120
340
THS

 

   
   

  

L I B R A R Y
PM higan State
University

‘- — W.r'“r..v—w

   

 

This is to certify that the
thesis entitled

THE EFFEITI‘ OF 'I'HAWING RATE ON
TEXI'URAL QUALITY OF FROZEN BLUEBERRIES

presented by
Abiodun Onotayo Oguntunde
has been accepted towards fulfillment

of the requirements for

M. S. degree in Food Science

 

 

    

Major professor

0-7639

THE EFFECT OF THAWING RATE ON
TEXTURAL QUALITY OF FROZEN BLUEBERRIES

Abiodun thayo OgImtmide

A.THESIS

Suhnitted to
Michigan State University
in partial fulfillment of the requirarmts
for the degree of

MSI‘ERCFSCIENCE

Department of Food Science and Human Nutrition

1978

(x-

Tf-IEEFFECI‘OF'I‘HAWWGRATECN
TEDCIURALQUALII'YOFFROZENBHJEBERRIES

BY

ABIODUN MAYO OGJNTUNDE

Blueberries of the Jersey variety (Vaccinium lamarckii) were indivi-
dually quick frozen in air-blast and by imnersion in each of prOpylene
glycol and sucrose solutions. Different thawing rates were obtained
during this study by varying the medium of thawing and the temperature
of the thawing medium. The effect of these thawing rates on the tex-
tural quality (determined using an objective method of neasurerrent)
after thawing of the frozen blueberries was found to be significant at
the 5% level and the relationship between the rate of thaw and texture
of the berries was analyzed and concluded to be a second-order polynomial.
The degree of correlation between the objective and sensory methods that
were used in evaluating texture of thawed blueberries was found to be
relatively low, though significant at the 5% level for berries thawed
in air-blast.

 

ToMonvingMother

The author would like to express his deep appreciatim to Professor
Theodore Wishnetsky for his suggestion of the topic and his guidance
and supportthroughoutthe course of this thesis.

The author also wishes to thank Dr. Mark A. Uebersax, Assistant
Professor of the Department of Food Science and Hmen Nutrition for the
valuable advice and suggesticns during the research.

Dr. Richard C. Nicholas, Professor of the Department of Food
Science and Human Nutritim and Dr. Dennis R. Heldman, Professor and
Chairman of the Department of Agricultural Engineering are also acknow-
ledged for the invaluable assistance they gave during this study.

Gratitude is extended to the Federal Institute for Industrial
Research, Lagos (Nigeria) , for the scholarhip awarded the author during
his graduate studies .

Finally, the author would like to express his gratitude to his wife,
Subuola, for her encouragenent and help throughout this study.

ii

II.

III O

VII.

WII O

m.

TABIEOFCIIVI‘ENTS

List of Tables .......................

List of Figures

Introduction ........................

Literature Review

Equiprent Used ..... f ................
Preparation of Samples .................

Freezing Procedures

Storage Procedures ...................
Thawing Procedures ...................

Results and Discussion ........

A. Effect of Factors Studied on Textural Quality

B. Effect of Factors Studied on Thawing Rate .......

C. Effect of Frozen Storage Period on Textural maluation .

D. Objective versus Subjective Methods of Texture

Evaluation ......................

Conclusions . . .

Page

iii

1].

ll
12
13
16
17
19

23
24
29

35

LISI' OF TABLES

Table Page

1 Means of Instron measurement (viz compression force
in GM) after thawing of blueberries which had been

kept in frozen storage at -lO°F for 2-3 days. ...... 25
2 3-way analysis of variance of the main effects and
interactions in Table l .......... . . . . . . . 25

3’ Thawing rates in °F/min (fron 20° to 40°F) of thawed
blueberries which. had been kept in frozen storage at

~109F for 2-3 days ................ . . . .. 26
4 3-way analysis of variance of the main effects and
interactions in Table 3 ........... . ...... 26

5 ‘ Means of Instron measurement (viz compression force
in CM) after thawing of blueberries which had been
kept in frozen storage at -lO°F for 60—68 days ....... 27

6 3-way analysis of variance of the main effects and
interactions in Table 5 .................. 27

.7 Thawing rates in 0°F/rnin (from 20° to 40°F) of thawed
blueberries which had been kept in frozen storage at
-100F fOr 60-68 days. ............ o o o o o o 28

8 3-way analysis of variance of the main effects and
interactionsinTable7 ....... ...........28

9 One-way analysis of variance on the effect of the
frozen storage period on the firmness (viz compression
force in GM using the Instron) of thawed blueberries. . . 34

10 Comparative data on firmness (as measured objectively
using the Instron and subjectively using a taste panel)
of thawed blueberries which had previously been frozen
and kept in frozen storage at -lO°F for 60-68 days.. . . .36

11 Que-way analysis of variance of the means of firmness
(as measured objectively using the Instron and subjec-
tively using a taste panel) of thawed blueberries 'after
60-68 days of frozen storage at -lO°F ........... 37

iii

Table

13

Page

' Correlation coefficients between the means of

objective and subjective firmness measurements
on thawed blueberries after a frozen storage period
of 60-68 days at -10°F ........... . ....... 39

Nth order regression analysis of the relationship
between Instron measurement (viz compression force
in GM) and thawing rates in °F/min of thawed blue-
berries ....... . ........... .......41

iv

Figure

LIST OF FIGURES

Rate of increase in temperature at the geometric
centers of blueberries which had been kept in frozen
storage at -10°F for 2-3 days and thawed in both
airéblast and propylene glycol solution (50% by

Page

weight) at 80°F and 100°F and also in room air at 70°F. . .30

Rate of increase in temperature at the geometric
centers of blueberries which had been kept in frozen
storage at -10°F for 2-3 days and thawed both in air-
blast and propylene glycol solution (50% by'weight)‘

at 80°F and 100°F. ...................

Rate of increase in temperature at the geometric
centers of blueberries which had been kept in frozen
storage at -lO°F for 60-68 days and thawed in both air-
blast and sucrose solution (22.5% by weight) at 80°F
and 100°F and also in room air at 70°F. ........

Rate of increase in temperature at the geometric
centers of blueberries which had been kept in frozen
storage at -10°F for 60-68 days and thawed in both air-
blast and sucrose solution (22.5% by weight) at 80°F

and 100°F .................. . . . . .'. .

. .31

. .32

.33

W10}!

 

Fruits and vegetables preserved commercially by freezing and frozen
storage muld have to be thawed before utilization. Thawing has, however,
been shown to be inherently slower than freezing when conducted under
comparable temperature differentials and in practice, the maximm temper-
ature differential permissible during thawing is much less than that
which is feasible during freezing due to the sensitivity of foods to
high temperatures.

Apartfronthefactthatthmvingtakes longerthanfreezing, an
additional concern is the temperature at which it occurs; i.e. the time—
tamperature pattern characteristic of thaving is potentially more detri-
mental than that of freezing, for during thawing, the temperature rises
rapidly to near the melting point and remains there throughout the long
course of thawing, thus affording considerable opportunity for occurrence
of chemical reactions, recrystallization and even microbial growth if
thawing is extrorely slow.

As a consequence of the above-mentioned factors, greater damage to
food texture coildbebroightaboutbythawingthanbyfreezing. Thawing
damage could, however, be lessened if: (1) more were known concerning
Optimum thawing procedures for various kinds of foods, (2) frozen foods
were marketed in a form (suitable size, shape and package) conducive to
good tl'iawing procedures, and (3) adequate instructions for thawing were
provided with each product.

The main purposes of this research were to investigate the effect
of various thawing procedures (applied on frozen blueberries) on the
textural quality of the berries and to determine the degree of correl-

ation between the objective and sensory methods of texture evaluation.

