Q15 43120-01
-2.1140—01

_;_ -2.4 1 60-01

 

 

 

43180-01 ’

 

 

 

 

 

 

 

 

 

 

 

-3.0200—01

 

Figure 6.3 The original and deformed Superior core model with a deformation
of 7.62 mm (0.30 inch). The scale shows on the left deformation

in inch.

150

-1 .3880+02

-1 .3860002

   
 
   

 

.. ~1.3850+02

 

-1.3840+O2
-1.332»02
4.301.402
4.380.412
2‘? 4.3780412

13:33 -1 3770002

 

-1 3750412

-1 3740412

 

Figure 6.4 The stress(o2) in the core model due to an imposed deformation of
7.62 mm (0.30 inch). The scale on the left shows stress in psi.

151

 

_ 41.050501

# 00475-01

40440-01

 

.. , -.. .
’iZ’I/fisrél

 

 

    

40410-01

 

 

60380-01

 

40350-01

 

£405”; W/flé

 

ti} -3.032o—01

.3 a
if} 40299-01 0‘

tie 50255-01

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5‘ -3.0230—01

. 43.020901 Y

 

Figure 6.5 The strain in the core model due to an imposed deformation of
7.62mm (0.30 inch). The scale on the left shows in/in.

152

3.2980000

2.4730000

5.
$05.55 s.

Rs.

\m§ .\\ “SAW V\\

0.0000000

w
a
2
a

’35 '1 .“90000

 

: 4.4730000

6.2980000

4.1 220000

 

 

Figure 6.6 The yield force of the core due to an imposed deformation of 7.62

lb.

1n

The scale on the left shows force

.30 inch).

mm (0

153

{it
1t
.‘9

c
it

 

 

Figure 6.7 The whole (Superior/Snowden) tuber model with 136 elelments and
mesh.

154

mm (0.40 inch) (Superior, fig 6.8) by the spherical indenter. The resulting contact
stress 2.51 Mpa (363.80 psi) and von Mises yield stress 1.80 MPa (261.80 psi) were
measured from the figures 6.9 and 6.10, respectively. The yield stress of Superior
variety was 0.95 MPa (138.22 psi). The predicted von Mises stress was 1.89 times
higher than the yield stress. This indicated that the tubers were severely stressed which
can easily be visualized from the von Mises stress contour bands shown in figure 6.10.
All four elements along the ordinate were above their yield stress indicating that elements
yielded. The elements just below the surface and near the top were highly stressed 2.50
MPa (363.80 psi). This situation resembled the actual collapse of the tubers observed
during experiments. The calculated, measured, and predicted stresses were 2.53 Mpa
(367.64 psi), 2.50 MPa (363.37 psi) and 2.51 MPa (363.80 psi), respectively (table
6.2). The error was within 2% which is in good agreement. The results verify the FEM
model (table 6.2).

The model of Snowden variety was subjected to 10.67 mm (0.42 inch) of
deformation (shown in appendix F.2 ). The corresponding bruising contact stress as
predicted by the model was 2.37 MPa (344.00 psi), whereas by analytical method, the
contact stress was 2.38 MPa (345.27 Psi). The predicted von Mises stress was 1.73
MPa (251 psi) which was 2.37 times higher than the yield stress. The stress condition
was similar to the previous models. The errors was within 2%.

When, a deformation equal to 5.08 mm (0.20 inch) (fig. 6.11) was imposed, the
model predicted a contact stress of 1.04 MPa (150.90 psi) (fig. 6.12) which was higher

than the dynamic bruising contact pressure of the Superior variety. The corresponding

155

5.1020—03
__ 6.541902

T” 45920-02

 

 

431640-01
-1.559.-or
4,974.01
.23000-01

47850-01

 

a? -3.1900-01
: 4.595501

40000-01 y

L.

Figure 6.8 The model for Superior variety shows the displacement bands as a
result of 10.16 mm (0.40 inch) imposed deformation. The scale
shown on the left is in inch.

 

 

156

2.01 55+01
_ -1.1 055501

__ -5.025o001

   
 
 
 
 
 
   

 

 

.asuuoi
‘ 4.2055502
4.6780002
gag 4.0700002

.31? 4.4620502

 

 

4.8540002

4.2460002

 

Figure 6.9 The contour stress band (02) due to imposed deformation of 10.16

mm (0.40 inch) in the Superior whole potato model. The scale on
the left shows stress in psi.

157

1.4005502
:15. 1.196.002

3:; 9.1 1 1.001

 

-. 6.267.001

. 3.4230001

5.7850000

 

 

Figure 6.10 The rsulting von Mises stress distribution with contour bands. The
scale on the left shows von Mises stress in psi.

158

2.61 80-03
-1 Jam
-3.791 0-02

63170-02

40005-02

53 -1.1 90001

 

. . -1 .392001

-1 .5950-01

-1.797o—01

 

101:1
Displacement y

 

Figure 6.11 The resulting displacement of the Superior potato model due to imposed
5 .08 mm (0.20 inch) deformation. The scale on the left shows
displacement in inch.

159

   

iobt
2nd Comp of Street

 

Figure 6.12 The stress (0,) distribution with contour band due to imposed deformation
of 5.08 mm (0.20 inch). The scale on the left shows stress in psi.

1.33M:
1.237.402
1.085.412
newt
\ 1 macaw

‘1 6.3090001

 

4193“!”

3.2780901

1 .7630e01

2.4730000

 

10M

 

Equivalent Von Mieee Streee

Figure 6.13 The resulting von Mises stress distribution with contour bands due to
imposed deformation of 5.08 mm (0.20 imch). The scale on the left shows
stress in psi.

161

contour bands of von Mises stresses are shown in figure 6.13. From figure 6.13, it
can be seen that the calculated maximum von Mises stress was 0.96 MPa (138.90 psi)
which is equal to the yield stress as found by core testing. This would result in tissue
failure and blackspot bruising in region where von Mises stress exceed or equal the yield
strength. From the contour band (fig. 6.13) the location of the concentrated von Mises
stress can be seen clearly at 5.08 mm below the surface of the model. The experimental
data indicate that the blackspot occurred was 2-7 mm below the skin in fresh tubers as
shown in figure 6.14. This has been reported by many researchers (Sawyer, 1960;
Scuder, 1950; Hughes, 1980; Chase, 1987). These results indicate that, the contact
stress that cause blackspot bruising under impact loading is higher than that found under
static loading. It can be safely extrapolated that if the contact stresses exceed the yield
strength, the blackspot bruise depth would extend from the surface to the point where the
von Mises equal the yield strength of potato tissue.
6.5 Conclusions

Finite Element Method can be used to study distribution of stresses in a loaded
potato tuber. The capability of FEM to produce pictorial illustration help in
understanding stress distribution in a loaded model. The von Mises stress distribution
shows the location of the highly stressed element where tissue failure occurs. The
Location was found at 5.08 mm below the surface of the potato model. The failure
occurs when von Mises stress is equal or higher the yield strength of potato. von Mises
stress can be used to investigate the tissue failure and its location in a loaded potato.

The static contact stress causing bruising being lower than those for impact condition.

162

 

Figure 6.14 A distinct blackspot bruise found approximately at 5—6 mm below
the contact surface of an affected potato tuber.

163

Table 6.1 Simulation of uniaxial compression and force at yield deformation
of cores of Superior and Snowden potatoes.

 

 

 

 

 

 

 

 

 

Superior Stress Deformation Strain Force
Potatoes 02 A e. F
(psi) (in) . (in/ in) (1b)
FEM 138.80 0.30 0.30 41.22
Analytical 139.93 0.31 0.31 40.50
Experimental 138.22 0.31 0.31 40.50 l
(10.52) (0.02) (3.08)
Snowden - - - -
potatoes
FEM 105.60 0.25 0.25 31.36
Analytical 105.67 0.25 0.25 31.00
105.65 0.25 0.25 30.96
Experimental (11.44) (0.03) (3.35)

 

 

 

 

 

 

Note: Numbers in the parenthesis are standard deviation

164

Table 6.2

Verification of measured quasi-static contact pressure on a whole

potato with the pressure predicted by FEM.

 

 

 

 

 

Superior FEM Analytical Experimental
potatoes

Maximum (363.80 psi) (367.64 psi) (362.37 psi)
contact pressure 2.51 MPa 2.53 MPa 2.49 MPa
Snowden — - -
potatoes

Maximum (344.00 psi) (345.27 psi) (339.52 psi)
contact pressure 2.37 MPa 2.38 MPa 2.34 MPa

 

 

 

 

165

 

7 SUMMARY AND CONCLUSIONS

7.1 Summary

Potato bruising is a complex process involving tissue failure due to induced
stress followed by bio-chemical reactions that form blackspot. The study described in
this dissertation discusses many aspects of potato bruising.

