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31293100642580 ., LIBRARY ‘

Michigan State
University

 

This is to certify that the
thesis entitled
CHEMICAL STABILITY OF FREEZE—DRIED
APPLE SLICES (var. SCHONE VAN BOSKOOP)
AT VARIOUS WATER ACTIVITIES

presented by
Poh—Lean P. Chan

has been accepted towards fulfillment
of the requirements for

M.S. dpgfiminFood Science

(TEQMWQ‘ C. C/RQMAW

Major professor

 

Date Outta,» m, WM _

0-7639

CHEMICAL STABILITY OF FREEZE-DRIED APPLE SLICES
(var. SCHONE VAN BOSKOOP) AT VARIOUS WATER ACTIVITIES

By
Poh-Lean P. Chan

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements

for the degree of

MASTER OF SCIENCE

Department of Food Science and Human Nutrition

1979

ABSTRACT

By
Poh-Lean P. Chan

"Schone van Boskoop” apple slices were freeze-dried to
a final moisture content of 4.30% on dry basis. During
storage the cells of freeze-dried slices were sensitive to
humidity. Accordingly, chemical stability was studied in
relation to water activity.

The freeze-dried slices were placed in desiccators of
saturated salt solutions of known water activities. A
sorption isotherm was prepared. Ascorbic acid, phenolics,
volatiles and texture were measured.

There was a correlation between ascorbic acid and total
phenolics retention.

l. At aw = 0.22 the drop of total phenolics caused
a drop in reduced ascorbic acid.

2. At aw = 0.3 to 0.66 reduced ascorbic acid was
degraded and the total phenolics changed very little.

3. At aw = 0.7 the total phenolics decreased and
browning began because ascorbic acid was no longer effective
as an antioxidant.

The best storage was at aw $0.24. Also volatile com—
ponents had the best retention; texture, flavor and color

were good.

 

Dedicated to my late parents

ii

ACKNOWLEDGMENTS

I wish to express my appreciation and gratitude to
Drs. Ramesh C. Chandan and Paul Tobback, Department of Food
Preservation, Catholic University of Leuven, Belgium, for
their encouragement and guidance in my graduate work.

I also wish to thank Drs. J.R. Brunner, C.M. Stine
(Department of Food Science and Human Nutrition) and
Dr. H.A. Lillevik (Department of Biochemistry) for their
advice and effort in reading this manuscript and for being
on my committee.

My special thanks to Dr. J.N. Cash for his editorial
help and assenting to be on my committee at a short notice.

Thanks are also due to Dr. M. Feys and the staff of
Department of Food Preservation, Catholic University of
Leuven, Belgium for their technical assistance while I was
doing my thesis.

Appreciation is extended to Drs. & Mrs. T.I. Hedrick
for their help during my years in East Lansing.

Finally, I wish to express my gratitude to my husband,
Toon, for his constant aid and encouragement during the

course of this study.

iii

 

TABLE OF CONTENTS

LIST OF TABLES.
LIST OF FIGURES
INTRODUCTION.
LITERATURE REVIEW

Dehydration of Fruits.
Advantages of Freeze- Drying. .
Utilization of Dehydrated Fruits . .
Effect of Water Activity on Chemical and
Physical Stability . . . .
Ascorbic Acid.
Total Phenolics.
Total Volatiles.
Texture. .

EXPERIMENTAL PROCEDURE.

Preparation of Freeze- dried Apple Slices
Analytical Procedures. . .

Moisture Sorption Isotherm

Moisture . . . .

Water Activity

Ascorbic Acid.

Total Phenolics.

Total Volatiles.

Texture. .

Organic Acids and. Sugars

CDNCNU‘l-DwN—J

RESULTS AND DISCUSSION.

The Sorption Isotherm of Freeze-dried Apple
Slices . . .
The Effect of. Water Activity and Time on
Ascorbic Acid of Freeze- dried Apple Slices
The Effect of Water Activity and Time on
Total Phenolics of Freeze-dried Apple
Slices . . ,
The Effect of Water Activity and Time on
Total Volatiles of Freeze- dried Apple
Slices . . . . . . . . .

iv

Page
vi

vii

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38

 

Page
The Effect of Water Activity on Texture of

Freeze- dried Apple Slices. . . . . . . . . 41
SUMMARY AND CONCLUSIONS . . . . . . . . . . . . . . 48
BIBLIOGRAPHY. . . . . . . . . . . . . . . . . . . . 5T

 

Table

LIST OF TABLES

Sugars and non-volatile acids of ‘Schone van
Boskoop' apples.

Chemical composition of freeze-dried apple
slices before equilibration. . . . .

Sorption data.

Comparative analysis of total volatiles of fresh
and freeze-dried apples.

Comparative analysis of total volatiles of

freeze-dried apple slices equilibrated at
different water activities .

vi

Page

27

28
29

39

42

 

LIST OF FIGURES

Figure

1

5a

5b

The relationship of moisture sorption isotherm

curve of freeze-dried apple slices with percent
retention of ascorbic acid and total phenolics

(as tannic acid) after two weeks equilibration

at 25°C.

Percent of ascorbic acid retained at aw = 0.22 -
0.24; aw = 0.43 - 0.45; aw = 0.6l - 0.70 with
storage time . . . . . . . . . . . . . . . . .

Percent of total phenolics (as tannic acid)
retained at aw = 0.22 — 0.24; aw = 0.43 - 0.45;
aw = 0.6l - 0.7l with storage time . . .

Total volatiles of fresh apples and freeze-dried
apples stored at aw = 0.22 - 0.24 at 20 - 250C
for 41 days. . . . . . . . . . . . . . . .

Textural profile of freeze-dried apple slices at
aw = 0.l8 and aw = 0.27 for 2 weeks at 25°C.

Textural profile of freeze-dried apple slicgs at
aw = 0.38 and aw = 0.60 for two weeks at 25 C.

Texture of freeze-dried apple slices after two

weeks equilibration at the different water
activities at 250C . . . . . . . .

vii

Page

.3l

34

37

4O

44

47

INTRODUCTION

The densest apple growing areas in Belgium are situated
in the southern parts of Limburg. About 70% of the total
apple production is Golden Delicious, but "Shone van
Boskoop", the second most important variety, takes 20% of
the total production.

Even though there is an over-production of apples in
Belgium, there have been fairly large quantities of apples
imported from other countries (France, South Africa, Italy)
during the recent past, which pushes the price of apples
grown in Belgium down to a very low level. The overall
quality of the Golden Delicious grown in Belgium is not
optimal so that this variety is not competitive with Golden
Delicious imported from other countries. For this reason
apple growers want to increase the production of other
varieties adapted to the local agricultural conditions.
"Shone van Boskoop" is one variety giving high production
with good quality and this cultivar is good as a table
fruit as well as for cooking. Along with the increased
production of Boskoop, there is also a demand for new
possibilities for processing of the fruit. The production
of freeze-dried apple slices could be one of the possibili-

ties.