II'I'ERAIUREREVEW

FoodtexturehasbeendesaribedbyFirmey (19691 tobethesun
total of kinaesthetic sensations derived from eating a food, for it
encorpasses the mouthfeel, the masticatory properties ard the sand.
According to Szczesniak- (1963), textural characteristics can be clas-
sified under three main graips: mechanical, geometric ard others. The
textural characteristics of fruits ard vegetables were defined by Mackey,
9311;, (1973), to be crispness, juiciness, firmness, toughness, meal-
iness ard fibrousness. These characteristics were found to be associated
with plant tissue, composition ard structure of cell wall constituents,
intercelluar birding tissue and the water relationships of the plant
tissues.

Fruits and vegetables have been preserved commercially by freezing
ard frozen storage. During the freezing of plant tissue, cells have
been fand to separate along the middle lamella, which is probably the
area of least resistance to mechanical forces from growing ice crystals
(Woodroof, 1938) . Weier ard Stocking (1949) discovered that any weak-
ening of the adhesive forces within the middle lamella brought abort
textural changes in plant tissues.

Gutschmjdt (1968) reviewed the principles of freezing ard low
tarperature storage applicable to fruits ard vegetables ard remarked
that the retention of "natural quality" in a plant tissue depended upon
maturityardqualityofrawproduct, amamtofhardlingbetweenharvesting
and processing, freezing, storage and thawing procedures.

Freezing has been defined by Fennema _e_t 31., (1973) to be a reduc-
tion in temperature , generally to 0°F or below and crystallization of
part of the water ard sore of the solutes . Histological techniques were

used by Reeve (1970) to ascertain that freezing a plant tissue led
totkeformatioioficecrystalsmichinttmm, gmcmredtlecellsof
thetissueardalsothatwhenplant tissuewas subjectedtofreezing
followedbythawing, thenetresultwasthewittdrawal ofmorewater
into the intercellular spaces during freezing than was reabsorbed during
thming, which finally led to the destruction of the colloidal complex
of cells.

Joslyn (1966) , Fennena and Powrie (1964) and Gutsclrmidt (1968)
reviewed an early theory that tissue damage during freezing results from
the withdrawal of water from cell protOplasm to form ice in intercel-
lular spaces. Theatrmmtofwaterwitldrawn fromtheprotoplasmofthe
cell of a fruit is generally high for fruits have large intercellular
spaces. This dehydration of the protoplasm would result in the defor-
matim of cellwalls ard overall loss in textural quality of the tissue.

Plant tissues contain protein-water gels ard carbohydrate-water
gels. Gutschmidt (1968) fond that water separated art of these gels
during freezing. Ponting, _e_t 511., (.1968) , reported that when colloidal
solutions in cell membranes became dehydrated, changes in penneability
and elasticity of the membranes occurred , so that loss of rigidity
resulted upon thawing; i.e. a frozen fruit on tlming appeared soft ard
Why. ’

Morris (1968) reviewed work on effects of freezing rate on hydro-
gels and he fond that rapid freezing resulted in less dehydration of
the colloidal material than slow freezing. He also fond that more
water was reabsorbed with less drip occurring during thawing of col-
loidal matter subjected to rapid freezing than to slov freezing. This
same researcher reviewed findings on protOplasm denaturation during

freezing and remarked that it was impossible, even with the fastest

freezing methods , to prevent protoplaem denaturation during freezing ard
he finally concluded that the only advantage of quick freezing over slow
freezing was the formation of smaller ice crystals which then resulted
in less tearing and disnrptioi of cell tissues.

Fennema ard Powrie (1964) reported that though cells in many fruits
are susceptible to mechanical rupture during growth of ice crystals,
resulting in "soft and limp thawed product with excess drip", rapid
freezing may however, minimize cell damage ard reduce the excessive
amount of fluid loss upon timing for small, uniformly dispersed ice
crystals are formed in the frozen tissue after rapid freezing.

Joslyn (1966) reviewed findings of several researchers on the
effect of freezing rate on texture of plant tissues and observed that
"quick freezing based on refrigerant temperatures of -33°F to -40°F,
results in less tissue rupture . "

Various techniques ard materials have been used for quick freezing
fruits. Bartlett and Brown (1964) referred to direct contact freezing
in sugar solutions and described their "polyphase process" for rapid
freezing to be a process which employed a heat transfer medium of invert
sugar solution that had been agitated and simultaneously chilled until
a disperse solid phase of very small ice crystals was formed. In 1969,
E. I. Dupont de Nerours introduced a system for direct contact freezing
of foods with liquid "freon" freezant (Anon, 1969). MacArthur (1945),
Fennema ard Powrie (1964) and Ponting, _e_t_ a1” (1968) reported that
rapid freezing of strawberries in liquid nitrogen was feasible.

The freezing process is usually followed by the frozen storage
period ard the time period that frozen foods can be maintained in "good
corditiou"deperdsa1trekirdofprcduct, rawitisprocessedand

packaged, and the storage terperat'nre. In the United States, 0°F is
generally accepted as a satisfactory storage temperature for frozen
foods, according to Fennema, et_a_.1_., (1973) and Fennema (1975); however,
theseauthorsalsostatedthatfrozenfoodsstoredatornearacon-
ventioial temperature of 0°F are not completely frozen nor are they
inert, for they deteriorate at a significant rate and the qaulity loss
incurred during a normal period of frozen storage generally exceeds that
causedbyanyotlerphaseofthefreezingprocessviaprefreezing
treatments, freezing and thawing. The loss of quality during proper
frozen storage of foods could be attrihited to chemical ard/or physical
means sincemdcroorganismscanmtgrawurderthesecorditions.

The major physical changes that occur during frozen storage are
recrystallizatim, and sublimation. Dyer (1951) ard Dyksm (1956)
reported that recrystallization was partially responsible for the dis-
appearance during frozen storage , of the quality advantages initially
apparent in rapidly frozen foods as compared to slowly frozen foods .
Sublimation of ice can occur during frozen storage of improperly pack-
aged frozen food and this can lead to a defect known as "freezer burn."
Recrystallization can be effectively controlled by storing products at a
constant low temperature ard for aminimal tinewhile sublimationcanbe
minimized by any means which effectively stops loss of moisture from the
product; e.g. application of an ice glaze (fish) or packaging with a
material which is highly impermeable to water vapor.

The chemical changes that occur in foods during frozen storage are
the degradation of pigments and vitamins , insolubilization or destabil-
ization of proteins, addation of lipids, reactions which lead to

diminished differences in quality between rapidly ard slowly frozen

foods, ardreactiorswhichcauseanincreaseintheamomtofthaw
endatefromtissueasthetimeof frozen, storageisectexded. Fermema
at 31;, (1973) reported that they fond that most of these chemical
reactions occurred slowly at 0°F ard declined further in rate as the
tetperature was reduced; and also that during the early stages of
freezing some reactions, including glycolysis , actually increased in
rate while others decreased in rate less than expected.

Thawing of food, which constitutes the last phase of the freezing
process before utilization, is believed to be potentially more damaging
than freezing because of the following reasons: (1) thawing of mm-
fluid foods (tissue, gels) is inherently slower than freezing when
comparable temperature differentials (i.e. difference in temperatures
betweenttefoodardtheccolingorteatingmedimm) areemployed
(Rinfret, 1960) due to the fact that during thawing, the temperature
rises rapidly to near the melting point ard remains there for a rela-
tively long time before it continues to rise; (2) temperature differen-..
tials frequently are less during thawing than during freezing , especially
with fruits. During thawing, foods are subject to damage by chemical,
physical , ard microbial means , althoigh microbial problems are negli-
gible in properly handled foods, and as a result of these considerations,
thawingskmfldberegardedasagreaterpotentialsoircecfdamagethan
freezing (Fennema. and Powrie, 1964), but Gitscmudt (1968), reported
thatthawingmayaffectthepropertiesoffniitstotresameextentas
freezing.

Luyet (1968a; 1968b) fond that during slow thawing, large ice
crystals developed at the expense of smaller ones; i.e. migration of
molecules from smaller to larger ice crystals took place . He called

this phenoremn, "migratory recrystallization" , and noted that it
occurred most rapidly at near freezing temperatures. Fennema and Powrie
(1964) also reported recrystallization occurring at a more rapid rate
near the freezing point of tissue. MacKenzie and Luyet (1967) fond
that recrys-tallization was more evident in rapidly frozen as corpared
with slowly frozen gelatin gels if these gels were thawed slowly.