It is difficult to identify internal bruise in a potato tuber. Therefore, it was
essential to develop a technique to identify bruise before any further study could begin.
Five strengths of tetrazolium chloride solutions were prepared. Five groups consisting
of 3 tubers in each group were impacted at 10 cm drop height. Each group of tuber was
dipped into the assigned solution for 1, 2, 4, 8, or 10 hrs. The tubers were removed
after the assigned period and peeled to find pink coloras an indicator of internal bruise.
Eight hours of submerging showed distinct pink color at the internal bruised area. The
strength of this solution was 1 gallon of distilled water at 30° C mixed with 3.5 g of
tetrazolium chloride. This method was used to identify internal bruises for the entire
study.

Dynamic contact pressure developed between the potato and impacting surface
was measured by dropping the tuber once onto a pressure sensitive film attached to a
steel platform. The pink color density of the pressure sensitive film indicated the

contact pressure. The film color was digitized using a computer vision system and the

166

corresponding contact pressure was determined from the calibration curve. The impacted
tubers were categorized as bruised and unbruised groups. It was found that as the
potential energy increased the contact pressure also increased to certain extent. Further
inrease in potential energy caused bruising but no further increase of contact pressure
was observed. The bruised tubers had higher contact pressure than that of the unbruised
tubers. It was found that there existed a threshold contact pressure when tubers started
to bruise internally. These were 148.86 psi (1027.60 kPa) for Superior and 127.73 psi
(880.89 kPa) for the Snowden potatoes.

In order to determine the relationship between contact pressure and mechanical
properties, the tubers used for free fall impact were also used for mechanical properties
tests. The modulus of elasticity, yield stress, yield strain, and strain energy were
determined by performing a compression test on a core sample. A 2.54 cm (1 inch)
long and 1.54 cm (0.61 inch) diameter core was cut out of each impacted tuber using a
metallic cylindrical borer. The core was compressed on an Instron testing machine until
it failed at a loading rate of (5 in/min) 12.70 cm/min. The differences of modulus of
elasticity, yield stress and strain, and strain energy of the bruised groups of Superior
variety were significantly higher than that of the Snowden variety.

To determine the correlation between drop height and ppo activity, the ppo
activity of all impacted tubers (bruised and unbruised) was measured and compared. A
small piece of impacted tissue from the stem end was taken after 24 hrs of incubation.
The tissues were mixed with buffer (pH 6) to stabilize enzymes and blended in an electric

blender for 15 seconds and filtered. Acetone of—20° C was added to precipitate enzyme.

167

The enzyme was diluted with sodium acetate and calcium chloride solution respectively
to precipitate any pectin. The mixture was centrifuged at 10,000 rpm to separate
impurities from the enzyme. The substrate catechol was added to the samples of the
extracted enzymes for activities test. The ppo activity of enzymes for 3 min tests was
performed in a Spectrophotometer. The activity was measured by the rate of change of
color formation in the solution. The activities (oxygen uptake) was determined from the
slope of the reaction curve. It was found that the ppo activities of bruised potatoes was
significantly higher than that of unbruised potatoes in both variety. There existed a high
correlation between the drop height and ppo activity of bruised potatoes. The ppo
activity of bruised Snowden potatoes was significantly higher than that of the Superior
potatoes. Increasing drop heights caused more bruising of the susceptible variety
releasing more substrate that resulted in higher ppo activities.

The color of the extracts of bruised or unbruised potatoes were the direct results
of the ppo activities. The measure of the color density is the optical density an absolute
value. According to Mapson (1963) concentration of PFC (tyrosinase) and enzymatic
browning of bruised potatoes were closely related. Dean’s et a1. (1993) method was used
to determine bruise susceptibility of potatoes using optical density. First, extracts of
each bruised group of potatoes was tested for color formation as measured by optical
density to determine bruise susceptibility. Tissues from bud-end and impacted stem-end
in equal amount was taken and mixed with potassium phosphate buffer to stabilize
enzyme during extraction. The tissues were blended immediately in a high speed electric

blender. The mixture was filtered and left 24 hrs for open air oxidation in a room

168

temperature. The oxidized filtrate became dense black (bruised groups). This filtrate
was centrifuged at 10,000 rpm for 30 min. A quantity of 10 ml homogenized filtrate was
diluted 1:3 ratio and measured for optical density (unbruised and bruised) in a
Spectrophotometer. It was found that the differences of optical density of bruised and
unbruised groups of tubers at all drop heights were significant in both varieties. The
results show that bruised Snowden variety had significantly higher optical density than
that of the Superior variety. This was expected since ppo activity of Snowden variety
was significantly higher than that of the Superior variety which resulted in higher optical
density. This indicated that the bruised Snowden variety had higher amount of substrate
(phenol) than of the Superior variety. Optical density and ppo activity were highly
correlated. Therefore, optical density may be used to determine bruise susceptibility of
a variety. The optical density method is simple and easy unlike measuring ppo activity.
According to Dean’s et al. (1993) bruise susceptibility scale the Superior variety was
ranked as "Moderately resistant" and Snowden variety as ”Susceptible".

In order to understand the role of minerals in bruising, the mineral contents of
each potato was determined in the laboratory. A small quantity (30 g) of tissues from
each impacted tuber was collected and oven dried at 75° C for 72 hrs. The dried samples
were ground and Calcium, Copper and Potassium contents were determined by the Soil
Testing Laboratory at MSU. The differences of the amount of Potassium, Calcium and
Copper between the varieties was found to be significant. The difference of Ca, K and
Cu contents between the bruised and unbruised potatoes were insignificant in both

varieties. Therefore, the contribution of minerals on bruising was unclear.

169

The onset of bruising or Internal bruising can not be detected by pressing a
spherical indenter into a potato. The hook in the force-deformation curve indicates only
a major localized tissue failure and there was no indication of onset of internal tissue
failure found on the force deformation curve. The reason for this is, as the indenter
starts to load the potato, the contact area increases continuously and monotonotally
causing a steady increase in force to deform the tuber. When the stress excwds the
ultimate strength of the tuber, localized major tissue failure occurs. This is the failure
shown by the hook on the force-deformation curve. Finney (1963) defined this point as
rupture point and he stated that potatoes did not exhibit yield point during a spherical
indenter test because of above reason.

In order to gain an insight into the internal bruising it is essential to understand
internal stress distribution within the tuber. The finite elelment method was used to
study internal stress of a statically loaded potato. A commercial numerical program
called MARC was used to create the potato models. The potato tuber was assumed to
be a homogeneous body. The modulus of elasticity, Poisson ratio, density and yield
strength were used to define the models. The boundary nodes on the ordinates were
allowed displacement only in the direction of the load application and the nodes on the
abscissa were allowed to move only in the x- direction.

The top node was subjected to deformations of 10.16 mm (0.40 inch)and
10.66 mm (0.42 inch) to simulate Superior and Snowden potatoes, respectively. These
deformations were determined experimentally by loading potato statically by a spherical

indenter. The pressure sensitive film was used to measure the contact stress between the

170

 

 

 

 

spheric
Mises
illusm
conce'
defor
belov
The

ma

of

ps

in

spherical indenter and the tubers. The simulated distribution of the contact stress, von
Mises stress and deformation were illustrated by contour bands. The pictorial
illustrations show a high stress concentration directly under the applied load. The
concentrated stress was about 3 times higher than the yield stress. Therefore, from the
deformed model it can be assumed that tuber was bruised severely which extended far
below the surface at the stem end. This was confirmed by experimental observation.
The error of contact stress and deformation were within 2% of the experimentally
measured values.

The simulated contact stress corresponding to 5.08 mm (0.20 inch) was
1040.73 kPa (150.90 psi) which was higher than the dynamic bruising contact pressure
of the Superior variety. The corresponding von Mises stress was 957.96 kPa (138.90
psi) which was equal to the yield stress as found by core testing. This would result in
internal tissue failure and consequently would cause blackspot bruising in the region
where von Mises stress equals the yield strength. The location of such von Mises stress
was at 5.08 mm below the surface-the location of commonly occurring blackspot.

7.2 Conclusions

External and internal bruise can be detected by using appropriate strength of
tetrazolium chloride solution. Using of pressure sensitive film is a viable method of
measuring contact pressure directly. There appeared to be a threshold value of contact
pressure when bruising occured. These were 880.89 kPa for Snowden and 1027.11 kPa
for Superior variety, respectively. The threshold bruising pressure was affected by the

variety, making some varieties more bruise susceptible due to a lower threshold value,

171

and the mechanical properties of potato tubers. If the contact stress during impact does
not exceed the threshold value, bruising may not occur.

The hook in the force-deformation curve generated by a spherical indenter
actually indicates a major localized tissue failure. The internal tissue failure occurs well
before the major failure since it exceeds the ultimate strength. The internal tissue failure
can not be traced in the force-deformation curve.