 

Freeze-dried apple slices are materials of a physio—
logical system, constantly undergoing changes. During
storage the cells are sensitive to such external influences
as temperature, humidity and oxygen, as well as the inter-
play of internal factors which are difficult to control.
The parameters of water content, equilibrium relative
humidity and water activity may be used to monitor the
chemical stability. Water is generally the major constitu-
ent of foods. Its concentration influences the palata-
bility, digestion, transportation and handling of various
foods. However, mobility of the water expressed in term
of water activity present in foods is involved in their
deteriorative processes and in the stability of dehydrated
food products. Since water activity is a useful expression
of water requirement for chemical and enzymatic stability,
the variables studied here are water activity related.

Model systems have Often been used by investigators
to determine the effect of water activity on chemical
stability (Acker, l962; Labuza, l973; Kirk et al., l977).
The primary Objective of this study was to determine the
effect of water activity and time on a real system -
freeze-dried apple slices. After determining the sorption
isotherm at 250C for the freeze-dried apple slices, the
changes in ascorbic acid, total phenolics and volatiles
were studied. The textural index was used as a measure of

the hygrosc0picity of the apple slices.

 

LITERATURE REVIEW

Dehydration of Fruits

Drying of fruits is an age-old practice dating back
to biblical times. Sun-drying was the method used from
those early times and it is regularly used now for drying
raisins, apricots and peaches. In the latter part Of the
nineteenth century attempts were made to use other sources
of heat for dehydration of fruits.

Apparently "evaporation" of apples was developed in
the early years of the twentieth century in western New
York State, using a one-kiln evaporator, which was eventually
followed by other types of evaporators (Van Arsdel and
Copley, l964). Low moisture fruit products like apple
nuggets, with less than three per cent moisture, were pro-
duced commercially by vacuum-shelf drying with a procedure
involving reduced pressure and temperatures ranging from
100 to 140°F.

With the advent of vacuum pumps and refrigeration
machines, freeze-drying has been made possible. Up to the
time of World War II freeze-drying had been regarded as an
occasional scientific tool but the opportunity for large—

scale use had not arisen. Flosdorf (l949) envisaged its

use for foodstuffs and other bulky materials but only
within the last two or three decades has freeze-drying
been developed and used extensively as a dehydration opera-
tion in food technology. The process has been applied to

fruits like apple and peach slices.

Advantages of Freeze—Drying

Theoretically, freeze-drying could be the best tech-
nique for preservation of foods if freezing behavior of the
material to be processed is studied carefully in order to
prevent drastic alteration during the course Of freeze-
drying. In other words, every given product has to be
processed in its own particular way and therefore the
process of freeze-drying has to be Optimized. Other criti-
cal steps Of the process include the technique of freezing
the material, as well as close1control of the sublimation
phase (Rey and Bastian, l962).

The major advantages of freeze-drying are structural
integrity, retention of volatiles, reduced browning and
preservation of vitamin activity as well as other properties
like good organoleptic qualities and good rehydration.

Shrinkage in freeze-drying is usually much less than
in ordinary drying because dehydration is achieved at low
temperatures. At these temperatures, the mobility of
interstitial concentrate is low, so that little change

occurs to the porous structure left by the subliming ice.

 

Flavor retention is much better in freeze-dried foods
than in other dehydrated foods, in spite of the high
volatility of the aroma constituents and the fact that
processing is done under vacuum. Thijssen and associates
(l968, l979) found that the diffusion coefficients of
volatiles and water dropped markedly as dissolved solid
contents increase, but that the diffusion coefficients for
volatiles drop to a considerably greater extent. Thus
above a certain dissolved solids content, water is removed
by diffusion at a significant rate, but other volatile
compounds are not. Flink, Karel, and associates (Flink
and Karel, l970a,b, l972; Kayaert et al., l975) have inter-
preted volatiles retention through the concept of micro-
region entrapment, in which volatile compounds are immo-
bilized within cages formed by association of molecules of
dissolved solids, such as by hydrogen bonding of carbo-
hydrate molecules. The degree of immobilization would then
relate to the water content, freezing conditions and other
variables.

The low temperature employed for dehydration in freeze-
dried foods helps to reduce browning, to preserve vitamin
activity and other nutritional properties of the food due
to the presence of given biological active compounds. This
low temperature process will prevent non-enzymatic browning
which causes loss of protein biological value. It will also

prevent formation of peroxides produced during lipid

 

oxidation with proteins or vitamins. Thus organoleptic
quality is not impaired.

Eisenhardt et al. (l968) compared the porosity and
rehydration of apples that were freeze-dried, air-dried
and explosion-puffed. They found that freeze-dried pieces
were most rapid in water uptake and this was related to
their highest amount of surface-connected pores. Freeze-
dried apple slices could be rapidly rehydrated compared to
the extremely slow rehydration rates of conventionally
dried pieces. Explosive-puffing greatly improved the

rehydration rates of conventionally dried pieces.

Utilization of Dehydrated Fruits

Dehydrated fruits are utilized mainly in bakery pro-
ducts, in cooked "sauces" and eating as a snack. As a
snack, apple slices prepared by freeze-drying and explosion-
puffed have the advantage of a crisp texture compared to
conventional air-dried slices. The crispy texture of the
dehydrated apple slices would lend itself readily for use
as a snack item as the banana chips in the tropical coun-
tries. Sonido et al. (l977) studied the equilibrium
relative humidity relationships of banana chips as a
systematic approach to the prOper selection of packaging
material for the shelf-life of banana chips. The shelf-
life of apple chips can likewise be determined by knowing

the moisture sorption isotherm and the effects of different

water activities on the chemical and physical properties.

Effect on Water Activity on Chemical and Physical Stability

Ascorbic Acid

 

Most of the work related to the influence Of a on the

w
nutritional composition of foods has been concerned with
ascorbic acid. The results have shown that this vitamin is
relatively stable at low aw levels.

Karel and Nickerson (l964), Jensen (l967), Vojnovich
and Pfeifer (1970) and Lee and Labuza (l975) have studied
the stability of reduced ascorbic acid in various low and
intermediate moisture foods and model systems as a function
of moisture content, water activity and storage temperature.
Results reported by these investigators showed that the rate
of destruction of reduced ascorbic acid increased as the
total moisture content and water activity increased. Lee
and Labuza (l975) have interpreted the increase in destruc-
tion rates to be the result Of dilution Of the aqueous
phase which resulted in a decreased viscosity and thus
increased mobility of reactants. But above a certain
moisture content, the aqueous phase viscosity would not be
expected to change very much so the rate of destruction
would approach a constant value. There is a greater loss
in the desorption systems than in the adsorption systems.

Kinetics data by Lee and Labuza (l975) and Kirk et al.

(l977) reported that the loss of total and reduced ascorbic

.—.—..--.:-——-

 

acid followed the first order reaction. They found that

the activation energies for a equal to or above the mono-

w
layer moisture content (aw 20.24) were not significantly
different for destruction Of ascorbic acid. Thus the
mechanism of the oxidation of ascorbic acid was not con-
sidered to be changed as aw increased. But Kirk et al.