Althoigh food quality coild be seriously impaired during thawing,
thetineinvolvedwaildbeshortcorparedtoanormalperiodof frozen
storage. As a result, loss of quality might be greater during frozen
storage (as caducted cormercially) than during thawing, ard the evidence
for this statement came from the observation made by Fermema ard Powrie
(1964) that some foods could be rapidly frozen and thawed (no frozen
storage) withth incurring appreciable damage .

Commercially, dielectric ard microwave heating had received sore
attention as possible means of thawing foods; for it had been fond that
provided the food material was reasonably lmogenais , dielectric or
microwave heating world enable more rapid and more uniform heating than
would be possible by thermal conduction (Copson, 1962; Cable, 1954).
Unfortunately, foods being thawed are not reasonably hotogenous, since
frozen and unfrozen phases exist simultaneously ard these phases heat at
markedly different rates when placed in a dielectric field, which often
leads to localized overheating before all areas have thawed.

Foruseinthehore, mostvegetablescanbethawedardcookedby
direct immersion in boiling water , but since high temperatures are
detrimental to the quality of most fruits , they therefore must be thawed
urder milder tetperature conditions than vegetables . Suitable tech-
niques that are presently used consist of placing unspened packages of

fruit at room temperature, in the refrigerator, or in cool to Slightly
warmwater until thawing is almost complete. Fruits should be consumed
prorptly after drawing because their color and texture deteriorate
rapidly at this point. D

In quality control, it is important to identify, measure, ard
control imdepetdently each of the significant components of quality
(Kramer ad Twigg, 1966). Texture is an important component of food
quality ad, in certain foods, may be even more important than flavor
and appearance (Szczesniak and Kleyn, 1963). Measurerent of food tex-
ture, therefore, plays a significant role in the food irdustry, via
product development and improvement, control of manufacturing processes ,
ardintheevaluatimofthequalityofthefinishedproducttobe
caisumed.

According to Finney (1969) , texture, or the kinesthetic character-
istics of foods is generally considered to relate to those attributes of
quality associated with the sense of feel, as experienced either by the
fingers, the hard or in the mouth. It includes such sensations as
hardness, tenderness, brittleness, mealiness, crispness, etc.,‘ but
excludes the sensations of temperature and pain. Objective measurements
of food texture, therefore, have involved predominantly an analysis of
the mechanical or rheological behavior of food materials; that is , their
deformation, strain, or flow characteristics when subjected to a mechan-
ical force.

Instruments used on solid foods can be divided into cutting , piercing ,
puncturing, compressing or shearing devices . Generally, emperical
texture testing systems are devices which contain for basic elerents,
via (1) a probe containing the food sample; (2) a driving mechanism for

imparting motion in a vertical, horizontal or rotational. direction; (3)
a sensing element for detecting the resistance of the foodstuff to the
applied force; ard (4) a read-art system for quantifying the resistance
of the specimen.

The deformation of- a food inder the influence of a force is frequently
used as a measure of quality. Brinton ard Bone (1972) classified
foods that deform to a small extent as "firm", "hard", or "rigid", while
foods that deform to a large extent are classified as "soft", "flaccid",
or "spongy". They also reported that softness may be associated with
good or poor quality departing on the food.

Hardness is a textural characteristic of foods evaluated organolep—
.tically during the "first bite" of the masticatory cycle (Brandt gt a_l_.,
(1963). At this time the chewing force is applied to the food in an
approximately linear manner which can be satisfactorily reproduced
instrumentally by oorpression testing with the Instron Universial Tensile
Tester (Shana ard Shenman, 1973). However, the relationship between
force ard cotpression exhibited by this instrument depends on the cross-
head speed utilized ard it is therefore necessary to closely simulate
the mechanical conditions prevailing during the initial stage of masti-
cation if instnmmental data are to be utilized to predict the sensory
evaluaticn of hardness. The importance of selecting the correct instru-
mental test conditions is further erphasized by observations that larger
chewingforcesareappliedtohardthantosoftfoodsandthattherate
of chewing for these two categories also varies. Studies with fruit ard
vegetables indicate that when the consumer judges firmness by squeezing
samples between the fingers, the rate at which the force is applied and
the maximum force also depend on product firmness (Voisey and Crete,
1973) .

10

Although various objective and subjective methods have been Pro-
posed for neasuring textural quality of fruits, the correlation of these
methodshas, inmost cases, beendifficultdueneixflytotle fact that
the rate of force application in sensory testing is significantly greater
than that custcmarily used in instrumental tests. It is however,
strongly desirable for one to law how selected objective methods compare
withhumansenses intreirabilitytodetectandquantjfytexturepara-

meters.

M'I'ERIAISANDNEII-IGDS

Raw Product

 

The Jersey variety of blueberries (Vaccinium lamarckii) was used in

 

this study. 'IWo sets of blueberries which weighed about 4 ngach were
parolesedduringthesecondhalfofthemmthinJuly, 1977 atthe
Municipal Market in Lansing, Michigan. Each of the two sets had been
hand-harvested a day prior to being brought to the Municipal Market at
DeGrandchamrp, SouthHaven, Michigan. The secondsetwaspurchaseda
week after purchasing the first set of berries.

Emiigrent Used

The equipment used in this study for freezing, frozen storage and

thawing were the following:

1) TWO identical "low Temperature-High Tempera " test chambers ,
Model SK-3105, produced by Associated Testing laboratories,
Inc., 200 Route 46, Wayne, New Jersey, 07470.

2) mo "Constant Terperature" Laboratory Baths (Cat. Nos. 4-8600
and 4-8600A) , each equipped with a "Quickest" Bimetal thema-
regulator (Cat. No. 4-23SF). These baths were manufactured by
American Instrumental Co., Inc., 8030 Georgia Avenue, Silver
Spring, Maryland, 20910. A mercury flemuneter, whose calibra-
tion was verified by immersion in ice water slurry, was used to
set the thennoregulators at the desired freezing and thawing
temperatures.

3) A Milti/Riter Recorder produced by Texas Instruments Incorporated ,
Houston, Texas . Thermocouples attached to this temperature
recorder were made of copper, 0.46 cm in diameter and constantan

ll

12

0.48 endiameter. Trechartspeedusedwas 1.0 inchperlminute
(2.5 emper minute) while the print speed obtaired With the
selected change gear ratio of 2.5:1 was 5 seconds between
points. The chart paper used had a temperature range of .-100°F
to 100°F. The calibration of the Multi/‘Riter Recorder was
checked and found to record tetperatnre values at 2"}? below
actualterperathevamesasrecordedbytremercurytlemumeter
mentioned above. Terperature values shown in tl'e various
tablesofdatawereallcorrectedtccampensatefortleabove
error (i.e. 2°F was added to the strip chart measurements).

4) The Instron Universal Testing Machine, Model TIBM, Serial No.
1950, produced by Instron Corporation, Canton, Massachussetts.
The diameter of the campression cell (probe) used was 5.5 cm.
The cross-head speed used was 20 cmv‘minute (high) while the
chartspeedusedwas30 em/minute. Theselectorofthe"1“ull
ScaleLoad" wassetonl, andonlythepeakheightrecordedon

thechartpaperwasmeasured.

Preparation of Samples

 

Inthelabcratory, sortingofeach4Kgsetofpurchasedberries
was done in order to remove bruised berries and foreign materials; and
also to have only berries with the minimum height (measured using cal-
ipers) of 1.10 cm and munimum width of 1.50 cm. The average and maximum
values of the height and width of the berries were not detenmined. Ten
berries removed at random from the remaining batch of each set of blue-
berries purchased were weighed (using a Mettler balance) and the average
weightperberrywas l.87gminthefirst setandl.909m_inthesecond
set. The remaining batch of blueberries from each set was divided into

13

tmportionsandeachportion (ttnxghoneoftheuvoportimswaskept

in the refrigerator for about 30 minutes, mile the other was being
processed) was rinsed with cold tap water and quickly arranged in a
single layer on a sieve No. 8 of the U. S. Standard Sieve Series which
had a pore opening of 2.38 mm. The sieve mesh was positioned horizontally
and shaken gently at intermittent intervals during the subsequent
draining which was done at an ambient temperature of 70°F for about five
minutes. Afterthedraim'ngtheberries frameachsetpurchasedat the
same time were individually quick frozen, first in air, followed by
immersion freezing using either propylene glycol solution or sucrose

solution as outlined below.