Bruise susceptible tuber exhibited higher ppo activity than that of less susceptible
variety. Optical density (OD) was a direct result of ppo activity. Therefore, OD could
be used to determine bruise susceptibility of a given lot of potatoes. There were
differences between the variety but more study is needed to understand the role of
minerals (Ca, Cu and K) in blackspot bruising.

Finite Element Method could be used to investigate internal stress distribution
within a loaded tuber. The capability of FEM to produce pictorial illustration helped in
understanding the stress distribution in a loaded potato model. The von Mises stress
distribution showed the location of the highly stressed element where internal tissue
failure occured. The element failure occurs when von Mises stress in that element is
equal or higher than the yield strength of a potato. The location of bruised tissue may
not necessarily be on the surface of contact. Under a given loading condition it occured
below the surface of the tuber where von Mises stress equal the yield stress of potatoes.

The varietal differences may be detected using the techniques described in this
dissertation. The Snowden variety was found to be more blackspot bruise susceptible

than the Superior variety.

172

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188

Br
Mtkg)

0.164
0.201
0.281
0.259
0.215
0.222
0.251
0.185
0.262
0.203
0.200
0.169
0.256
0.238
0.207
0.282
0.196
0.271
0.299
0.207
0.175
0.25
0.2
0.176
0.16
0.15
0.3
0.177
0.277
0.196
0.186
0.188
0.167
0.211
0.159
0.154
0.242
0.222

Appendix A.1: Mechanical Properties of Bruised Superior Potatoes.

Br Br
Mgh(Nm) Dtmrn)

0.24519 65.03

0.3005 68 4
0.42011 76.88
0.38722 7347
0.32143 69.59

0.3319 71.16
0.37526 76.57
0.27658 67.49
0.97925 74.33
075873 67.93
0 74752 68.00
0.63166 65.18
0.95683 74.57
088955 74.27
0 77369 68.94
1.05401 79.33
073257 68.56
1.01289 75.41
1.11755 80.32
0.77369 68.77
0.65408 65.18
149504 77.3
1.19604 68.74
1.05251 65.36
0.95683 62.83
0.89703 71.14
1.79405 78.72
1.05849 65.69
1.65851 76.29
1.17211 69.17
1.11231 69.54
1.12427 65.83
0.99889 64.78
1.26182 68.94
0.95085 64.17
0.92095 63.17

1.4472 71.72

1.3276 72.00

Br
YldF(|b)
33.00
35.00
45.00
44.00
43.00
45.00
44.00
42.00
40.00
43.00
38.00
39.00
46.00
42.00
37.00
36.00
37.00
41.00
43.00
38.00
40.00
38.00
41.00
40.00
44.00
39.00
41.00
39.00
38.00
43.00
39.00
42.00
36.00
42.00
43.00
38.00
45.00
40.00

Br
Deflm)
0.29
0.27
0.34
0.33
0.32
0.32
0.33
0.33
0.31
0.33
0.30
0.30
0.34
0.31
0.29
0.30
0.30
0.32
0.31
0.28
0.29
0.34
0.36
0.35
0.34
0.29
0.31
0.34
0.33
0.31
0.33

Br

Br

Br

Senrgy YIdS(psi) E(Psi)

4.785
4.725
7.650
7.260
6.880
7.200
7.260
6.930
6.200
7.095
5.700
5.850
7.820
6.510
5.365
5 400
5.550
6.560
6.665
5.320
5.800
6.460
7.380
7.000
7.480
5.655
6.355
6.630
6.270
6.665
6.435
7.140
5.400
7.350
6.665
6.080
7.200
6.000

189

112.628
119.454
153.584
150.171
146.758
153.584
1 50.171
143.345
136.519
146.758
129.693
133.106
156.997
143.345
126.280
122.867
126.280
139.932
146.758
129.693
136.519
129.693
139.932
136.519
150.171
133.106
139.932
133.106
129.693
146.758
133.106
143.345
122.867
143.345
146.758
129.693
153.584
136.519

388.372
442.422
451.717
455.063
458.618
479.949
455.063
434.378
440.383
444.720
432.309
443.686
461.755
462.402
435 448
409.556
420.933
437.287
473.412
463.189
470.754
381.450
388.699
390.054
441.678
458.986
451.393
391.488
393.009
473.412
403.351
421.602
409.556
409.556
473.412
405.290
479.949
455.063

Br
E(kglcm" YId(kg/c

27.304
31 .1039
31 .7573
31.9926
32.2425
33 7422
31.9926
30.5384
30.9606
31.2655

30.393
31.1928
32.4631
32.5086
30.6136
28.7933
29.5931
30.7429
33.2826
32.5639
33.0958
26.8173

27.327
27.4222
31.0516
32.2684
31.7346

27.523

27.63
33.2826
28.357 1
29.6402
28.7933
28.7933
33.2826
28.4934
33.7422
31.9926

Br

7.91817
8.39805
10.7975
10.5576
10.3176
10.7975
10.5576
10.0777
9 59778
103176
9 11789
9.35783
11.0374
10.0777
8.87794

8.638
8.87794
9.83772
10.3176
9.11789
9.59778
9 11789
9.83772
9.59778
10.5576
9.35783
9.83772
9.35783
9 11789
10.3176
9.35783
10.0777

8.638
10.0777
10.3176
9 11789
10.7975
9.59778

Measur
DyP(kg/cu\'

9.10
10.25
10.99
10.89
10.50
10.99
10.79
10.25
10.50
1 1.10
10.53
10.55
1 1.25
10.25
10.16
9.98
9.88
10.50
1 1.60
10.45
1 0.59
9.67
10.00
10.02
1 1.00
10.50
1 1.49
9.89
9.50
10.80
9.76
10.76
9.80
10.30
12.34
9.50
1 1.20
10.49

Appendix A.2 Mechanical Preperties of Unbruised Superior Potatoes.

NbR Nbr Nbr NoBr NoBr NoBr

NoBr NoBr Meat NoBr
M(kg) mgh(nm) D(mm) F(lb) Def(tn) Stenergy YldS(psi) E(Psi) NoBrP YIds(kg/cz
0 kglcm‘Z

0.247 0.36928 66.9 53.00 0.37 9.805 180.887 488.885 10.10 12.7171
0.161 0.2407 63.02 44.00 0.33 7.260 150.171 455.063 9.35 10.5576
0.244 0.36479 72.77 49.00 0.34 8.330 167.235 491.869 9.70 11.7573
0.223 0.33339 63.9 43.00 0.31 6.665 146.758 473.412 9.38 10.3176
0.219 0.32741 71.72 49.00 0.39 9.555 167.235 428.809 9.80 11.7573
0.181 0.2706 67.89 44.00 0.33 7.260 150.171 455.063 9.50 10.5576
0.15 0.22426 75.42 40.00 0.31 6.200 136.519 440.383 8.20 9.59778
0.2 0.29901 68.07 46.00 0.36 8.280 156.997 436.102 10.15 11.0374
0.25 0.37376 65.45 48.00 0.34 8.160 163.823 481.831 10.00 11.5173
0.345 0.51579 67.9 52.00 0.36 9.360 177.474 492.984 10.50 12.4771
0.195 0.29153 68.17 45.00 0.33 7.425 153.584 465.405 9.00 10.7975
0.223 0.33339 71.41 45.00 0.33 7.425 153.584 465.405 9.25 10.7975
0.223 0.83349 69.79 46.00 0.34 7.820 156.997 461.755 9.00 11.0374
0.203 0.75873 69.64 41.00 0.32 6.560 139.932 437.287 8.00 9.83772
0.214 0.79985 71.85 39.00 0.30 5.850 133.106 443.686 8.50 9.35783
0.191 0.71388 76.9 38.00 0.29 5.510 129.693 447.217 8.00 9.11789
0.25 0.9344 69.87 39.00 0.29 6.890 133.106 458.986 8.65 9.35783
0.198 0.74005 77.9 41.00 0.32 6.560 139.932 437.287 8.35 9.83772
0.15 0.56064 75.89 39.00 0.30 5.850 133.106 443.686 8.20 9.35783
0.2 0.74752 64.17 49.00 0.34 8.330 167.235 491.869 10.20 11.7573
0.145 0.86713 78.48 45.00 0.35 7.875 153.584 438.810 8.00 10.7975
0.15 0.89703 78.27 39.00 0.29 5.655 133.106 458.986 8.50 9.35783
0.14 0.8372 69.22 40.00 0.30 6.000 136.519 455.063 8.15 9.59778

190

Appendix A.3 Mechanical Properties of Bruised Snowden Potatoes.