(1977) reported that the activation energies at a of 0.l0

w
for total ascorbic acid and reduced ascorbic acid to be
significantly different from the activation energies for

the other aw levels. These activation energies were inter-
preted to describe a possible change in the mechanism of
ascorbic acid destruction under these storage conditions.
Although the greatest stability for total ascorbic acid was
observed on model system at low storage temperature and

water activity below 20°C and 0.24 respectively, Kirk et al.
(l977) implicated from their experimental rate constants

that dissolved oxygen content was a primary factor in the
storage stability Of ascorbic acid in dehydrated low and
intermediate moisture foods of neutral pH. Karel and
Nickerson (I964) reported that all the absorbed water,
however, including that adsorbed in a monolayer, appeared

to be available for the reactions resulting in destruction

of ascorbic acid in orange crystals. Their results suggested
that reduction of moisture content to the lowest possible
content was necessary for prevention Of ascorbic acid

losses and that oxygen had little effect on the destruction

of ascorbic acid in orange crystals. This indicates that
ascorbic acid decomposition may proceed at and below the
BET monolayer.

The preservation of ascorbic acid in foods for extended
periods, therefore, requires that the food be stored at low

temperatures and at low a levels as are practicable.

W

Total Phenolics

Enzymatic discoloration of fruits is due to the action
of a copper-containing enzyme complex on phenolic substances
in the tissues of fruit. Walker (l962) reported that the
major browning substrate of apple was chlorogenic acid.

Only a few phenolic compounds besides chlorogenic acid and
l-epicatechin, have been identified as apple polyphenol
oxidase substrates. Shannon and Pratt (T967) found that
esculetin and dihydroquercetin to be substrates for apple
polyphenol oxidase.

Acker et al. (l967a) studied a polyphenol oxidase
preparation from potatoes, using cathechol as the substrate.
The oxidation was followed by measuring transmittance-color
change of the mixtures. They found that as aw levels
increasedthere was a corresponding decrease in percent
transmittance. They concluded that water serves both as a
medium for the enzyme reaction and as a vehicle for the
substrate.

Optimal and minimal aw levels for the activity of

polyphenol oxidases have not been determined in model

 

l0

systems or in foods (Troller and Christian, l978). Acker
(1962) cited a number Of findings indicating that this
class of enzymes is relatively inactive in dried food
products in the 5 to l2% moisture range. Generalizations
on the effects of aw limitation on enzyme-substrate reac-
tions are hazardous because reaction rates also depend on

the degree of enzyme binding to the substrate and the

nature of the substrate.

Total Volatiles

 

Many measurements have been made of volatiles reten-
tion as a function of time during rehumidification of
freezeédried substances (Flink, l969; Flink and Karel,
l970a, l972; Chirife et al., l973). Results typically show
that rehumidification at relative humidities below 30%
gives very little volatiles loss, yet volatiles loss is
substantially complete over a few hours at relative humi-
dities above 70%. Rehumidification at moderate relative
humidities can give substantial but not total volatiles
loss, with the rate loss becoming very small and perhaps
zero after an initial period of more rapid loss.

Two basic mechanisms have been proposed for the
interpretation Of volatiles loss and retention. One is
based upon selective diffusion analysis (Menting et al.,
l970b; Chandrasekaran and King, l972b). The other is the

concept of microregion entrapment in which volatile

 

11

compounds are immobilized within cages formed by associa-
tion of molecules of dissolved solids, such as by hydrogen
bonding of carbohydrate molecules (Flink and Karel, 1970a,

b, 1972; Chirife and Karel, 1973b, 1974a,b; Chirife et al.,
1973; Kayaert et al., 1975). Volatiles loss during rehu-
midification occurs more slowly than does water uptake.

Flink and Karel (l970a,b, 1972) found that the seeming
asymptotic retention of flavor volatiles at longer times for
moderate relative humidities can be attributed to break-up
of some, but not all, of the micro-region cage bonds,
releasing some of the molecules of the volatiles material

but keeping other molecules Of the same compound immobilized.
Chirife and Karel (1974a,b) further showed that at a constant
moisture content in the freeze-dried solid, the loss Of
volatiles would not occur unless some critical temperature
was exceeded. These authors have implicated "collapse"

(i.e. loss of structure) as the determining factor in the
release of encapsulated materials during rehumidification or
exposure to high temperatures.

The selective diffusion and micro-region entrapment
concepts are macro and microscale interpretations of the
general phenomenon of volatiles retention in freeze-dried
foods (King, 1971; Flink, 1974). The selective diffusion
approach is a potentially quantitative mathematically for-
mulated model based on transport concepts, while the micro-

region entrapment approach is more qualitative and is based

12

upon molecular concepts (Omatete and King, 1978). They
investigated the relationship of the rate characteristics

of "collapse" to the rate characteristics of volatiles

loss through the selective diffusion concept in terms of

the influences of these variables on the dimensionless
Fourier group Dt/LZ. Their results showed that with the
resulting increase in volatiles loss, there was an increased
value of Fourier group.

To and Flink (1978) through other observations of the
state of the freeze-dried materials, studied the state of
the freeze-dried materials after "collapse" from a practical
standpoint. They concluded that it was probably not
"collapse" per se which caused the loss of entrapment
ability but rather some additional changes in the structure
Of the material. They then related the "collapse" and
recrystallization behavior to volatiles loss and found that
recrystallization of matrix solutes following "collapse"
was a major cause of loss of entrapment ability in the dry
state during rehumidification or exposure to temperatures

above critical levels.

Texture
Texture of food is very important in its acceptance
and rejection. Evaluation of the effect of relative humi-

dity or a level has met with the difficulties of objec-

W

tively quantifying texture. In spite of the limitations of

l3

textural measurements, a number of studies have shown
convincingly that the moisture condition of dried and semi-
dried foods plays an important role in determining texture.

Kapsalis (1967) found that increases in equilibrium
relative humidity of intermediate moisture foods above the
BET monolayer of water (20% R.H.) to a moisture level of
66% produced increases in both hardness and cohesiveness
of most samples.

Heldman et al. (1972) used the Instron Universal
Testing Machine to determine hardness and chewiness of pre-
cooked and freeze-dried beef. They found that the hardness
and chewiness of the samples increased as the relative
humidity was increased until a moisture content in the 40
to 50% R.H. range was reached. Further increases in mois-
ture decreased both hardness and chewiness. Results for
these parameters were similar at 80% and 0% R.H.

Sonido et a1. (1977) related the effect of moisture
level to the crispiness of fried banana chips via the use
of organoleptical test. For plain banana chips they found

that at moisture levels of 0.78 to 1.37% (a $0.25), the

w
chips were very crispy; at 2.2% (aw <O.38) - criSpy; at
3.1% (aw <O.5) slightly crispy; at 0.45 to 0.65% (aw <O.7) -

not crispy; and at 9.1% (a 20.7) - wet.

w
Rheological scientists and instrument manufacturers

have developed machines to provide data on such parameters

as toughness, crispiness, bioyield strength, etc. According

14

to Troller and Christian (1978) the most meaningful data
relating moisture to texture have been drawn from studies
with dried and semidried meat.