Freezing Procedures

 

The freezing procedures used were the following:

Air-blast freezing—the air inside a "Irnw Temperature" test chamber
was cooled down to -40°F and maintained at this constant temperature
throughout the duration of the freezing treatment.

Immersion freezing in propylene glycol solution—propylene glycol
(trade name—Dowfrost) was mixed with distilled water at room temperature
to obtain a 50% (by volume) of Dowfrost solution which was equivalent to
53. 5% (by weight) of Dowfrost solution (obtained from the conversion
chart for aqueous solutions of Dowfrost that was supplied by the Dow
Chemical Company, manufacturers of Dowfrost) . The Dowfrost solution was
poured inside the "Constant Temperature" Laboratory Bath (Cat. No. 4-
8600A). to reach a height of about 25 em, and tie solution was cooled
down to -20°F and maintained constant at this temperature with the aid

of the bath.

l4

Immersion freezing in sucrose solution—sucrose crystals were mixed
with distilled water at room temperature to obtain a 30% (by weight) of
sucrose solution which was checked for accuracy using the Abbe refrac-
tometer . The sucrose solution was put into a cylindrical pot of diameter
33 cmandheight 24 cm. Thispotwasmade ofanenamel-ooatedmetal
(0.23 cm thick) and the pot which had been filled to a height of 15 cm
with the sucrose solution was held securely in the "Constant Temperature"
Laboratory Bath (Cat. No. 4—8600) which also contained 50% (by volume)
Dowfrost solution at a constant temperature of -20°F and this Dowfrost
solution reached a height of 18 cm on the exterior of the pot and through
this arrangement the sucrose solution was able to be cooled down to 0°F.
Awocdenpaddlewasinsertedinthepotamiwasusedtobreakupand
redissolve the hydrated sucrose molecules as they crystallized out of
the solution throughout the pot. The manual , uninterrupted stirring
(till freezing was completed) with the paddle, imparted same motion to
the hydrated sucrose molecules and thereby, kept the tetperature of the
sucrose solution (which was continuously measured using the temperature
recorder), at the desired temperature of 0°F.

Tomonitorthefreezingrateineachoftheabovethreemedia, the
exposed ends (which were about 0.55 cm long) of thermocouples were
pushed into the blueberries (from the point of attachment of the berry
fruit to the stalk) in order to reach the geometric center of each
fruit, which was determined by halving the average height of the berry
ardthisuasfomritobeapproximatelyOfiSembetweentheendoftte
berry'sattackmenttothestalkardtheinsideoftleberry. Markers
(small paper cellotapes) were put on the tlemocouples at distances of

15

0.55 cm from the exposed tips before these tips were inserted into the
berries, while the other ends of the thermocouples were attached to the
Multi/Riter Recorder. Freezing rate was determined by observing the
time taken for the terperature at the geatetric center to drOp from
ambient temperature (70°F) to the temperature of the freezing medium
used.

The sorted and washed blueberries (from the first set purchased)
weighing about 3.7 Kg were divided into two batches. One batch was
spread out to form a single layer inside a 0.64 cm mesh-wire rectangular
cage, 35 cm X 50 cm, and 9 cm high, which also had a lid. Three berries,
each containing a thermocouple, were gently placed on the floor of the
cage and part of each thermocouple wire inside the cage was taped to the
wall of the cage in order to prevent movement of each of the three
berries. The lid of the cage was closed before the cage was positioned
in the center of the air-blast freezer. In this position, the cage was
parallel to and also in the zone of the incoming blast of cold air
(-40°F) for 10 minutes.

The second batch was spread out to form a single layer inside a
wire-mesh cage similar to the above, which also had a similar arrange-
ment of three berries into which thermocouples had been inserted. The
cage, with the lid closed, was then immersed in propylene glycol solu—
tion (50% by volme) at -20°F for 4.5 minutes and the excess solution
adhering to the sides of the cage and berries were quickly drained by
gently tapping the sides of the cage .

The berries containing the thermocouples in each frozen batch of
berries were removed from the "freezing cage" and put into a small tin

can (8 cm high and 10 cm diameter) which was then covered with aluminum

l6

foilmiletheremainingfrozexberrieswerepoured framtlecageinto
a30-lbcapacitytincanwithalidandbothtincansvereplaoedinthe
storage chamber for 2-3 days at -10*°F.

The sorted and washed blueberries (from the second set purchased)
weig'zing about 3.75 Kg were also divided into two batches. One batch
was frozenintteair—blastfreezerasdescribedaboveforttebatchof
berries from the first set purchased. The second batch was spread out
in a (single layer inside the "freezing cage" (as described above for the
berries frozen by immersion in Dowfrost solution) and the cage was then
immersed in 30% (by weight) sucrose solution at 0°F for four minutes,
followed by quick draining of the excess sucrose solution at the surface
oftheberriesandcagebygentlytappingtkesidesofthecage. Tl'e
berries containing the thermocouples in each frozen batch were prepared
for storage as described above for the first set of berries purchased,
while the remaining berries per frozen batch were poured into a 30-1b
capacitytincanwithalidwhichwasthenplacedinthestoragechamber
for 60-68 days at -10°F.

Storage Procedures

 

Thestoragetincansmichhadpreviouslybeenplacedintte "Low
Temperature-High Temperature" test chamber for about 30 minutes were
thentransferred, usuallywithin 60 seconds aftertheberrieswouldhave
beenpackedintotl'ecans, backintothetestchamberwhichwasmain—
tained at a temperature of -10°F. In the first set of experiments, the
storage period was 2-3 days while in the second set of experiments, the

storage period used was 60-68 days.

17

The herperatureoftletestchamberwasreoordedusingamercury
themumeter (Taylor Co.) and for the storage period of 60-68 days, the
temperature was observed to fluctuate from -lO°F to -7"F occasionally.
During sampling of the berries for the set of thawing operations,-the
freezer (test chamber mentioned above) door was opened and closed as
quidtlyaspossibleinordertomimmizetheetountofwarmerairfram
outside entering the freezer. Though the freezer temperature was ob-
served to warm up to -2°F to -3°‘F, it however, usually returned to -10°F
withinafewmumtes. Smefireicecrystalsmereobservedalongtke
inside walls of the cans kept in frozen storage for 60-68 days and it
wa8believed thatthesewere formedbythecondeisationofwatervapor
inside the cans , followed by crystallization of the liquid water .

Thawing Procedures

 

Samples of the frozen berries (each sample used for a Specific
thawing treatment weighed about 300 g) were reroved from storage when
needed and each sample was spread out to fonm a single layer inside a
0.64 emmesh-wire rectangular cage, 35 cm X 50 cm, and 9 cm high with a
lid and this served as the "thawing cage". Also three berries con-
tairfingthenrocouplesmichhadbeenfrozeninasimilarmermerastte
BOOgsampleabove, wereplacedonttefloorofthecageandpartof
eachtl‘enrooouplewireinsidettecagewastapedtotlewalloftlecage
inordermpreventmovetentofeachoftl'e threeberries. Thelidof
thecagewasclosedbeforethecagewasplacedinanyofthethawing
media viz air-blast 50% (by volume) Dowfrost solution and 22.5% (by
weight) of sucrose solution. The thawing procedures used were the

following:

18

Air-blast thawing—the air inside a "Low Terperature-High Tetper-
ature" testchamberwaswameduptothedesiredthawingtetperatureard
maintained constant at this temperature throughout the subsequent
thawing treatment. The thawing temperatures used were 80°, 90° and
100°F.