Br
Mrg‘.
239
277
275
229
229
205
207
221
204
245
244
242
220
215
237
214
166
218
189
189
257
204
234
203
199
197
207
233
210

Br
01mm)

Br
YldFtlbt
35

Br

08111111
0.28
0.22
0.25
0.26

Senrgy
(tn-lb)
4.900
2.970
3.750
4.030
4.455
5.075
4.030
4.000
4.930
4 455
4.590
3.105
4.590
3750
5 040
4.760
3.875
4.160
4.930
5.250
3.750
5.400
6.200
1.980
3.105
3.875
4.930
4.320
4.320
3.105
4.320
4.820
5.250
3.625

Yldstrss
P91
119 454
92.150
102.389
105.802
112.628
119 454
105.802
109.215
116 041
112.628
116.041
92.150
116.041
102.389
122.867
116.041
105.802
109.215
116.041
119.454
102.389
122.867
136.519
75.085
92.150
105.802
116.041
109.215
109.215
92.150
109.215
112.628
119.454
98.976

Etes:

P81
426.621
418.864
409.556
406.931
417.141
411.910
406.931
436.860
400 141
417.14 1
429.781
400.653
429.781
409.556
438.810
414.432
423.208
420.058
400.141
398.180
409.556
409.556
440.383
417.141
400.653
423.208
400.141
404.500
404.500
400.853
404.500
402.243
398.180
395.904

PE
(Nrm
0.38
0 41
0 41
0.34
0.40
0.34
0.31
0.39
0.30
1.1C
0 91
0.90
0.82
0.80
1.35
0.80
0.62
0.81
0.71
0.71
0.96
0.76
0.87
0.76
0.74
1.18
1.05
1.39
1.01
0.88
1.30
1.67
1.52
1.43

MeesP Ybs Elect
kglcm‘Z Kglcm‘2 11976192
8.90 939905 29.9931
7.20 64785 294477
7.50 719933 297933
9.00 743826 286088
9.25 791917 29.3265
9.00 9.39905 28.9588 '
8.20 743929 29.8099
9.90 787822 307129
9.50 915911 291314
9.00 791917 29.3265
9.90 815811 30.2152
9.75 64785 28.1674
950 915911 30.2152
9.80 719933 29 7933
11.25 9.939 30.95
9.75 815811 291361
9.80 743828 297531
9.94 787822 29.5316
9.50 815811 291314
9.50 9.39905 27.9935
9.90 7.19933 29.7933
9.80 8.638 29.7933
10.10 9 30.9606
7.20 5.27979 29.3285
7.75 64785 28.1874
9.90 7.43929 29.7531
9.10 9.15911 29.1314
9.50 7.67822 28.4379
9.90 7.67822 29.4379
8.10 6.4785 28.1874
8.80 7.67822 29.4379
9.50 7.91917 29.2792
9.50 9.39905 27.9935
9.20 6.95839 27.8336

Appendix A.3 (cont’d)

191

 

0.25

Appendix A.3 (cont’d)

5 250
5 580
4 420
3 750
4.320
4 455
4 125
4,590
3 480
4 320
4 930
3 900
1 890
4 930
3 600
4 000
3 750
3.360
3.105
4.030
3.360
2.970
3.875
3.750
3.220
3.625
3.750
3.840
3 450
4 420
3.600
5.510
3.220
4.420
3.600
3.750

119 454
122 867
116 041
102.389
109.215
112.628
112 628
116 041
98 976
109.215
116 041
102 389
71 672
116 041
102.389
109.215
102.389
95.563
92 150
105.802
95.563
92.150
105.802
102.389
95.563
98.976
102.389
109.215
102.389
116.041
102.389
129.693
95.563
116.041
102.389
102.389

192

398 180
396.345
445.311
409 556
404.500
417.141
450 512
429.781
412 400
404.500
400 141
393.804
398 180
400 141
426.621
436 860
409.556
398 180
400.653
406.931
398.180
418.864
423.208
409.556
415.492
395.904
409.556
455.063
445.170
446.311
426.621
447.217
415.492
446.311
426.621
409.556

0.99

10.00
9.30
9 80
8 80
8.50
9.50
10.00
11 00
9.50
10.25
10.35
7.50
6.50
8.50
8.25
10.00
7.90
7.10
7.50
8.00
7.60
7.90
7.75
8.50
8.20
9.50
9.45
10.00
9.24
10.50
10.10
10.45
9.30
10.00
9.50
9.80

8.39805
8 638
815811
7 19833
7.67822
7.91817
7.91817
8.15811
6.95839
7 67822
8.15811
7 19833
5 03883
8 15811
7 19833
7.67822
7.19833
6 71844
6 4785
7.43828
6 71844
6 4785
7 43828
7.19833
6.71844
6.95839
7.19833
7.67822
7.19833
8.15811
7.19833
9.11789
6.71844
8.15811
7.19833
7.19833

27 9935
27 8645
31.3773
29 7933
29 4379
29 3265
31 8727
30.2152
29 9933
29 4379
29 1:114
27 8959
279935
29 1314
29.9931
30 7129
29 7933
27.9935
29 1674
29 6088
27.9935
29 4477
29 7531
29.7933
29.2106 '
27.8336
29.7933
31.9928
31.2971
31.3773
29.9931

31.441
29.2108
31.3773
29.9931
29.7933

0.31
0.21
0 24
0.25
0.22
0.23
0.25
0.24
0.22
0.26
0.23
0.26
0.25
0.25
0 19
0.20
0.22
0.27
0.25
0.25
0 24
0.25
027

Appendix A.3 (end)

5.735
2.625
3 240
3 875
2.860
3 450
3.625
3 600
3.080
4.160
3.105
4.030
4 000
3 875
2.280
2 400
2.860
4.590
4 000
3.625
3 600
3.750
4.320

126.280
85.324
92 150
105.802
88.737
102.389
98.976
102.389
95.563
109.215
92.150
105 802
109.215
105.802
81.911
81.911
88.737
116 041
109.215
98.976
102.389
102.389
109.215

407.354
406.306
383 959
423.208
403.351
445.170
395.904
426.621
434.378
420.058
400.653
406.931
436.860
423.208
431.112
409.556
403.351
429.781
436.860
395.904
426.621
409.556
404.500

193

1.38
1.01
0 45
0.90
0.99
0 77
1.01
1.26
1.14
0.97
0.93
1.05
2.05
1 14
1.05
1.30
1.25
1.78
1.40
1.30
1.22
1 11
1.00

10.50
8 10
7.50
8.59
7.25
8.60
8.25
9.50

9.60
7.90
9.75
9 75
9.24
7.70
7.50
8.50
10.00
9.30
8.00
9.10

9.58

8 87794
5 99861

6 4785
7 43828
6.23855
7.19833
6.95839
7.19833
6 71844
7.67822

6 4785
7.43828
7 67822
7.43828
5 75867
5.75867
6.23855
8 15811
7.67822
6.95839
7.19833
7 19833
7.67822

28 6385
28 5648
26 9937
29 7531
28.3571
31.2971
27.8336
29.9931
30.5384
29.5316
28.1674
28 6088
30 7129
29.7531
30.3088
28.7933
28.3571
30.2152
30 7129
27.8336
29.9931
28.7933
28.4379

NBr
M19:
176
214
226
284
225
298
234
215
217
228
198
248
206
239
170
184
198
286
173
180
198
186
324
203
188
180
177
183
168
200

Appendix A.4

Nbr

01mm)

73
75
74
83
70
79
70
72
77
74
75
74
70
72
80
78
80
84
71
75
75
70
87
71
68
70
77
71
72
72

Nbr

YIdF

LD
44
36
35
43
47
34
38
35
35
43
33
39
42
35
36
37
38
41
34
39
33
38
35
31
34
40
40
44
30
35

Mechanical Properties of Unbruised Snowden Potatoes.