A practical approach is to observe the sample during
the deformation test. This direct sample observation can
provide much insight into the interpretation of texture

measurements. The influence of moisture content or a on

w
the perception of food texture is obvious. Chemical reac-
tions, hydrogen bonding, colloidal aggregation and changes
in hydration among different sorptive groups are some of

the possible mechanisms for structural changes in freeze-

dried materials during rehumidification.

EXPERIMENTAL PROCEDURE

Preparation of Freeze-dried Apple Slices

Apples of the variety "Schone van Boskoop" were sliced
in 2 mm thichnesses with an electrical cutter. The untreated
2 mm slices were cored, frozen in liquid nitrogen for 15
secs and freeze-dried for 24 hours in a "Secfroid"a freeze-

dryer under the following conditions.

Start Finish
Platen temperature - 15°C 300C
Chamber vacuum 0.2 Torr 0.05 Torr
Condenser temperature - 600C - 80°C
Conditioner temperature - 30°C 20°C
Vacuum pump O.l Torr 0.05 Torr

Analytical Procedures

 

Fresh apples were tested for organic acids, sugars and
total volatiles. After freeze-drying, samples were tested
for moisture, ascorbic acid, total phenolics and total
volatiles. The freeze-dried apple slices after two weeks of
equilibration in the different water activities at 25°C were
measured for moisture sorption isotherm, ascorbic acid, total

phenolics and texture. For extended storage another lot of

 

aFreeze-dryer - Secfroid 1024 CH Ecublens - Lausanne, Suisse.

15

 

16

samples were stored at aw = 0.22 - 0.24, 0.43 - 0.45, 0.61 -
0.70 at 20 - 25°C and ascorbic acid, total phenolics and

total volatiles were measured.

Moisture Sorption Isotherm

 

The moisture adsorption isotherm at 25°C of freeze-
dried apple slices was determined gravimetrically by
exposing the samples to atmospheres of known relative
humidities.

Saturated salt solutions were used to obtain standards
of prescribed water activity levels as reported by Rockland
(1960) and Gall (1967). Separate desiccators were used for
the different equilibration salt solutions. Freeze-dried
apple slices were then allowed to equilibrate in the salt
atmospheres for two weeks. The moisture content of the
equilibrated samples was determined by Karl Fischer titra-
tion. Water activity of the samples was measured by an

electrolytic hygrometer.

Moisture
The chemical method of Karl Fischer titration was
employed (A.O.A.C., 1975). Material used were:
Kark Fischer Reagent (Merck)
Water-free Methanol (Merck)
Sodium Tartrate Dihydrate (Merck)
The Karl Fischer reagent was standardized against Sodium

Tartrate Dihydrate which has 15.66% water. A blank was

 

17

determined against the solvent water-free methanol. A
ZOO-mg sample of freeze-dried apple slices was (Hspersed

in lO-ml of water-free methanol and electrometric titration
was carried out. The presence of excess iodine, detected
potentiometrically indicated the end-point of the reaction.
Triplicates were done on each sample. Results were reported

as percent of moisture on 100 g dry weight.

Water Activity

 

In the Sina-scope apparatus6 the electrolyte is
Lithium Chloride solution. The operation of this instru-
ment requires that an alternating current be passed through
this saturated Lithium Chloride solution which is suspended
onto an inert carrier. The measuring cell is made up of
two resistances sensitive to relative humidity. The resis-
tances are connected in a bridge with one resistance as a
reference and the other is brought in contact with the
relative humidity Of the sample to be measured. Heat is
applied to the cell and eventually the water vapor pressure
(w.v.p.) of the Lithium Chloride solution is equal to the
w.v.p. of the sample. The direct reading of water activity

can be read from the dial indicator.

Ascorbic Acid

 

The ascorbic acid level in the freeze-dried apple

slices was evaluated according to the A.O.A.C. method (1975).

 

aSina Instrument, Zurich, Switzerland. AG type: SMT - 0, Nr.
7-0113.

 

18

Ascorbic acid was extracted from 2-3 9 sample with a
metaphosphoric acid - acetic acid solution and titrated with
2, 6, dichloroindophenol sodium salt solution till a faint
pink persisted for 30 sec. Results were reported as mg
ascorbic acid per 100 g of dry weight so that the ascorbic
acid levels retained are comparable at the different water

activities.

Total Phenolics

 

The method for extraction of total phenolics in the
presence of reducing agents according to Khanna et al.
(1968) was used.

a. Acetone Extraction in the Cold

Four to five gram samples of freeze-dried apple slices
were extracted in a Sorvall mixer with 25 ml of acetone
containing cysteine chloride as a reducing agent. Twenty-
five m1 of 50% acetone was used for each of the first two
extractions. The residual solid was extracted twice with
two 25 m1 aliquots of anhydrous acetone (cysteine chloride
was not added). Finally the residue and container were
rinsed with acetone. The total volume of the extract was
about 140 m1 and contained 4 x 10'6 moles of cysteine
chloride.

Total phenolics content in every extract was determined
in suitably diluted samples, with the Folin-Denis reagent

using tannic acid as standard (Goldstein, 1963).

19

b. Determination of Total Phenolics as Tannic Acid
Five milliliters of the total extract were diluted to
100 ml with distilled water. To 2 ml of the diluted sample
solution 2 m1 of 10% Folin-Denis reagent (complex phos-
photungstomolybdate) were added followed 3 min later by
2 ml of l N sodium carbonate. The absorbance was read 20
min later against a blank at 725 nm wavelength. The con-
centration of cysteine chloride at this stage was about
4 x 10'8 moles which was sufficient enough to prevent oxi-
dation in the sample solution but not enough to give a
color to Folin-Denis reagent. From the standard curve for
tannic acid, total phenolics were reported as mg tannic

acid per 100 g dry matter.

Total Volatiles

 

A simple method for the analysis Of apple volatiles
was developed by Feys et a1. (1977). This is a modified
vacuum cold trap distillation in which the aqueous solution
of volatiles is ready for direct GLC analysis.

a. Preparation of Sample

Thirty grams of freeze-dried slices were homogenized
in 270 ml of distilled water and 15 m1 of 4% TCA (tri-
chloracetic acid) in a Sorvall mixer. The TCA prevented
the formation of volatile aldehydes during homogenization
of the apple tissue. Two milliliters of antifoaming agent

were added to the mixture after blending.

20

b. Vacuum Cold Trap Distillation

The jar containing the apple pulp was gently heated to
prevent cooling as a result of solvent evaporation under
vacuum. A stream of nitrogen (15 ml/min) was passed
through the apple pulp. Between the jar with the apple pulp
and the cold traps with liquid nitrogen, a cooling spiral
was placed, cooled to -l7°C by means of a cryostat (Haake,
KT 33) with methanol as cooling liquid. This was to trap
and condense the water. Finally, the whole system was
connected to a vacuum pump (Cenco Hyvac 7). The mean
volume of the distillate was 16 ml i 4 m1 and the distilla-
tion time was one hour.

c. GLC Analysis

This was carried out using a Hewlett-Packard 5720 A
instrument with flame ionization detector, connected to a
Hewlett-Packard 3380 A integrator. The column used for the
identification of the volatiles was a 20% D.E.G.S. on
chromosorb P-AW, 60-80 mesh, 20' x 1/8”, SS. Column tem-
peratures were programmed at 90 and 110°C, with a N2 flow

of 9 ml/min.