Immersion thawing in propylene glycol solution—50% (by volume)
Dowfrost solution was made using pure Dowfrost and distilled water at
roomtemperature. This solutimwaswarmeduptotkedesiredtrwiro
temperature and maintained constant at this temperature inside the
Constant Laboratory Bath (Cat. No. 4—8600) . The thawing temperatures
used were 80°, 90° and 100°F.

Immersion thawing in sucrose solution—sucrose crystals were mixed
with distilled water at room temperature to obtain a 22.5% (by weight)
of sucrose solution which was checked for accuracy using the Abbe
refractcmeter. The sucrose solution was wanted up to the desired thawing
temperature and maintained constant at this temperature inside the
laboratory bath (Cat. No. 4-8600) . The thawing temperatures used were
80°, 90° and 100°F.

To monitor thawing rates for each theving operaticm, the three
thermocouples in the "thawing cage" described above, were attacked to
the Multi/‘Riter Recorder. The thawing rate for a berry, in °F/minute
ms calculated by determining the time in minutes for the telperatureat
the geometric center of the berry towanm up from 20° to 40°F (for this
tetperature interval represents the approximate range where most of the
ice to water change of state occurs). Each thawing Operation was
replicated so that the average of six thawing rates (each obtained from
a berry) was calculated and used in this study.

19

Texture Measurements

The blueberries from the first set purchased were thawed in air-
blast and propylene glycol solution. After a thawing Operation, the
berries were immediately packed into several small almimmm cups (6cm
dieteter and 4 1/2 on high) and transferred within five mu'mtes to the
room rousing the Instron Universal Testing Machine in an ice chest. The
Instron had previously been calibrated and set ready for measuretent
before the berries were thawed. A total of ten berries (chosen at
random) wereremovedoneatatimefremtheenpsinthe icechestand
placedonaWhatmanNo. l filterpaperof 11.0 cmdiemeterwhichwas
then placed on top of the gage (i.e. the platform of the Instron on
which samples are placed) . The Instron measuretent on the first through
the last of the ten berries was usually accomplished in. about seven
mLiImtes. Theremairmngberriesweretlendiscardedaftertrelnstron
measurement had been demon the ten berries.

Nosample holdervasused, andallmeasuretentsweremadewitha
full scale load of l kilogram and a cross-head speed of 20 cm/mu'mte.
Eachberrywasplacedwiththepointof attackmenttothe stalk facing
upwards and was compressed till the height was reduced from the initial
value (which was about 1.10 cm) to 0.10 cm. Tie maximum compression
foroecorrespandingtothemaximmmpeakheightwasrecordedingrams in
the results. No sensory evaluation of the firmness of the thawed
blueberries was conducted on the first set of pmrchased berries .

Theberriesframthesecondsetpurchasedwl'uchhadbeenstored for
60-68 days were thawed in air-blast and sucrose solution. The sucrose

solution was used in the second set of experiments, because the berries
after thawing were analyzed for textural quality by a taste panel.

20

The 30% (by weight) sucrose solution was used during the freezing phase
in order to obtain a cooled sucrose solution at 0°F while the 22.5% (by
weight) sucros'esolutionwasusedduringtlethawingphase, sothatthe
thawing medium was isotonic with the soluble solids content of the
berries, whichwasdeterminedusingtheAbberefractatetertohavean
average value of 22.5° Brix. hmediately after each thawing operation,
the berries were packed into trays, covered with aluminum foil and were
kept in the refrigerator (temperature, 40°-42°F) for about 15 minutes
before samples were taken out for sensory evaluation by taste panel.
After the completion of sensory evaluation, which took about 30 minutes,
the berries remaining in the refrigerator were packed into small alumi-
numcups andtransferredtothe Instronmachine inanice chestwithin
10 minutes.

TheInstronmeasuretentswereconductedas specifiedaboveforthe
firstsetofexperiments, butthetimelagbetweentheendofthawing
operationandthebeginningofthe Instronmeasuretentintl'esecondset
of experiments was roughly one hour. Though ten berries from each
thawing operation were compressed using the Instron Machine, each thaving
Operation was replicated so that an average of twenty Instron measurenents
wascalcmnlatedandusedinthissoady. Thecross-headspeedoftle
Instronwassetatzo exv‘mimntes, forthisvaluehadbeenfoundby
Vibbert (1976), to enable the Instron simulate as closely as possible,
themechanioal conditions prevailingduring the initial stageofmasti:
cation.

The sensory evaluation was conducted using ten panelists to evaluate
subjectively firmness of the blueberries after each thawing operation
(which was replicated) on the second set of purchased berries , by cutting

21

withteethandbypressingbetweenthetlmmbandafinger. These
subjective methods have been described by Finney (1969) to be related to
kinesthetic characteristics or texture of foods . Each panelist was
supplied with four blueberries for each of the teeth-cutting and finger-
pressing evaluations and was told to evaluate the firmness using a nine-
point hedonic scale (Amerine 93 a” 1965) as follows:

Ectrerely finn = 9
Very firm = 8
Moderately firm = 7
Slightly finm .= 6
Neither firm nor soft = 5
Slightly soft = 4
moderately soft = 3
Very soft = 2
Extrerely soft = l

'Ihesamplesof instructionandscore sheetscanbefoundintte
Appendix -

Results were statistically analyzed by one-way analysis of variance
(Kramer and Twigg, 1966) for difference between mean scores and signifi-
cantly different samples identified using Duncan' 5 Multiple Range Test.

Calculations for determining the standard deviation, the regression
analyses and the coefficient of correlation were done using the Wang
System 2200 NB (situated in the Department of Agricultural Engineering)
manufactured by Wang Laboratories, Inc. , Tedtshlry, Massachussetts;
while the critical values for correlation coefficients were checked up

from "Statistical Tables" compiled by Rohlf and Sokal, 1969.

22

The three-way analysis of variance of the results was calcuated
using a pocket calculator and following the method described by Sokal
and Rohlf, 1969.

RESULTS AND DISCUSSICN

 

Effect of Factors Studied on TexturalJQuality

 

Statistical analysis of the factors studied (viz freezing medium,
thawing medium, and tetperature of thawing medium), revealed that only
the nature of the freezing medium and the interaction between the
thawing medium and temperature had significant effects on the Instron-
measured firmness of timed blueberries after 2—3 days of frozen storage
(see Tables 1 and 2) . The difference between the effects of freezing in
propylene glycol solution and air-blast on firmness was highly signifi-
cant, with higher firnmess values being obtained with berries frozen in
the glycol solution. The reason for this increase in firmness after
thawing might be due to the fact that faster freezing is achieved using
propylene glycol solution than air-blast. With rapid freezing , the
course of ice crystal formation and growth is altered, thereby resulting
in higher firmness values than slow freezing.

Similarly, statistical analysis of the above mentioned factors
revealed that only the nature of the thawing medium and the interaction
between the thawing medium and temperature had significant effects on
the Instron-measured firmness of thawed blueberries after 60-68 days of
frozen storage (see Tables 5 and 6). The difference between the effects
of thawing in sucrose solution and in air—blast on firmness was highly
significant. Frozen blueberries thawed in sucrose solution were found
to be consistently firmer (using the Instron) than those thawed in air-
blast, butthis trendwasnotobservedwith the subjectivemethods of
texture evaluation as will be detailed later in this discussion. The

reason for this increase in firmness of berries thawed in sucrose

23

24

s01ution is not lonown to the author. Drip loss measurements and changes
in berry weight during thawing , which might have offered clues to the
reasons for the observed difference in firmness, were not conducted in

this ecperiment.

Effect of Factors Studied on Thawing Rate

 

Thefreezingmedium, thethawingmedium, andthe temperatureof the
thawing medium respectively, along with the interactions between two or
all of these three factors, were found to have significant effects on
the thawing rate (from 20° to 40°F) obtained when thawing berries which
had either been kept in frozen storage for 2—3 days or for 60-68 days
(see Tables 3, 4, 7, and 8).