Nbr
Defun)
0.33
0.29
0 28
0 31
0 34
0.26
0 30
0.29
0.27
0.32
0.27
0 30
0.33
0.26
0.27
0.28
0.29
0.29
0.26
0.29
0.26
0.28
0.26
0.25
0.27
0.30
0.29
0.31
0.24
0.27

Nbr

Smgy

(In-lb)
7.26
5.22
4.90
6 67
7.99
4 42
5 7O
5 08
4 73
6.88
4 46
5 85
6.93
4.55
4.86
5 18
5.51
5.95
4.42
5.66
4.29
5.32
4.55
3.88
4.59
6.00
5.80
6.82
3.60
4.73

Nbr
Ystrss
PSI
150171
122.867
119454
146 758
160 410
116 041
129 693
119454
119454
146 758
112.628
133106
143345
119454
122.867
126.280
129.693
139.932
116.041
133.106
112.628
129.693
119.454
105.802
116.041
136.519
136.519
150.171
102.389
119.454

194

Nbr
Elasn
P51

455 063
423 679
426.621
473 412
471 793
446.311
432.309
411.910
442 422
458 618
417.141
443.686
434.378
459 438
455.063
451.000
447.217
482.523
446.311
458.986
433.185
463.189
459.438
423.208
429.781
455.063
470.754
484.421
426.621
442.422

Nbr

PE

( Nm)
0.29
045
0.34
0.42
0.34
0.45
0.35
0.32
0.32
0.34
0.74
0.93
0.77
0.89
0.20
0.25
0.90
0.43
0.26
0.30
0.30
0.28
0.48
0.30
0.28
0.67
0.66
0.68
1.00
0.87

NBr
YldS
Kglcm‘Z
10 5576
8 638
8 39805
10 3176
11.2774
8 15811
9 11789
8.39805
8.39805
10.3176
7.91817
9.35783
10.0777
8 39805
8.638

8 87794
9 11789
9.83772
8.15811
9.35783
7.91817
9.11789
8.39805
7.43828
8.15811
9.59778
9.59778
10.5576
7.19833
8.39805

N81
Elasl
kg/cm‘Z
31.9926
29 7862
29 9931
33.2826
33 1688
31.3773
30.393
28.9588
31 1039
32 2425
29.3265
31 1928
30.5384
32.3002
31.9926
31.7069
31.441
33.9232
31.3773
32.2684
30.4545
32.5639
32.3002
29.7531
30.2152
31.9926
33.0958
34.0566
29.9931
31.1039

NBrP

Ptess

kglcm‘Z
7.50
6 78
6.00
8.00
7.80
6 90
6.00
5.90
6.20
7.00
7.45
7.00
7.80
7.87
5.50
7.00
8.00
8.24
5.90
5.78
6.90
7.60
7.80
6.80
7.25
8.78
8.80
9.50
6.60
7.89

Appendix A.5

 

Figure A.5 Yielded cores of fresh Superior potato tubers.

 

Figure A.6 Load-deformation curve at yield of 9 Superior potato core.

195

Appendix B.l Calculated and Measured Quasi-Static Contact Pressure for Superior
potatoes.

3: radnus 01 contact
where 3‘3: [3'3 14'P(k1+k2)(R1°R2]/{4(R1+R2)}
k2 ot sphere as zero

Max Pr=1.5P/314 a Tmu eq.231 page 413

R2=sphore
R1=potatoe page 413
Calulated by Timoshenko FIImPr FIImPr
F(LB) mue1‘2 K1 R1 R2 3‘3 8 Pmaxmsn kglcm‘2 Gray Psu kglcm‘2 Damn)

124 0 2401 0.0006 1.56789 0 44 0 06066 0.39293 383.673 26 9718 99 377.758 26.5578 0 45
119 0.2401 0.0006 1.51234 0 44 0.05778 0.38659 380.362 26.739 99 45 375.157 26 3749 0 44
107 0 2401 0 00059 1.5748 0 44 0.05084 0 37047 372.427 26 1812 101 366.323 25 7539 0 4
126 0 2401 0 00063 1 4679 0 44 0.0633 039854 378.952 26 6399 99.26 376.253 26.452 0.48
118 0 2401 0 00059 1 47638 0 44 0 05593 0 38243 385 421 27 0946 98 66 379 733 26 6967 0 43
116 0 2401 0.00058 1.55512 0 44 0 05474 0.3797 384.368 27.0206 98 67 379.675 26 6926 0 42
115 0 2401 0.00059 1 5748 O 44 0.05471 0 37962 381.212 267987 98.68 379.617 26 6885 0 42
85 0 2401 0 00058 1 6789 0 44 0 04067 0 34389 343.356 24 1375 106 339 106 23 8404 0.34
95 0.2401 0.00062 1.5748 0 44 0.04746 036205 346.221 24.3389 105 67 340 844 239626 0.35
110 0.2401 0.00059 1 45901 0 44 0.05142 0.37187 379.995 26 7132 99 91 372.515 261892 0 41
112 0 2401 0.0006 1 49568 0 44 0.05377 037745 375.55 26 4007' 100.52 369.038 259447 0.42
124 0.2401 0 00062 1.50346 0 44 0.06151 0.39474 380.159 26.7247 99.15 376.889 264967 0.46
98 0.2401 0.00059 1.52346 0 44 0 04656 0.35977 361.693 25 4265 103 355.205 24 9722 0.38
85 0.2401 0.0006 1.79346 0 44 0 04238 0 34865 334.041 23.4827 108 328.751 231124 0.33
75 0.2401 0.00061 1.45669 0 44 0.03622 0.33086 327.288 23.008 110 318.69 224051 0.32
Avg 0.37197 367.648 25.8452 101.798 362.37 25 476 0 40333

Std 0.01899 19.2578 1.3538 3.66801 19.9321 1 4013 004756

196

PoMR11
1.58903
148977
1.5748
147896
147638
1.55512
1.5748
16789
189658
145901
149568
1 45368
152346
1.89765
178654

1.59536
0 14624

Palsson

0 49
0 49
0.49
0 49
0.49
0 49
0.49
0 49
0 49
0.49
0.49
0 49
0.49
0 49
0 49

0 49
1.7E-10

Appendix B.1 (end)

1IR1
0.62931
0.67125
0.635
0 67615
0.67733
0 64304
0.635
0 59563
0.52727
0.6854
0.66859
0.68791
0.6564
0.52697
0 55974

0.63167
0.05301

410

4.57143
4.57143
4.57143
4.57143
4.57143
4 57143
4.57143
4 57143
4.57143
4.57143
4.57143
4.57143
4.57143
4 57143
4.57143

4.57143
ERR

Cal-F 11mm

R1(00131

1.58903
148977

1.5748
147896
147638
155512

1.5748

16789
189658
145901
149568
145368
152346
189765
178654

1.59536
0 14624

1IR1‘
0.62931
0.67125
0.635
0.67615
0.67733
0 64304
0.635
0.59563
0.52727
0.6854
0.66859
0.68791
0.6564
0.52697
0.55974

0.63167
0.05301

Regressron Output:

Constant

Std Err of Y Est
R Squared

No. of Observations
Degrees of Freedom

X Coeffiaenfls)
Std Err 01 Coef

197

Sort

2 41455
2.43186

2 4169
2.43387
2.43436
2.42023

2 4169
2 40056
2.37191
2.43767
2 43076

2.4387
2.42574
2.37178
2.38556

2 41542
0.02204

-0.4822
0.30751
0.95977

13

1.00707
0.05718

165.772
164.542
170.688
157.809
167.452

171.17
170.503
173.025
165 448
169.093
166.055
160.396
168.834
170.305
167.203

167.22
3.97229

0.76

0.76
2.4E-10

 

M99 PR1.5PI314 0

Potatoes.
19110 80231 9998 413

“0 8‘3. [3'3 14'P181982XR1'R2W41R1OR2»

HdWOIM

WMTWO

Appendix B.2 Calculated and Measured Quasi-static Contact Pressure for Snowden

FhPr F”:

F110) M122 “3 R1 R2 .‘3 I Mug" I’m-"‘2 my 9‘. “amaz Mn)
90 0.2401 000058 140552 044 0.03507 0.32734 358.855 25.0724 105 344395 243123 0.42
90 02401 0.0006 145322 044 004325 0.35101 349949 245308 102 380.725 25.3803 048
99 02401 00008 1.1755 044 004042 0.3432 380.957 25.3749 104 349.782 24.8998 945
93 02401 000088 11890‘ 044 004855 0.35972 343.327 24.1354 102 380725 25.3803 991
8‘ 02401 000058 11697‘ 044 003921 033892 388339 25.7532 103 355205 249722 949
99 02401 000059 1.25929 044 004001 034203 383429 255495 103 355205 249722 947
79 “‘0‘ °°°°57 13"" °“ 003" 032618 35471 24.9357 104 349782 245998 0.39
93 02401 00008 121905 044 003792 033597 351.285 24.9935 104 349782 24.5998 944
90 02401 00006 1.25964 044 004115 034524 380718 25.3579 103 355205 24.9722 049
94 02401 000055 1.3283 044 003615 033088 387.012 25.9005 102 360.725 25.3903 943
91 02401 000084 135202 044 004542 035879 341491 24.0084 107 333992 234739 9.49
92 0.2401 000083 1.29287 044 004518 0.35812 349544 24.3918 108 339108 239404 95
80 02401 000077 199784 044 003987 033918 250.958 17.8209 129 24032 199954 9.32
98 02401 000057 145984 044 003934 03401 355177 24.9995 104 349782 245998 944
92 02401 000059 12744: 044 003849 033187 359095 25033 104 3‘97“ "a“ 041
75 02401 000054 199788 044 003433 0.32502 339154 23.9421 109 329 751 231124 939
95 02401 000059 1.28973 044 00379 0.3359 359979 25299 104 349782 24.5998 945
6‘ 02401 000053 1.29 044 00249 029202 341712 240219 105 344.396 24.2123 9.35
90 02401 00008 1.39394 044 004288 034941 352.151 24.7557 105 344.398 24.2123 947
70 02401 000057 1.99077 044 003349 0.3223 321.923 22.9309 111 313755 21059 0.35
90 02401 000057 134193 044 004035 034299 385473 25.9923 102 380725 25.3803 945
70 02401 000053 189564 044 003092 031395 339471 23.9844 107 333992 234739 9.38
95 0.2401 000059 151493 044 0.0393 0.33999 351.273 24.9941 104 349.782 24.5998 943
85 02401 000087 199785 044 003841 0.33148 292.831 19.9998 120 272.572 19.1929 933
79 02401 000054 1.32005 044 0.03321 0.32145 395.23 25.9752 103 35.1205 24.9722 941
93 0.2401 000055 1.31072 044 003558 032998 389.831 25.7737 103 355.205 24.9722 943
99 02401 000059 139925 944 004041 034315 381057 25.3919 103 355.295 24.9722 01.5
90 02401 000084 134912 044 004522 035828 339742 238132 109 329751 231124 0.49
80 02401 000081 1.38951 044 0.02992 0.30895 304.211 21.3857 114 299424 210506 0.54