Texture

The Universal Texture Machine Wolpert Amsler, Machine
TZM with a maximum force of 10 KN (kilo-newton) was used to
measure the texture of freeze-dried apple slices after

their equilibration at the different relative humidities.

 

21

The 2 mm thick apple slices were placed over a hollow
base with a diameter of 8.5 mm which was centered under a
100 N load consisting of an 8 mm diameter puncher. With a
load speed of 25 mm/min the puncher ruptured or deformed the
apple slice. Energy required to rupture or deform the
slices was recorded on a lO-volt full scale chart with a
chart speed of 200 mm/min. Readings for each sample were
obtained in triplicate. The area under the curve up to the
bioyield point was measured with a planimeter in cmz. The
bioyield point is a point on the force-deformation curve at
which there occurs an increase in deformation with a decrease
or no change in force. This area (cmz) is directly related
to the energy of force required to rupture or deform the

apple slice. The greater the area the greater the energy

or force required to rupture or deform the apple slices.

Organic Acids and Sugars

 

The method of Heatherbell (1974) was used for the
separation, identification and quantitative analysis of
fruit sugars and non-volatile organic acids.

a. Extraction and Separation of Fruit Sugars and Acids

Twenty five grams of apple were blended for five min
in a Sorvall mixer with 150 ml of 80% ethanol. To a 5 ml
aliquot in a 50 ml centrifuge tube were added 10 mg of
tartaric acid, 1 ml of saturated neutral lead acetate

solution and 10 m1 Of 85% ethanol. After mixing thoroughly

 

22

the extract was allowed to stand for 45 min and centri-
fuged. The supernatant contained the sugars, the remaining
precipitate contained the acids as lead salts. The tar-
taric acid which was added was the internal standard for
the organic acids. The precipitate containing the acids
was then washed twice with 80% ethanol and once with di-
ethylether. The residual ether was evaporated before
oven-drying. The dried precipitate was powdered with a
spatula and transferred to a vial for silylation. Inositol
(0.1 m1 of 0.2% w/v) was added to the supernatant contain-
ing the sugars as an internal standard. The supernatant
was then partitioned between 120 ml of chloroform -
methanol - water, 8:423 (v/v). Only trace amounts of
sugars were detected in the lower methanol - chloroform
layer and this was therefore discarded. The methanol -
water layer was made up to 100 ml with 80% ethanol. A 1 ml
aliquot was used for silylation.

b. Conversion to Trimethylsilyl Derivatives

One milliliter of pyridine, 0.5 ml M.S.T.F.A. (N-
methyl-N-trimethylsilyl-trif1uoroacetamide), 0.2 m1 T.M.C.S.
(chlortrimethylsilan) were used. The total contents were
heated at 80°C for 30 min.

c. GLC Analysis

The trimethylsilyl derivatives were analyzed on a

Varian 1800 gas chromatograph, with a 1.5% SE-30 + 1.5%

 

 

23

SE - 52 U- AW DMCS column, 80 - 100 mesh, 3 m x 3 mm, with

a temperature program from 100 to 230°C (4°C/min).

RESULTS AND DISCUSSION

The Sorption Isotherm of Freeze-dried Apple Slices

The equilibrium moisture content at different relative
humidities with their standard deviations are presented in
Table 3. These data were used in plotting the moisture
sorption curve shown in Figure 1.

The sorption isotherm of a food material is best
described as a plot of the amount Of water adsorbed as a
function of the relative humidity or activity of the vapor

space surrounding the material at a constant temperature.

 

7 Relative Humidity

' ' - : E. :
Act1v1ty - a Po 100

W

where P = water vapor pressure exerted by the food material

P0 vapor pressure of pure water at temperature T0

T0 equilibrium temperature of system

There are five types of adsorption isotherm in the
classification of Brunauer, Deming, Deming and Teller
(BDDT), also known as the Brunauer, Emmet and Teller (BET)
classification (Brunauer et al., 1935). The sorption

isotherm of a typical food material is sigmoidal in shape

and is classified as type II in the BET classification.

24

25

This general sorption isotherm has 3 regions as
discussed by Labuza (1968). Thus, in theory, the course
of water sorption by a dry material is first by the forma-
tion of a monolayer, followed by multilayer adsorption
and condensation in pores (capillary effects), followed by
dissolution of soluble components. These 3 regions may
overlap in the sorption isotherm and will differ in
magnitude among foods depending upon chemical composition
and structure.

The effect of composition of a food on the shape of
the sorption isotherm has been studied by Salwin (1963),
who found that for starchy foods like potatoes, beans,
corn and flour, the sorption isotherm has a sigmoidal
shape. These starchy foods have the greatest water holding
capacity in the low relative-humidity region.

Protein foods - meat, fowl, eggs, fish and milk have
a lower water-holding capacity. Beef has approximately
75% protein 20% fat and only 1% carbohydrate (on a dry
basis) and its moisture sorption properties change with
temperature. As the temperature increases from 40 to
lOOOF, the isotherm becomes progressively less sigmoidal
and more convex toward the vapor-pressure axis.

Green peppers or group of foods that have high sugar
content as well as high-molecular weight constituents have
moisture sorption curves sigmoidal in shape at 40°F. But

at higher temperatures the solution effects of the sugars

 

26

predominate and the curve becomes more convex toward the
vapor-pressure axis, except at very low vapor pressures.

The peach is a typical high-sugar item, with 67% sugar,
25% other carbohydrate and 4% protein and has a sorption
curve which is entirely convex toward the vapor-pressure
axis, which is typical of high-sugar items. Silvia and
Chirife (1979) also came to the same conclusion for freeze-
dried apple slices. “Boskoop" apples have about 77%
sugar(refer to Table l)and the relationship between water
activity and equilibrium moisture content at 25°C is illus-
trated in Figure 1. It has the characteristic shape of
high-sugar food isotherms. But Karl and Nickerson (1964)
found that the water sorption for orange crystals to have
a sigmoidal shape and that this corresponded to the data on
adsorption of water by sugars and pectin, which are also
the major components of the orange crystals. It seems then
that not only chemical composition but the structure of
the dried materials has a bearing on the sorption isotherm
curve.

The convex-shaped sorption isotherm of high-sugar item
like freeze-dried apple slices, corresponds to the type III
BET classification. The sigmoidal-shaped sorption isotherm
which is typical of starchy products is type II in the BET
classification. And by applying the BET theory Of monolayer
adsorption which is the safe moisture limit, Salwin (1963)

found the monolayer moisture value for these products to be

 

 

27

Table 1. Sugars and non-volatile acids of 'Schone van
Boskoop' Apples

 

 

Sugars g/lOO g.d.wt. Acids g/lOO g.d.wt.
Fructose 36.42 Phosphoric 0.12
Glucose 18.09 Maleic 8.18
Sacdharose 23.09 Citric 0.25
Ascorbic 0.10
Quinic 0.28

 

28

Table 2. Chemical composition of freeze-dried apple slices
before equilibration

 

 

Components Amount

Aw , 0.22

Moisture 4.30% d.wt. ‘
Ascorbic Acid 33.65 mg/lOO d.wt.
Total Phenolics 1620.90 mg/lOO

as mg Tannic Acid g.d.wt.