The difference between the effect of freezing in propylene glycol
solution and in air-blast on thawing rate was highly significant, with
higher tlmving rates being obtained with berries that had been frozen
in glycol solution. Similarly, the difference between the effect of
freezing in sucrose solution and in air—blast on thawing rate was highly
significant, with higher thawing rates being obtained with berries that
had been frozen in the sucrose solution. These results indicated that
immersion freezing produced faster thawing than air-blast freezing under
identical frozen storage and thawing procedures.

TheresultsinTables 3and7ontheeffectofthethawingmedium
on thawing rate agree with the statistical analysis in Tables 4 and 8
respectively, which shoved that thawing in either of the solutions
(propylene glycol and sucrose) produced faster thaw than thawing in
airardthiscouldbeduetothefactthatthesethawingsolutionshave

higher convective heat transfer coefficients than air .

25

Table 1. Means* of Instron measurenent (viz compression force in GM)
after thawing of blueberries which had been kept in frozen
storage at -10°F for 2-3 days.

 

 

 

 

 

 

Temperature 'of the thawing median
Freezing median Thawing medium 80°F 90°F 100°]?

Air-blast Air-blast 255 i 88 279 i 92 215 i 71
Propylene glycol.)

-40°F/10 min. (50% solution) 290 i 34 318 i 67 237 i 77

50% Propylene Air-blast 287 i 53 304 i 64 328 i 62
glycol solution Propylene glycol.)

-20°F/4.5 min. (50% solution) 295 i- 61 319 .t 33 299 i: 51

 

 

 

 

 

 

 

* (1 canpressicn/berry x 10 berries x 2 replicates thawed; n = 20)

Table 2. 3-way analysis of variance of the main effects and interactions

 

 

 

 

 

 

 

 

 

 

 

 

in Table 1.

Source of variation df SSA NB F3
Total 239 1,131,097
Replicate 1 992 992 0.23
Freezing medium (F) 1 31,648 31,648 7.41**
Thawing medium (M) 1 141 141 0.03
Tamer“ it‘mlfegfm (T) 2 21,940.6 10,970.3 ' 2.57
P x M 1 15 15 .00
1? x T 2 6,429.7 3,214.9 0.75
Mac T 2 81,568.3 40,784.2 9.54**
F x M x T 2 18,385.0 9,192.5 2.15

227 969,977.1 4,273.03

 

 

 

 

 

 

** Significant at 1% level.

 

Table 3 .

26

Thawing rates* in °F/min (fron 20° to 40°F) of thawed blue-

berries which had been kept in frozen storage at -10°F for

 

 

 

 

 

 

 

2-3 days.
I Teuperature of the thawing medium
Freezing median Thawing medium 80°F 90°F 100°F
Airéblast Air-blast 4.13i0.87: 5.01:0.71a 8.3 1:0.34
-40°F/10 min. Propylene glycol.)

(50% solution) 16.88i1.57 33.1li0.22 56.10 i 0.99
50% Propylene Air-blast 5.39:1.09b 6.57:0.10b 9.85 i 0.72

glycol solution P 1 ene glycol. .
-20°F/4.5 min. (50% solution) 18.46.452.17 503213.22 158.41 i 0.74

 

 

 

 

 

* (3 berries/experiment x 2 replicates thawed; n = 6)
+ (Means of thawing rates followed by like letters alog the rows in the
above Table are not significantly different P 2 0.05, Duncan, 1955)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 4. 3-way analysis of variance of the main effects and interactions
in Table 3.
Source of variation df SS NS Fs
Total 23 9930.93
Replicate l 0.03 0.03 0.01
Freezing median (F) 1 114.80 114.80 28.42**
Thawing medium (M) 1 6325.48 6325.48 1565.71**
Tegferiture a? an) 2 1936.27 968.14 239.64**
{ F x M 1 49.05 49.05 12.14**
F x T 2 85.04 42.52 10.52**
M x T 2 1292.70 646.35 159.99**
P x M x T 2 83.14 41.57 10.29**
Error 11 44.42 4.04

 

 

** Significant at 1% level.

 

 

27

Table 5. Means* of Instron measurenent (viz caupression force in GM)
afterthawingofblueberrieswhichhadbeenkeptinfrozen
storage at -10°F for 60-68 days.

 

‘Teuperature of the thawing medium

 

 

 

 

 

Freezing median Thawing median 80°F 90°F 100°F
Air-blast Air-blast 459 i' 60 419 r 48 448 i 67
-40°F/10 min. Sucrose
(22.5% solution) 436 i- 72 469 i 47 479 i 56
30% Sucrose Air-blast 446 i 73 436 i 61 441 i 60
solution S
0°F/4 min. (22.5% solution) 487 i 52 514 i 34 462 i' 77

 

 

 

 

 

 

* (1 compression/berry x 10 berries x 2 replicates thaned; n = 20)

Table 6. 3-way analysis of variance of the main effects and interactions

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

in Table 5.
Source of variation df SS DB Fs
Total 239 973,219.3
Replicate 1 2,528.5 2,528.5 0.70
Freezing median (F) 1 9,337.5 9,337.5 2.59
Thawing nediunn (M) 1 64,977.5 64,977.5 l8.00**
~ Tm“ imj‘efim (T) 2 244.3 122.1 0.03
F x M 1 11,718. 11,718 3.25
F x T 2 19,793 9,896.5 2.74
M x T 2 31,486 15,743 4.36*
F x M x T 2 13,899£8 6,949.9 1.93
Error 227 819,234.8 3,608.9

 

* Significant at 5% level.
** Significant at 1% level.

 

 

Table 7 .

28

Thawing rates* in 0°F/min (from 20° to 40°F) of thawed blue-

berries which had been kept in frozen storage at -10°F for

 

 

 

 

 

60-68 days.
Temperature of the thawing medium
Freezing medium Thawiig nedium 80°F 90°F 100°F
Air-blast Air-blast 8.49:0.31 10.73 r022;10.80:».0.25&l
-40°F/10 min. Sucrose
(22.5% solution) 24.94r0.24 40.84 r0.32 55.28r0.40
30% Sucrose Air-blast 6.95:0.20 8.92 r0.20 10.9lr0.22
solution
0°F/4 min. Sucrose , 35.50r0.29 55.71 i0.45 68.09r0.50

 

 

 

 

 

 

 

 

* (3 berries/experiment x 2 replicates thawed; n = 6)

+ (Means of thawing rates followed by like letters alog the rows in the
above Table are not significantly different P 2 0.05, Duncan, 1955)

3-way analysis of variance of the main effects and interactions

 

 

 

 

 

 

 

 

 

 

 

Table 8.

. in Table 7.

Sonrce of variation df SS DB Fs
Total 23 10,491.28 -.
Replicate 1 0.06 0.06 0.02
Freezing median (F) 1 121.20 121.20 30.84**
Thawing medium (M) 1 7,270.54 7,270.54 18.50"

Emu 8"“;ng (T) 2 1,544.41 772.21 l96.49**
F x M 1 56.08 56.08 14.27**
F x T 2 87.80 43.90 11.17**
M x T 2 1,282.36 641.18 163.15**
F x M x T 2 85.60 42.80 10.89**
Error 11 43.23 3.93

 

 

 

 

 

 

 

** Significant at 1% level.

29

In both.Tables 3 and 7, increasing the temperature of thawing
nediunfron80°tolOO°F1edtohigherthawingrates. Thetrendcb-
served showed that increasing the temperature caused increase in the
rate of heat transfer and hence, rate of thawing.

These observations made on the effects of thawing:medium.and
thawing temperature on thawing rate support previous observations by
Fennema and Powrie (1964) that the rate at which a food material freezes
or thaws is influenced by several factors amongst which are the temper-
ature differential between the product and the cooling or heating medium;
and also the means of transferring heat energy to, from, and.within the
the product (conduction, convection and radiation).