”‘7‘” “‘0‘ 000059 ”1013 044 0.039 0.33554 1452‘ 241721 108.139 339.522 239898 042821

997728 ERR 4.9805 0.2319 1.7E-10 000495 001509 257913 191245595517 25.90? 199949 994991

mm 99189811 1811 4741 711188191 1011' 8811 14181":
140552 049 071149 4.57143 1.40552 071149 244834 177.818 0.78
145322 049 089913 4.57143 1.45322 0.89913 2.43879 184178 0.78
1.1755 049 0.8507 4.57143 1.1755 0.9507 2.50456 160.424 0.78
1.19807 049 084099 4.57143 1.18907 0.84099 2.50089 14818 0.78
1.1807? 049 085497 4.57143 1.18977 0.95497 2.50822 185.57 0.78
115829 049 07941 4.57143 1.25029 0.7941 248188 188.878 0.78
119417 049 0.7727 4.57143 119417 0.7727 247322 172179 0.78
1.21905 049 0.92099 4.57143 1.21805 0.82099 249287 181.778 0.78
115984 049 079375 4.57143 115984 0.79375 2.48172 183.825 0.78
1.3283 049 075399 4.57143 1.3263 0.75399 248584 177.471 0.78
1.35102 049 0.73983 4.57143 1.35202 0.78983 2.45992 154.129 0.78
110187 049 0.77359 4.57143 110287 0.77359 2.47359 154.084 0.78
1.9U75 049 0.50333 4.57143 1.98875 0.50333 2.3819 133.183 0.78
1.45984 049 0.8851 457143 145984 0.8851 2.43755 172.989 0.78
1.27442 049 078487 4.57143 1.27442 0.78487 247808 189.003 0.78
1.58789 049 0.8379 4.57143 1.58789 0.8378 241806 183.822 0.78
118873 049 0.79919 4.57143 118873 078819 247949 188.446 0.78

119 049 0.77519 4.57143 1.29 077519 247423 185.153 0.78

1.38804 049 0.73317 4.57143 1.38594 0.73317 245719 182.819 0.78
1.88077 049 0.52899 4.57143 1.99077 0.52899 2.37259 179.808 0.78
1.34193 049 074519 4.57143 1.34193 074519 248209 171.119 0.78
1.80984 049 0.52753 4.57143 1.89584 0.52753 2.37202 194.307 0.78
1.51483 049 0.98014 4.57143 1.51493 0.88014 2.42729 173.177 0.78
1.45822 049 0.89913 4.57143 1.45322 0.88813 2.43879 140.017 0.78
1.52085 049 0.75709 4.57143 1.32095 0.75709 2.4889 181.39 0.78
1.31072 049 078294 4.57143 1.31072 0.78294 2.48927 177.471 0.78
1.38025 049 0.71991 4.57143 1.38925 071991 2.45175 171.119 0.78
1.34812 049 074177 4.57143 1.34812 074177 248089 152.853 0.78
1.58897 049 0.63736 4.57143 1.58897 0.83736 241798 182.847 0.78
140142 049 0.72859 457143 140142 0.72959 245423 167.356 0.78
0.20575 24810 0.0999 ERR 0.20575 0.0999 0.03691 12.7418 0

198

Appendix C.1 Calculated and Measured Dynamic Contact Pressure for Superior

Potatoes.
Horsfield(1972)
P max: 0.899(Wh)‘(1l5) (1lR)“(3/5)(E“0.8)
calculabd
(1IR) (1IR)*.6 Elas(E) (E‘0.80) Wh (wh)“.2 Ptmax) F(max) Elas(E)
P81 P81 Kg/cm‘2 Kglcrn‘Z
0.68455 0.7966 488.45 141.598 3.2604 1.26664 128.586 9.04009 34.3388
0.80605 0 87866 439.00 130.008 2.1252 1.16273 119.54 8.40409 30.8633
0.698 0 80596 479.92 139.816 3.2208 1.26355 127.962 8.99624 33.7402
0.70497 0.81078 454.66 133.706 2.9436 1.24101 121.079 8.51233 31.9643
0.70826 0.81304 464.14 135.931 2.8908 1.23653 122.993 8.64685 32.6308
0.74823 084027 473.00 138.003 2.3892 1.19028 124.223 8.73335 33.2537
0.67355 0 7889 450.00 132.608 3.696 1.29881 122.287 8.59726 31.6387
0 74626 0 83895 454.67 133.707 2 4288 1.1942 120.562 8 47595 31.9648
0.75164 0.84257 497.68 143.733 2.1384 1.16417 126.889 8.92075 34.9885
0.70916 081367 435.72 129.231 2.9304 1.2399 117.339 8.24936 30.6327
0 74523 0.83825 481.41 139.963 2.574 1.20815 127.57 8.96867 33.8448
0.71133 0.81516 454.67 133.707 2.9436 1.24101 121.735 8.55844 31.9648
0.78117 086228 331.82 103.824 2.1648 1.16703 94.1223 6.61714 23.3278
0 74264 0.8365 480.34 139.715 2.6532 1.2155 127.852 8.98845 33.77
0.66075 0.77987 451.72 133.013 3.7092 1.29974 121.344 8.53091 31.7576
0.69142 0.80139 455.06 133.8 3.4188 1.27872 123.401 8.67553 31.8824
0.72992 0.82787 458.62 134.836 2.838 1.23198 123.587 8.6886 32.2427
0.71385 0.81689 511.84 147.02 2.8304 1.2399 134.019 9.42202 35.8813
0.66342 0.78176 455.06 133.8 3.3132 1.27072 119.825 8.4101 31.8828
0.75263 0.84324 434.38 128.812 2.442 1.1955 116.959 8.22267 30.5384
0.7793 0.86104 438.80 129.511 5.577 1.4102 141.53 9.95011 30.7157
0.68122 079428 440.00 130.245 8.448 1.53232 142.668 10.0301 30.8338
0.68391 0.79616 443.30 131.026 7.854 1.51014 141.781 8.96776 31.1856
0.64036 0.76534 479.53 139.525 9.306 1.56226 150.142 10.5555 33.7127
0.63243 0.75964 540.89 153.834 8.867 1.58065 166.025 1 1.8722 38.0288
0.72782 0.82844 474.86 1 38.46 7.359 1.48061 153.513 10.7825 33.3815

199

0 72941
0 70695
0 74778
0.72697
0 77723
0 68271
0 68339
0 74778
0 79159
0 73681
0 74094
0 67359
0 73866
0 77935
0 714

0 64727
0 64905
0 77328
0 6658
0 67659
0 78419
0 79154
0.73383
0 65716
0 73909
0 77722
0.8084
0 64526
0.73438
0 73049
0 77167
0.73691
0.80422
0.70823

0 82753
0 81214
0 83997
0 82587
0 85967
0 79532
0 7958
0 83997
0 86916
0.83256
0 83535
0 78893
0 83381
0.86107
0 817

0 77029
0 77155
0 85704
0 78344
0 79103
0 86428
0.86913
083053
077733
0.8341
0.85966
0 88019
0 76885
0.83091
0.82826
0.85597
0 83262
0.87745
0.81303

500 84
443.30
423.31
466.03
511 50
477 40
435.06
470.34
431.93
436 90
473 00
462 78
470.34
431.26
436.90
477 40
506 89
487 14
436 48
539.00
501 47
461.35
400.00
473.00
418 94
465.00
457.91
495.00
511.50
477 40
459.00
404.94
479.53
454.67

Appendix C. 1 (End)

144 464
131.026
126.278
136.374
146 919
139 029
129 074
137.382
128.331
129 511
138.003
135.612
137.382
128171
129.511
139 029
145.858
141.294
129 411
153.204
144 609
135.278
120 684
138.003
125.234
136.133