 

 

29

Table 3. Sorption curve data

 

 

Aw % Moisture on Standard Deviation
Dry We1ght
0.14 0.98 0.013
0.21 2.40 0.680
0.26 5.00 0.035
0.32 7.75 0.084
0.43 10.75 0.007
0.53 14.25 0.148
0.64 22.25 0.145
0.74 34.80 0.205
0.76 42.50 0.705
0.81 49.50 0.401
0.86 70.50 0.030

 

 

30

approximately 6% (approximately 15% RH). For freeze-dried
apple slices the moisture content is approximately 1.0% at
15% RH. The moisture content exerts a high vapor pressure,
even at very low percentages, and is available for deterio-
rative reactions. The BET equation is then not applicable
for the type III BET classification. Complete dehydration
is then essential for good stability in freeze-dried

"Boskoop" apple slices.

The Effect Of a“ and Time on Ascorbic Acid of Freeze-dried

 

Apple Slices

 

The percent of ascorbic acid retained in the freeze-
dried apple slices after 2 weeks of storage at 25°C for
the different aw is shown in Figure l. The maximum reten-
tion seemed to be at aw = 0.19 and 0.31 with the minimum
retention at aw = 0.66. The dip in percent of ascorbic
acid retention at aw = 0.23 could be due to other factors
besides aw. The exact route of ascorbic acid degradation
is highly variable and dependent upon the particular system.
Some factors which can influence the nature of the degrada-
tion mechanism besides water activity include sugar concen-
tration, enzymes, initial concentration of ascorbic acid,
and the ratio of ascorbic acid to dehydroascorbic acid
(Fennema, 1976).

Destruction of ascorbic acid as a function of water

activity has been studied based upon both kinetic and

 

96 of Ascorbic Acid and

Total Phenolics (as Tannic Acid) Retained

31

 

 

 

 

100 ‘T «100
t '* t
30.. 1‘80
6° 4r “'60
A
404* ‘ 040
20 a. I. 4’20
0 t ‘ t % e s + L :
0'1 0:3 (15 ()7
A w
Figure l. The relationship of moisture sorption isotherm
curve of freeze dried apple slices A; with
percent retention of Ascorbic Acid 0; and total
Phenolics (as tannic acid)fi'after two weeks

equilibration at 250C.

96 moisture on dry weight

 

 

 

physicochemical measurements. Many Of these studies have
been conducted in model systems with a pH less than 2 or

in high concentrations of organic acids and therefore may
not duplicate the exact pattern in a particular ascorbic

acid—containing food product.

It seemed from Figure 1 that at aw = 0.23 the limiting
factor for ascorbic acid destruction is not only aw and
moisture content. It may well be the initial concentration
of ascorbic acid or the ratio of ascorbic acid to dehydro—
ascorbic acid. Ascorbic acid exists, as the reduced form
and oxidized form (dehydrO-ascorbic acid). The oxidized
form is readily reduced to ascorbic acid and thus has
essentially the same activity as ascorbic acid. The
analytical procedure used here did not take into account
the dehydro-ascorbic acid.

At aw 0.31 to 0.66 the percent loss of ascorbic acid
increases with increasing water activity and moisture con-
tent. This effect is due to an increase in water content
which increases the mobility of the reaction species in the
aqueous phase which also becomes significantly less viscous
and diffusion is enhanced (Lee and Labuza, 1975). Also
phenolases can oxidize ascorbic acids in the presence of
an appropriate substrate (Joslyn & Ponting, 1951).

Above a critical moisture level, as in this case aw =

0.66, water may act to dilute the concentration of ascorbic

acid, thereby reducing the percent loss of ascorbic acid as

 

33

can be seen in Figure l for aw = 0.74.

The time effect during storage at 20 - 25°C for the
different water activities on the percent of ascorbic acid
retained is shown in Figure 2. The best retention is in
the aw = 0.22 - 0.24 where 70% Of ascorbic acid is retained
after 122 days. For aw = 0.43 - 0.45 and aw = 0.61 - 0.70
the loss of ascorbic acid is 50% after 20 days. The des—
truction is complete at approximately 50 days for aw = 0.61 -
0.70, whereas the retention at 43 - 45% RH is approximately
30% at 60 days and complete destruction comes at approxi-
mately 80 days.

The greatest ascorbic acid losses occur at the highest

water activity during storage.

The Effect of aW and Time on Total Phenolics of Freeze—dried

 

Apple Slices

 

The percent of total phenolics retained in the freeze-
dried apple slices after 2 weeks of storage at 2500 for the
different water activities is shown in Figure 1. The per-
centages lie within the 80% to 95% range. Acker et al.
(l967a) showed that in a model system Of polyphenol-oxidase-
cellulose-catechol, the enzyme activity increased with
increasing water activity. In real systems this may not
be so simple. In Figure 1 the total phenolics retained
are less at aw = 0.22 than at aw = 0.27 and 0.3.

Freeze-dried apple slices have a porous structure which

leads to a greater susceptibility to oxygen action.

34

 

 

 

 

 

no .. um. o
d 8o .
e
.m
a
a mo . 4
R A. .
d 0 Wall HIV
.m
A no L.- O
.m o
.m ,
o 8 .. w o
u .u a. O
A
M no e .
x o
G v a 4 L. - p . o v n a a r
no so no mo So So
qm<m

mamcwm m. umwnmza ow >mnoxcso >oma amamszmq we m2 u o.mm - o.me.»w m2 n o.ew - o.em ow

m: u o.md I o.uo O. is”: mwowmmm flsam.

35

Enzymatic systems like polyphenol oxidase are oxygen depen-
dent in their reactions. At aw = 0.22, 88% of total
phenolics are retained compared to 95% at aw = 0.27 and 0.30.
This could be due to the higher diffusion of oxygen into
the more porous structure at aw = 0.22, thus resulting in

a lower percent of total phenolics. The colorless quinones
formed at this stage may then be reduced to the dihydroxyl
state, probably by ascorbic acid which is concurrently
oxidized to dehydroascorbic acid. This could account for
the dip of the curve of ascorbic acid at aw = 0.23 in
Figure 1.

At aw = 0.3 to 0.66 the percent Of total phenolics
differ by 5% only. There is not much enzymatic reaction in
this range, probably due to some inhibitory action of
ascorbic acid. As can be seen ascorbic acid decreases from
92% to 17%. NO browning occurs at this stage after a
2-week storage because the colorless quinones formed have
not reached the indole, 5, 6 quinone stage.

At aw = 0.7 and above there is a decrease in percent
of total phenolics retained. Here it seems that the ascor-
bic acid is not effective or has lost its ability to act
as an antioxidant due to the dilution effect at this water
activity and moisture content. 0n the other hand, the
moisture content at this water activity may serve as a
medium for the enzyme reaction and as a vehicle for sub-
strate mobility. There is some slight browning of the

apple slices here.