Selected thawing curves from which the thawing rates were calculated
are illustrated in Figures 1, 2, 3, and 4. The curves are designated
by two letters and a number in which the first letter represents the
freezing medium.(air-blast, 50% by weight propylene glycol solution
or 30% by weight sucrose solution) and the second.letter represents the
thawingzmedium.(aireblast, 50% by weight propylene glycol solution or
22.5% by weight sucrose solution) while the number represents the thawing
temperature. It could be Observed from these graphs in general, that
curves which represent thawing in solutions (viz propylene glycol and
sucrose solutions) are steeper in slope than corresponding curves which
represent air thawing: i.e. the forner yielded faster thawing rates than

the latter due to the reason given above.

Effect of Frozen Storage Period on Textural Evaluation

Differences in Instron measurements were observed for thawed blue-
berries subjected to the same freezing and thawing treatments but
different frozen storage periods at -10°F (see Table 9).

Tenperature of blueberries , in °F

50

‘40-

30

20

10

30

 

 

8
N
(F g" Q
q? «9 @999
. é é” P13. 10
(P
E!
First letter represents freezing medium.
Second letter represents thawing median.
, A = Air, P = Propylene glycol solution.
/ The number represents the teuperature of
0 l 2 3 4 5 6 7

Timeinthannring,inmirmtes

Figure l . Rate of increase in tenperature at the gecmetric
centers of blueberries which had been kept in frozen
storage at -10°F for 2-3 days arnd thawed in both air-
blast and propylene glycol solution (50% by weight)
at 80°F and 100°F and also in room air at 70°F.

flrnperature of blueberries, in °F

 

31

 

8° 8
q)
50]» E" 43'
Q
1:53
g
.. 0
40 qfl~%
30«-
207'
K21
1°"’ First letter represents freezing'mediumm
, Second letter represents thawing medium.
' A.= Air, P = Propylene glycol solution.
0 The number represents the temperature of
thawing’medium.
-10- 7L % L L .L f i
o l 3 4 5 6 7
Time in thawing medium» innndnutes
Figure 2. Rate of increase in temperature at the geometric

centers of blueberries which had been kept in frozen
storage at -10°F for 2-3 days and thawed in both
air-blast and propylene glycol solution (50% by
weight) at 80°F and 100°F.

 

Teuperature of blueberries, in °F

 

 

32

 

 

Ker
10 / First letter represents freezing median.
Second letter represents thawing medium.
A = Air, S = Sucrose solution. The
nunber represents the temperature of
0« - thawing medium.
'10 i t : e 4 : ,4
0 l 2 3 4 5 6 7
Time in thawing median, in minutes
Figure 3 . Rate of increase in tenperatnrre at the geanetric centers

of blueberries which had been kept in frozen storage at
~10°F for 60-68 days and thawed in both air-blast and
sucrose solution (22.5% by weight) at 80°F and 100°F
andalsoinroonairat 70°F.

Tenperature of blueberries, in °F

 

 

 

33

 
 

 

O
O
H
an
50 (F U) QQ
<50
5?
40 .
30 9
20
521’.
10 First letter represents freezing median.
Second letter represents thawing median.
A = Air, S = Sucrose solution. The
narber represents the terperature of
0 ‘ ' thawing median.
-10 + e i t i i f
0 l 2 3 4 5 6 7
Time in thawing median, in minutes
Figure 4 . Rate of increase in temperature at the gecmetric

centers of blueberries which had been kept in frozen
storage at -10°F for 60-68 days and thawed in both
air-blast and sucrose solution (22.5% by weight) at
80°F and 100°F.

34

Table 9. Onedway analysis of variance on the effect of the frozen
storage period on the firmness (viz coupression force
in GM using the Instron) of thawed blueberries.

 

‘Tfeatment imposed

Frozen Storage

mans of Instron

 

 

 

 

 

 

 

 

 

 

on the blueberries Period :measurement in gm. 5

Frozen in air-blast, 2 '— 3 days 255 88

thawed in air-blast **
at 80°F 60 - 68 days 459 60

Frozen in air-blast, 2 - 3 days 279 92

thawed in air-blast *
at 90°F 60 - 68 days 419 48

Frozen in air-blast, 2 - 3 days 315 71

thawed in.air-blast **
at 100°F 60 - 68 days 448 _ 67

 

 

+ Storage temperature was -10°F
* Significant at 5% level
** Significant at 1% level

35

Berries stored for 2-3 days had lower Instron values (less firm)
thanberries stored for 60-68days. Thismighthavebeenduetothe
fact that more dehydration occurred in berries stored for 60—68 days
(for sore fine ice crystals were observed along the inside walls of
the storage tin cans). Dehydration might have become manifested as
increase in firmness or Instron measurenent, for Karel (1975) remarked
that the most cannon quality defects of dehydrated foods include tough,
"woody" texture, slow and inccmplete rehydration and loss of juiciness
typical of fresh food; and that though the physiccchenical basis for
these changes is as yet not fully understood, it is thought that in
the case of plant materials , loss of cellular integrity and crystalliz-
ation of polysaccharides such as starch and cellulose is , in fact,
promoted by renoval of water.

It should be pointed ont, however, that berries stored for 2-3
days came from a different batch of raw material than those stored
60-68 days. Textural differences between the two may, therefore, have
resulted frcm raw material differences rather than storage changes .

Objective Versus Subjective Methods of Texture Evaluation

Orne-way analysis of variance of the data in Table 10 (see Table
11) showed that the objective method enployed, viz conpression force
ingmcbtainedbyusingtheInstronwasverygood for itdetecteddif—
ferences in means of measurements which are significant at both the 5%
and 1% levels of significance, while the subjective methods employed.
viz sensory evaluation by taste panel using teeth to out thawed berries
or pressing thawed berries between fingers did not detect differences
in firmness to be significant. However, the values obtained (using the

 

36

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table 10. Comparative data on finnnness' (as measured objectively using the
Instron* and subjectively“ using a taste panel:) of thawed blue-
berries which had previously been frozen and kept in frozen
storage at -10°F for 60-68 days.

Means of. firmness measurements

Temperature Instron Taste panel Taste panel

Freezing ' Thawnng' of the (Conpression (Ontting with (Pressing be-
medium median thawing median force in Q) teeth) tween fgggp’ s)
Boon air 70°F 432 e 7st 2.88 + 1.73 3.25 ; 1.03 f

nefri erator 42°F r438 t 8881) 2.38 e 0.92 3.25 .+. 1.49,

Alr‘blast 80°F 459 e 60Ab 3.12 4: 1.5L 3.79 i 1.10

Air-blast 90°F 419 i 48131D 2.35 i. 1.14 3.40 r 1.29

-40°F/10 min. Eb

100°F 448 r 67 2.41 .4: 1.05 3.50 e 1.23

Sucrose 80°F 436 : 72‘3b 2.37 e 1.41 2.34 e 1.08

(22.5% 90°F 469 r 74AID 2.45 e 1.18 3.55 t 1.05

solution) 100°F 479 : 56Aa 2.34 t 1.17. 3.09 i 1.45

80°F 446 t 733b— 3.34 r 1.25 3.64 t 1.52

Air-blast 90°F 436 2 611375— 2.40 i 1.05 3.15 t 1.10

30% Sucrose Bb
0

solution 100 F 441 i: 60 2.65 r 1.29 3.49 r 1.31

0°F/4 min Sucrose 80°F 487 i 52”“3 2.65 i 1.12 2.91 i 1.33
(22.5%: __ 90°F 5.14:34M 1.95 r 0.75 2.85 r 1.29#

”Int-1°“) 100°F 462 t 77Ab 2.21 t 1.06 2.77 .+. 1.40

 

 

 

 

 

 

 

 

 

* (1 compression/berry x 10 berries x 2 replicates themed;

extremely firm 8 9, extrenely soft = 1)

11+

(10 panelists/taste panel 1: 2 replicates thawed; n = 20)
(Means followed by big like letters are not significantly different,

n=20)

(Use of a nine-point hedonic scale to rate responses of taste panel,

viz

P 2 0.05, while means followed by small like letters are not significantly

different P 2 0.01, Duncan, 1955)

 

37

Table 11. One-way analysis of variance of the means of finnness (as
measured objectively using the Instron and subjectively
using a taste panel) of themed blueberries after 60-68 days
of frozen storage at -10°F.