13447
143.115
146.919
139.029
134.726

121.874

139.525
133.707

6 699

7 062
6.303
7.326

6 534

9 108

8 646

6 699

5 214

6 831

6 468

8 943

6 831
5.775
13 4112
17.2128
15.3648
9 3456
14.6256
13 1472
8.8176
8.3952
11.0352
15.3648
10.5600
9.2928
8 448
16.2624
10.3488
9.8208
10.5

111408
8.1312
12.7776

200

146286
147838
144514
148927
145558
1.55555
153944
146286
1.39134
146858
145263
154987
146858
142007
168071
176673
172705
1.56358

17101
1.67404

15455

15304
161643
172705
160226
1.56181
153232
174677

15958
1.57917
160043

1.61951
1.52066
1.66452

157.394
141.586
137.957
150.959
165 457
154.801
142 314
151.928
139 672
142.515
150 715
149.236
151.403
141.054
160.052
170 283
174.922
170409
156 043
182.589
173.845
161.942
145.815

166.74
150.632
164 499
163.228
172.984
175.327
163.661
166.108

147.905
167.552
162.852
avg
Std

11.0654
9.95399
9 69888
10.613
11.6322
10.8831
10.0052
10.6811
9 81944
10.0193
10.5958
104919
10.6442
9 91659
11.2523
11 9715
12 51
11 9804
11.50
12.8367
12.2219
11.3851
10.2513
11.7224
10.59
11.5649
114755
12.1614
12.3261
11.7012
11.678

10.3983
11 7795
11 4491
10.2234
1.38602

35.2109
31.1656
29.7603
32.7636
35.9603

33.563
30.5863
33.0666
30.3665
30.7157
33.2537
32.5352
33.0666
30.3192
30 7157

33.563
35.6362

34.248
30.6862
37.8937
35.2553
32.4348
28.1215
33.2537

29.453
32.6912
32.1928
34.8003
35.9603

33.563
32.2694

28.4686
33.7128
31.9648

11.0654
9.95399
9.69888
10.613
11.6322
10.8831
10.0052
10.6811
9 81944
10.0193
10.5958
10 4919
10.6442
9.91659
11.2523
11 9715
12.2977
11.9804
10.9704
12.8367
12.2219
11.3851
10.2513
11.7224
10.59
11.5649
11 4755
12 1614
12.3261
11.506
11.678

10.3983
11.7795
11 4491
10.2078

1.3706

Appendix D.l
Table D.1 Mineral Contents of Superior and Snowden Potatoes.

Variety: Superior
Date of harvest: September 17, 1993.

Bruised at 6 inch drop height

Cu(ppm) Ca(%) K(%)
9.70 0.05 2.04
10.40 0.08 2.43
9.65 0.07 2.14
8.70 0.06 1.78
8.60 0.05 1.74
9.50 0.08 2.24
12.05 0.04 1.81
8.90 0.06 1.84

Unbruised at 6 inch drop height

7.70 0.05 1.97
7.65 0.06 2.35
9.70 0.06 2.03
6.95 0.05 2.54
6.60 0.05 1.89
6.90 0.05 1.93
6.15 0.06 1.93
6.45 0.07 1.83
9.15 0.04 1.81
8.95 0.06 1.91
8.70 0.06 1.86
7.35 0.04 1.77

Bruised at 15 in drop height

9.50 0.05 2.03
11.75 0.06 2.32
8.75 0.06 2.16
13.10 0.07 2.37
9.15 0.07 2.39
10.15 0.06 2.12
9.55 0.05 2.17
7.40 0.05 2.13

201

6.25
7.65
9.10
7.35
10.55

0.05
0.04
0.04
0.04
0.05

1.95
2.02
1.89
1.83
2.22

Unbruised at 15 inch drop height

7.50
13.00
10.15
8.10
7.70
9.90
5.50

0.05
0.06
0.04
0.05
0.06
0.06
0.05

2.05
1.89
2.20
2.06
2.12
2.25
2.10

Bruised at 24 inch drop height

9.40
7.35
7.75
8.90
7.75
8.90
9.55
8.15
8.90
9.35
6.25
12.05
8.45
9.80
7.10
7.50

0.05
0.06
0.06
0.04
0.05
0.05
0.07
0.06
0.06
0.05
0.06
0.06
0.06
0.07
0.05
0.05

2.25
2.19
2.29
2.16
1.57
2.10
2.13
2.27
2.15
2.31
2.12
2.20
2.13
2.09
2.53
2.32

Unbruised at 24 inch drop height

6.10
6.75
8.20
8.55

0.06
0.06
0.06
0.06

202

2.23
2.13
2.09
2.11

Variety: Superior
Date of harvest: September 30’1993.

Bruised at 6 inch drop height

Cu(ppm) Ca(%) K(%)
10.95 0.05 2.26
9.55 0.06 2.18
10.25 0.05 2.33
11.00 0.05 2.47

Unbruised at 6 inch drop height

9.85 0.06 2.04
8.10 0.04 2.21
10.65 0.06 2.34
7.15 0.06 2.36
12.10 0.06 2.52
10.40 0.06 2.19

Bruised (all) at 15 inch drop height

11.20 0.05 2.20
10.10 0.04 2.46
7.40 0.04 2.22
7.55 0.04 2.34
9.40 0.05 2.34
12.15 0.05 2.30
9.20 0.04 2.21
6.70 0.05 2.13
8.20 0.06 2.30
11.15 0.06 2.45

Bruised at 24 inch drop height

7.65 0.09 2.46
10.95 0.07 2.39
9.40 0.06 2.16
10.20 0.06 2.22
11.15 0.07 2.62

203

9.85 0.07 2.18

9.10 0.08 2.67
11.55 0.06 2.2

12.75 0.07 2.72
8.75 0.06 2.42

Variety: Snowden
Date of harvest: September 24, 1993.

Bruised at 6 inch drop height

5.10 0.03 1.55
6.15 0.03 2.06
6.10 0.04 1.96
6.55 0.04 1.74
5.45 0.03 1.83
7.45 0.04 1.86
4.10 0.03 1.87
5.05 0.02 1.82
4.75 0.02 1.75
5.85 0.03 1.77

Unbruised at 6 inch drop height

4.85 0.04 1.79
5.00 0.03 1.68
6.50 0.05 1.98
5.40 0.04 1.73
4.20 0.03 1.84
5.40 0.03 1.72
6.05 0.03 1.72
6.05 0.04 1.92
7.20 0.05 1.84
6.95 0.04 1.82

Bruised at 15 inch drop height

6.95 0.04 1.61
8.45 0.04 1.65
8.55 0.05 1.82
10.55 0.04 1.54
8.70 0.05 1.67
7.70 0.04 1.85

204

7.50
8.10
8.60
9.05
8.50
10.20
8.50
8.80
7.45

0.04
0.04
0.07
0.04
0.05
0.05
0.05
0.05
0.04

1.69
1.74
1.81
1.98
1.81
2.07
1.62
1.64
1.93

Unbruised at 15 inch drop height

7.30
5.35
7.00
7.70
6.70

0.03
0.03
0.04
0.04
0.03

1.59
1.56
1.82
1.66
1.53

Bruised at 24 inch drop height

9.35
6.80
8.40
12.40
8.60
6.45
7.70
10.30
5.70
5.45
9.40
11.05
9.45
8.35
9.45
10.00

0.05
0.04
0.04
0.03
0.04
0.04
0.04
0.05
0.04
0.04
0.06
0.04
0.04
0.04
0.05
0.05

1.68
1.57
1.44
1.65
1.44
1.45
1.60
1.78
1.55
1.92
1.45
1.84
1.87
1.75
1.97
1.67

Unbruised at 24 in drop height

9.25
9.40
8.70
7.75

0.05
0.04
0.05
0.05

205

Variety: Snowden
Date of harvest: October 22, 1993.