36

The time effect during storage at 20-250C for the dif-
ferent water activities on the percent of total phenolics

retained is shown in Figure 3. At a = 0.61 - 0.70 the

w
enzymatic reaction is greatest. This is expected since
water acts as a medium for substrate and enzyme mobility.
At aw = 0.43 - 0.45 lesser enzymatic activity occurs than
at the higher relative humidity. A maximum color develop-
ment occurs at 30 days. The higher relative humidity has

a slightly darker reddish-brown hue than the lower relative
humidity. The total phenolics levels off after 30 days Of
storage. This may be due to the fact that the substrates
for polyphenol oxidasein the apple slices are exhausted

or that the enzyme activity decreases during storage. It
may also be due to end product inhibition of the enzymatic
reaction which prevents further enzyme activity.

In the lowest water-activity there is less loss in
total phenolics during the same period of storage. The
moisture content here is low to slow down enzymatic reac-
tion. Nevertheless, minor activity could revive during
long-term storage. At 122 days the total phenolics drop
to 73%. No browning occurs in these apple slices. This
could be due to the 70% retention of ascorbic acid at this

stage where the ascorbic acid could act as an antioxidant.

 

37

 

no-nm.n

 

 

 

.0 t

Phenol ice (as Tannic Acid) Retained

#0 e

% of Total
n
10

 

1h

[P
4

<>

P b
‘

 

 

4|
1

0 no .0 no no doc .90
au<u

mdmc1m u. umxnmsw oi wowed ezmzodsnm Amm amazsn mn.av smamsamg me e u o.mm - o.mp v;

a: u o..u - o..m on a: n o.ms - o.qoezs~: meoeaem ease. z

 

38

The Effect of aw and Time on Total Volatiles of Freeze-

Dried Apple Slices

 

Feys et al. (1977) identified the volatiles Of "Schone
van Boskoop" by using different GLC columns and known com-
pounds as standards. Their results showed that "Boskoop"
is a typical alcohol type variety because more than 90% of
the total amount of volatiles are alcohols.

There is a considerable loss in volatiles after freeze-
drying as shown in Table 4. Acetone seemed to have the
highest retention. 0n the whole flavor volatiles retention
in freeze-dried products is much better than other methods
of dehydration. Kayaert et al. (1975) and Thijssen et a1.
(1968, 1969) have studied flavor, retention in freeze-dried
substances in detail and have developed the theories of
micro-region entrapment and selective diffusion respectively.

Gas-liquid chromatograms of fresh and freeze-dried
apples (Aw=0.22-0.24) with respect to their volatile com-
pounds areshown in Fig. 4. It may be seen that there were
no new volatiles formed as a result of the freeze drying
treatment. The data indicated that freeze-dried apple
slices possess an aroma and flavor profile comparable to
that of fresh apples.

Flink and Karel (1972) and Chirife, Karel and Flink
(1973) found that exposure Of freeze-dried substances con-
taining trapped volatiles, to various levels of relative

humidity would give decreased volatile content, which was

39

Table 4. Comparative analysis of total volatiles of fresh
and freeze-dried apples.

 

 

Components Fresh Freeze-dried %
ug/lOO g D. Wt. ug/IOO g D. Wt. Retention
Diethylether 11.6 - 0
Acetaldehyde 2865.5 310. 10.83
n-Propanal 107.4 - 0
Acetone 253.7 116. 45.85
i-Propanol 229.1 52. 22.71
n-Butanal 122.9 28. 22.95
Ethanol 26380.0 5626. 21.33
n-Propanol 443.8 132. 29.80
Ethylbutrate 659.4 - 0
i-Butanol 723.3 175. 24.20
n-Butanol 9385.5 2516. 26.81
n~Pentanol 129.3 - 0
n-Hexanol 1348.0 458. 33.98

 

 

 

Fresh Apples

( u'sN
33
\l
.4
A

 

filo. (s

90 C.

 

 

 

 

" w
11. so 4:
”14’3701

13 493

 

 

 

 

 

 

 

’3 07"”
_2.aa
3.8?

- 4.5?_1

 

 

 

 

 

 

 

 

lj Freeze-dried Apples
3 stored at aw = 0.22 -
f 0.24
l
90°C 1109C
7 l
3 ‘3‘. 1
3 2 i
a i ll
.: l
i‘ 4 i
— \2 H 25 I"
a D i ll 3331 '
'5 a v x 1' '2:
1

 

Figure 4. Volatiles Of Fresh Apples and Freeze-dried
Apples at aw = 0.22 - 0.24 at 20 - 25°C for 41
days. 1) Diethylether, 2) acetaldehyde, 3) n-
propanal, 4) acetone, 5) i-prOpanol, 6) n-
butanal, 7) ethanol, 8) n-propanol, 9) ethyl-
butrate, 10) i-butanol, 11) n-butanol, 12) n-
pentanol, 13) n-hexanol, ?

 

41

lower as the relative humidity was increased. From Table 5,
it seemed that the data agreed with their findings. As
expected in the lowest aw = 0.22 — 0.24 storage, the reten-
tion of volatiles was the best for the same period of time.
The highest aw = 0.61 - 0.70 had the highest loss in vola—
tiles in terms of concentration (see Table 5).

After 30 days of storage at a = 0.43 - 0.45 there

w
were complete loss of i-propanol, n-propanol and i-butanol.

After 30 days storage at a = 0.61 - 0.70 there were no

w
detectable levels of i-propanol or n—propanol left in the
samples. At these two storage water activities the apple
slices became sticky and the pieces fused at the surfaces
with subsequent "collapse" or loss of structure (Tsourorflis
et al., 1976; To and Flink, l978b). These authors had
implicated 'collapse' (i.e. loss of structure) as the

determining factor in the release of the encapsulated

materials such as volatiles

The Effect of Water Activity_on Texture of Freeze-dried

 

Apple Slices

 

The texture of apples is derived from the pectin,
hemicelluloses, cellulose, pentosans and hexosans and their
interactions and interconnections (Hulme, 1958). The
behavior of these polysaccharides, which attract and retain

water, is governed by the sorption isotherms.

42

 

 

 

 

m.mk m._m P.N__ _.m__ e.mm~ m._em m.mom m.ewm - e.er _oeexe=-e
N.Nmm o.wme e.eoe e.ewe m._m_P 0.5me2 m.e~ep o.eoem 0.0mm o.mem_ _oeee=m-e
m.NN N.em m.em - N.ma e.om_ m.mm~ e.mmw _.mmm m.em_ Foeeezm-e
- - - - e.oe m.me o.em_ o.m~. - m.mm _oeeaoea-e
e.mmm m.~em_ o.eme_ m.emm o.ooem m.kmmm o.mmm_ m.mmme e.mmm o.oemm Poeeeem
- - - m._N - m.em N.em - - - Feeaesm-e
- - _.m - - - N.e_ “.mm - N.mN _oeeaoca-w
m.mm e.mm~ m.mep m.ope e.e_N k.eme m.eom m.mm e.em_ o.NON eeoeee<
. I 1 1 1 u 1 I 1 . Pmcmqoca-c
m.ee e.ae $.54 - - - N.Nm m.mh_ _.mm o.ne_ meseeefieoee<

om om o, om om op om Fe om Pm
mseo .e; .o
m oo_\m:
oa.o - _e.o me.o - me.o em.o NN.o meeeeoanu

3

<

 

mmwpw>wuum swam: ucmcmwwwn an
umpmgnwpwzcm mmowpm mFaam um_cvsmNmmce mo mmFmeFO> Fmpou we mwmxpmcm m>wumcmnaoo

.m mFQMP

43

In this study puncture test were used to give a graphic
representation of the texture, which ranged from brittle to
rubbery as shown in Figures 5a and 5b. At aw <0.25,
moisture content is less than 5.0% and the apple slices
are crispy. The structure of the slices is probably com-
parable to that of an amphorous dried matrix. The polysac-
charide hydroxyl groups can form intermolecular hydrogen
bonds in the absence or presence of low.polar solutes. This
dried matrix does not require much energy to cause a rupture
point on the apple slice.

At values of aw between 0.25 <a <0.4, the increased

w
layers of water molecules form hydrogen bonds with the
hydroxyl groups of the polysaccharides. These layers of
water molecules adjacent to the hydroxyl groups of poly-
saccharides are therefore partially ordered and immobilized.
Formation of this atmosphere of associated water assists in
dissolving or dispersing some of the large molecules of
polysaccharides and this rehydration gives a viscous texture
to the apple slices. This could account for the increased
amount of energy required to puncture the surface of the
apple slice.

At aw >0.74 and higher, the moisture content does not
necessarily disrupt all the intermolecular hydrogen bonding
among the polysaccharide molecules. This fraction then is

the insoluble fraction which can be responsible for the

rubbery texture. Epstein et al. (1969) referred to the

44

 

 

 

a = 0.18
40.. w
20 __ [Bioyield point
C
3
3 O
G.) .
z o o ' _.
{810y1e1d p01nt aw - 0.27

40 1-

2o .- fl
0 L
mm

Figure 5a. Textural profile of freeze-dried apple slices

at aw = 0.18 and aw = 0.27 for two weeks at

250C.

FORCE

 

r

 

 

 

 

45

 

 

 

 

 

 

aw - 0.38
40 r
Bioyield point

20 -
i;
C
O
a I
:0
z: 0 60
v 3:.

w

8‘0-
:2:
0
LA.

20 -

_ {Bioyield point 1
0
mm

Figure 5b. Textural profile of freeze-dried apple slices

at aw = 0.38 and aw = 0.60 for two weeks at
250C.

46

insoluble fraction of polysaccharide as the cause of adhe-
sion of dried apple slices. At this stage of hydration
there is no indication of initial cell rupture in the apple
slice. The force-deformation energy required here is the
same as for the brittle apple slice (Figure 6) since a
considerable plastic flow deformation takes place at aw =
0.74 after the bioyield point. A bioyield point occurs

on a force-deformation curve (Figures 5a and 5b) at which
there occurs an increase in deformation with a decrease or
no change of force.

For the preservation Of textural quality the best

storage condition is at a aw of less than 0.25.

47

 

 

 

 

54)»

N

E T

U

8 :10«»

‘U

:2

_ l.

a

h

3

up!

3

#1 to»

i t t e
a b c d e
0 (12 04 (L6 (l8
AW
Figure 6. Texture of freeze-dried apple slices after two

weeks equilibration at the different water
activities at 25°C. Regions a = brittle;

b = crispy; c = fairly crispy; d = soft; e =
rubbery.

 

 

SUMMARY AND CONCLUSIONS

"Schone van Boskoop" apples were freeze-dried for 24
hours and the final moisture content was 4.30% on a dry
basis. The freeze-dried apple slices were put in different
desiccators with known relative humidities for two weeks for
equilibration at 25°C. The moisture sorption isotherm was
then determined. The curve was typical of one with high—
sugar content because Boskoop apples had about 77% sugars.
The BET theory does not apply to this type of sorption
isotherm which corresponds to type III in the BET classifi-
cation. The moisture here exerts a high vapor pressure even
at very low percentages and is available for deteriorative
reactions.

There wasa.correlation of ascorbic acid and total
phenolics retention with water activities. For the same
period of storage ascorbic acid content affectaithe total
phenolics or vice versa. In the low aw = 0.22 where the
porosity of apple slices was conducive to oxygen diffusion,
the drop in total phenolics caused a drop in reduced ascor-
bic acid. No browning occurred here because ascorbic acid
was an effective antioxidant. As the ascorbic acid was

degraded in the a = 0.3 to 0.66, the total phenolics

w
changed very little. When the ascorbic acid was diluted

48

49

in the higher a = 0.7, the total phenolics decreased and

w
there was some browning because the dilution effect on
ascorbic acid made it ineffective in preventing browning.
No browning occurred as long as ascorbic acid was effective
as an antioxidant. During extended storage periods the
greatest ascorbic acid losses occurred for the high aw =
0.61 to 0.70. Complete destruction occurred at 30 days and
reddish—browning occurred then as the loss of total phenolics
leveled Off. The same was true for aw = 0.43 to 0.45,
though the loss Of ascorbic acid was slightly slower and
total phenolics retention was slightly greater. Complete
ascorbic acid destruction occurred at 80 days but the
reddish-brown hue appeared at 30 days. The reddish-brown
hue was a little less than in the higher water activity
samples. Also the total phenolics levelled off. Once the
brown color became apparent the total phenolics levelled
Off. This could be due to substrates exhaustion for the
enzyme polyphenolase or the enzymatic end-products had an
inhibitory effect on further enzymatic oxidation during
prolonged storage.

The best storage was at aw $0.24 when the ascorbic
acid degradation and total phenolics reactions were slow
because no browning occurred and the flavor was good.

Freeze-dried apple slices just after freeze-dring

lost 50% or more of the volatile components typical of

Boskoop apples. Acetone had about 4 % retention, n-hexanol

50

had about 34%, acetaldehyde had 11% and the rest of the

alcohol-type volatiles and ethylbutyrate had an average

retention of 25%. During storage the best retention of

volatiles was in the aw = 0.22 to 0.24. High losses of

volatiles occurred at aw = 0.61 to 0.70 and at aw = 0.43
to 0.45 the retention was not much better.

The texture of freeze-dried apple slices at each dif-
ferent water activity were measured. At the low aw = 0.2
the porosity of the slices caused the texture to be
brittle. As water activity increased to about 0.3 there
had been some solubilization of the components of the
slices to cause them to be less brittle but crispy. As
more moisture was absorbed, the slices became sticky, the
texture was rubbery and the slices did not break but rather
just deformed under the force applied to them. From this
observation, it was concluded that the porosity of the
cells in the apple slices gave a crispy texture.

For freeze-dried apple slices to maintain their quality
and acceptable chemically and physically, they have to be
kept at as low a water activity as possible. Complete dry-

ness is essential for good stability of dried apple slices.

 

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