Bruised at 6 inch drop height

Cu(ppm) Ca(%) K(%)
3.30 0.01 1.51
3.15 0.02 1.57
3.15 0.02 1.67
2.40 0.02 1.77
4.55 0.02 1.96
4.25 0.03 1.91
2.60 0.02 1.69
2.95 0.01 2.09
3.30 0.03 1.75
2.70 0.03 1.81

unbruised at 6 in drop height

2.50 0.02 1.64
1.80 0.02 1.92
2.75 0.02 1.68
2.65 0.01 1.75
2.60 0.01 1.64
2.70 0.01 1.65
2.50 0.02 1.66
3.05 0.01 2.09
2.90 0.02 1.38
4.00 0.02 1.62

Bruised at 15 inch drop height

4.20 0.02 1.67
3.25 0.02 1.80
3.80 0.02 1.66
3.50 0.02 1.66
3.30 0.01 1.73
3.35 0.02 1.80
3.60 0.01 1.59
4.45 0.02 1.88
6.75 0.02 1.93
3.75 0.01 1.92
4.35 0.02 1.60

'§

3.75
4.05
2.85
4.25

0.01
0.02
0.01
0.01

1.90
1.60
2.05
1.70

Unbruised at 15 inch drop height

4.05
2.95
3.70
6.65

0.01
0.02
0.02
0.01

1.68

Bruised at 24 inch drop height

9.35
3.55
4.60
5.25
3.20
5.56
5.55
4.75
3.80
6.50
3.65
9.65
7.05
4.50
5.80
4.10
5.90

0.03
0.02
0.02
0.04
0.02
0.01
0.01
0.03
0.02
0.04
0.02
0.04
0.04
0.02
0.03
0.02
0.03

1.65
1.86
1.94
2.27
1.86
2.03
1.61
2.0

2.14
1.96
2.19
2.02
1.83
1.70
1.93
1.67
1.88

Unbruised at 24 inch drop height

5.50
4.95

0.04
0.03

2.15
1.84

207

Appendix D.2

Table D.2 t-test for the differences of mineral contents of Superior and
Snowden varieties (a=0.05).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Mineral Significance t-values Probability
Calcium "' 4.83 0.00
Copper * 11.42 0.0
Calcium * 14.27 0.0
Appendix D.3
Table D.3 t- test for the differences of the bruised and unbruised tubers impacted at
various drop heights (a=0.05).
Date in Minerals 6 inch 15 inch 24 inch Variety
1993
Sept 17 Calcium ns(1.5) ns(0.72) ns(0.62) Superior
Sept30 Calcium ns(0.53) *(5 .7) all bruised Superior
Sept 24 Calcium ns(1.40) ns(1.58) ns(0.64) Snowden
Oct 22 Calcium ns(1.05) ns(1.02) ns(-1.31) Snowden
Sept 17 Potassium ns(-0.08) ns(0.37) ns(0.35) Superior
Sept30 Potassium ns(O. 34) ns(-1 .84) all bruised Superior
Sept 24 Potassium ns(0.32) ns(1.74) ns(0.93) Snowden
Oct 22 Potassium ns(0.85) ns(0.31) ns(0.50) Snowden
Sept 17 Copper *(3.79) ns(0.42) ns(1.6) Superior
Sept30 Copper ns(0.76) ns(-1.09) all bruised Superior
Sept 24 Copper ns(. 18) *(3.15) ns(0.09) Snowden
Oct 22 Copper ns(1.75) ns(0.49) ns(0.04) Snowden

 

 

 

 

 

 

 

 

Note: Value in the parentheses is t- value and “ is test for significance.

208

Appendix 12.]

Fuji film pressure verification
Superior Potato Core Tests

 

 

 

 

1E
14- ‘
13~ R-=0.97
12
5 11‘ a
510 1"
h 9. It ,7, "‘
E & r
‘5
32 3
a.
33 5‘
g 4‘
3‘
2-
1.4
10 12 14 16 18 20 22 24 26
Yield ForceGCg)
A C“ I! WEI)
Figure 13.1 Compressive Pressure Verification for Superior Potato Core

Specimen Using Pressure Sensitive Films During Uniaxial
Tests.

209

 

Appendix 13.2

Pressure-Deformation trend in Snowden
l’otnto tubers. 1993

400
370‘

 

I)

310~ ’7‘
280‘
250-
220~
190-
160*
130~

l 00 - T T I I
0.3 0.35 0.4 0.45 0.5 0.55 0.6
Deformationfin)

Measured Contact Pressure (Psi)

 

 

 

A W K menuhfll

Figure 13.2 Pressure-Deformation Trend on Whole Snowden Potato Tubers
(sperical indenter).

210

Appendix F.l

Simulation of force-deformation.
Snowden whole Potato tubers.l993.

 

lltrfi

Force (lb)
i

40"

20"

 

 

 

o 0.05 o. 1 Ft 5 0:2 0:25 03 0.35 014 0.45
Deformation (in)

— FEM made! * Expt-rum

Figure F.l Simulation of Force-Deformation for Snowden Whole Potato Tubers.

211

Appendix F.2

5.487e-03
_ 4.706.432
__ 1951.-02

4222.01 \g

 

 

.. 4.547.411
f, 4.073.431
.-‘. -2.‘98.-°1

313: 4.924601

 

4.349.411

> 47750-01

42000-01

 

 

Figure F.2 The deformed model and its deformation contour lines due to 0.42
inch imposed displacement (Snowden). The scale shown on the left
is in inch.

212

Appendix F.3

2.9170001

4.1450000

4.5460001

 

6.2780001

 

w 4.20:..02

  
        
 
 

§ 4.5740002

   
   
 
 

x§ otg®\> I “ .

. k. «A \ \‘\\\

1.947.412 WNW%“ ‘\--
m§§ r m‘. II -

77 2.3200002

  

 

4.6940002

-3.0670602

{1.4400002 Y

 

 

Figure F .3 The resulted contour stress distribution within the model due to 0.42 inch
of imposed displacement (Snowden). The scale on the left shows stress in
psi.

213

Appendix F.4

__ 2.7830902
__ 2.5100002

_ 2.2380002

 

 

1 .9650902
‘ 1.6920402
.: 1.420.412

1.147.602

  
  
 

4‘ 8.7410001

 

 

5:51 6.01 4.4-01

:6 3.2870001

5.6040000

 

Figure FA The corresponding von Mises contour stress distribution in the model due
to 0.42 inch of imposed displacement (Snowden). The scale on the left
shows stress in psi.

214

Appendix F.5

__ 1.7360-03

__ 4.8440412

,_. 43610-02

 

_7 43780-02

 

3 63960-02
14139-02
f; -8.931o-02
10450-01

:1 4.1970411

 

-.? 43480-01

45000-01

 

 

Figure F.5 The deformed model with the contour deformation due to imposed
displacement of 0.15 inch (Superior). The scale on the left shows
deformation in inch.

215

__ 2.0289900
_ 4.307on1

._. ~2.81 60901

 

4.3250001

 

: -S.8350+01
3.3440901
a": 08540401
4.036.432
,Egi 4.1870902
...5. 4.3380412

- -1 .4 890002

 

 

Appendix F.6

i \
\~ ‘ “
\\\‘.
\ "
‘.
\ \ ‘

.\\_\\ H .. ' '* ......
.............

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure E6 The resulting contour stress distribution in the model due to the imposed
displacement of 0.15 inch (Superior). The scale on the left shows stress in

216

Appendix E7

1 .0580+02
9.5390411
8.5000401

7.4629001

 

6.4230001
I . 5.3840001
r: 4.345.411

. _. 3.3060001

 

5: 2.267.901

1.2290001

1.8980400

 

 

Figure R7 The corresponding von Mises contour stresses with bands within the model
due to 0.15 inch of imposed displacement. The scale on the left shows
stress in psi.

217

Appendix G.1 Stepwise Analysis of Variance of BS (Bruise susceptibility)

SI'EPWISE AMIXSIS OF VARIANCE OF 89

 

mom C11". GIVULA'I'IVE ”Um m‘ WW' 5
SCXJRCE SS 13? SS 16 R-SQJARED CP P
(131817563? 1093.9
CP 8.63768-01 1 8.6376E-01 8.6376E-01 -0.0006 1320.6 2
E 3.5812 2 4.4450 2.2225 0.0215 279.8
OD 105.94 '3 110.318 36.795 0.9021 15.6
PPO 1.0508 4 111.44 27.859 0.9102 5.0
12351111211. 10.625 131 122.06 9.3176E-01
(255$ 1331111051) 132 MISSINS {PSI-‘5 0
DEGREES OF FREEIDd 12']
013321111; F 333.0 P VALUE 0.0000
W R SQUARE!) 0.9102
R SQUARE!) 0.9129
REED. MEAN SQUARE 8.3668—02

218

Appendix G.2 Unwighted least square linear regression of BS (Bruise susceptibility)

WEIGH‘ED LEAST SQUARES LINEAR REGRESSION 01? BS

PREDICIUR

VARIABLES @EFFICEINI‘ STD ERROR S'IUDENT'S T P
CIDDE'IANI’ ~1.2228E-01 9.8934E-02 -1.24 0.2188
CP -2.7922E-06 3.0978E-06 -0.90 0.3691
E - 1.2776E-08 9 .7676E-09 - 1 .31 0 . 1933
CD 5 . 0348 4 . 0024E-01 1.2 .58 0 . 0000
PPO l . 0951E-01 3 . 0901E-02 3 . 54 0 . 0006
CASES INCLUDED 132 MISSIIG @515 0

DEGREES OF FREEIIN 127

OVERALL F 333.0 P VALUE 0.0000

ADJUSTED R SQIARED 0.9102
R SQUARED 0.9129

RE’SID. MEAN SQIARE 8.366E-02

219

"‘11111111111111S