 

 

 

 

 

Method used in measuring firmness F3
Instron (coupression force in gun) measurements **
Taste panel (cutting with teeth) ratings ns
Taste panel (pressing between fingers) ratings ns

 

 

 

** Significant at 1% level

38

hedonic rating scale) for finger-pressing were, in the majority, fournd
tobeconsistentlyhigherthanthecorrespondingvaluesobtainedfor
cutting of thawed berries by teeth.

The one-way analysis of variance also showed that the meanscf
firmnessmeasurenents usingthelnstron, forberriesthawedinroomair
and refrigerator fitted into the second group of thawed berries that
were less firm.

The values obtained for the coefficients of correlation between the
objective and subjective methods of texture evaluation were significant
at the 5% level when frozen blueberries were thawed in air-blast but
were not significant when berries were thawed in sucrose solution (see
Table 12) . The degree of correlation between the Instron measurements
and finger-pressing evaluation of air-thawed berries was found to be
higherthanthatbetweenthelnstronmeasurementsarxiteeth-cuttiig
evaluation of the air-thawed berries. This might have been due to the
fact that the finger-pressing method was more similar to the mode of
operation of the Instron in that both measured compression forces only;
while the teeth-cutting method was not able to detect firmness alone
for several complex sensationshadbeenfoundtobeinvolvedinoral
evaluation of foods.

It could also be seen in Table 12 that the coefficient of correla-
tion between the two subjective methods of texture evaluation (viz
teeth-cutting ard finger-pressing) is only significant for blueberries
thawed in air-blast only.

The relationship between Instron measurements arnd thawing rates
(in Tables 1, 3, 5, and 7) was analyzed using linear, Nth order, geo-

metric and exponential regressions. Only the Nth order regression

 

39

Table 12 . Correlation coefficients between the means+ of objective and
Subjective finmess measurements on thawed blueberries after
a frozen storage period of 60-68 days at -10°F.

 

-w. a? Cf‘m’

 

 

 

 

 

 

 

 

 

Pair of means of firmness being ' Sfteimgf Coeff. of m
, correlated ation Correlation estimate
ggjfrggdvgécggggfigggggigs for b°th 0.101 0.286(ns) 0.383
igjfiggfizzé teath’CUtting f°r Only 0.40 0.641(*) 0.365
.:3::::2_Z:éw::;th’cutting f°r °“1Y 0.142 0.377(ns) 0.244
ggifirgirzsAngigggigngifiisgngé; 0.072 0.229(ns) 0.438
:3:§F§§£Z:fia£iflgér‘9resSing f°r 0.470 0.686(*) 0.177
ggigrgficzgéeff?ger‘pressing f°r 0.127 0.357(ns) 0.415
gggtggfifitzigfi Zia iiflgiéefifiiiifig 0.338 0.581(ns) 0.431
gzfitggjgtzigfitgzéifiéfiger'presSing 0.463 0.681(*) 0.311
ggftggigttgggrgzé_finger‘Pressing 0.045 0.214(ns) 0.477

 

 

 

 

 

 

+ The means of firmness measurements used in calculating the correlation
coefficients are based upon the data in Table 10.

* Significant at 5% level.

4O

analysis was reported (see Table 13) for it was the only analysis that
gave significant results at the 5% level, for thawed blueberries which
had been kept in frozen storage at -10°F for 60-68 days. This Nth order
regression analysis revealed that the following equation (using the
data in Table 13) illustrates the relationship between texture evalua-
tion (represented by Y) in gm compression force using the Instron and

thawing rate in °F/minute (represented by X):

y = 421.84 + 2.09x- 0.02142.

41

Table 13 . Nth order regression analysis of the relationship between
Instron measurerent+ (viz cotpression force in GM) and
thawing rates* in °F/mnin of thawed blueberries

 

 

 

 

 

 

 

 

 

Frozen storage period at -10‘F

3993:9555“ data 2 - 3 days 60 - 68 days
0 Deg. Coefficient 270.44 421.84
1 Deg. Coefficient 3.62 2.09
2 Deg. Coefficient -6.13 -0.02

F 3 (Regression Table) ns *

Coeff. of Determination 0.30 0.54

Coeff. of Correlation 0.55 (ns) 0.74 (*)
Standard error of estimate 25.77 19.60

 

 

 

 

 

* Significant at 5% level
+Themeansof Instronmeasuretentandthawing ratesused inthis
analysis are based upon the data in Tables 1, 3, 5, and 7.

CCNCLUSIQ‘IS

 

Blueberries which had been hand-harvested at DeGrandchamp, Sonth
Haven, Michigan, were frozen, stored and thawed using various methods,
ard subsequently evaluated (using both objective and subjective methods
of measurement) for textural quality.

It was fond that only the freezing median amongst the factors
studied, within the ranges and under the conditions that were tried,
had a significant effect upon the final texture of the thawed blue-
berries after 2-3 days of frozen storage. It was also fomd that only
thethawingmediamhadasignificanteffectuponthe finaltextureof
thawed blueberries after 60-68 days of frozen storage.

Differences in Instron-measured firmness due to the interaction
between the thawing median and tenperature of thawing median indicated
that the relationship between these two factors was not constant, but
instead, varied with the test coditions that were used.

All the variations in the factors studied, however, exhibited
significant effects on the thawing rates. For instance, differences
in thawing rate were associated with differences in freezing median,
thawing median, and terperature of thawing median regardless of the
period of frozen storage. Differences in thawing rate due to the
interaction between two orall of the three factors mentioned above
indicated that the relationships between these factors were not con-
stant, but instead, varied with the test coditions that were used.

Generally, faster thawing rates were obtained for the blueberries
frozen in liquid media than in air-blast. Faster thawing rates were
also obtained for the blueberries thawed in liquid media than in

42

43

air-blast. Faster thawing rates were also obtained for the blueberries
thawedinliquidmediathaninair—blastandthiswasattributedto
the fact that liquid media have higher convective heat transfer coeffi-
cients than air. Increasing the thawing temperature from 80° to 100°F
alsoledtohigherthawiigratesduetoincreaseintherateofheat
transfer with increase in temperature .

Under the test coditions used, the objective method (Instron
measurement) was more reliable for evaluating texture of thawed blue-
berries than any of the subjective methods , viz sensory evaluation
(cutting with teeth and pressing with fingers) by taste panel. The
objective method detected significant differences in means of measure-
ments at the 1% level of significance, while the latter subjective
methods did not detect any significant difference in the firmness of
the thawed berries. Of the two subjective methods studied, the finger-
pressing method consistently gave higher values of firmness measurement
using the hedonic rating scale, than the teeth-cutting method.

Though the correlation coefficients for objective versus sensory
methods of texture evaluation were significant at the 5% level for
berries thawed in air-blast, the values were relatively low and, con-
sequently, did not indicate very good correlation between objective
and sensory methods. The coefficient of correlation between the two
subjective methods of texture evaluation was found to be significant
only for blueberries thawed in air-blast.

When subjected to an Nth order regression analysis, the relation-
ship between Instron measureents (Y) and thawing rates (X) was found
to be significant at the 5% level, but only for those berries that had

44

been stored for 60-68 days. The Nth order analysis indicated that
the relationship could be expressed using a second order polynomial,
viz Y = 421.84 + 2.09x - 0.02xz.

l.

45

SUGCESI'ICNS FOR FURTHER STUDY

Investigatingifaonangeintheweightperberrycconrswhen frozen
blueberries are subjected to a thawing method, and the possible
effect of a change in weight on texture measurement.
Investigatirginagreaterdetailthanwasdoneinthis study;
thawing of frozen blueberries in air-blast, for this median would
hopefully facilitate better the measurements of drip loss and
weight change than a liquid median such as propylene glycol or
sucrose solution.

Determination of the part of the thawing regime which mostly limits
texturalqualityofblueberrieswhenthawedinstillairatroom
temperature; forthisportionofthethawingcurve, ifitcanbe
detected would represent the critical area that needs to be more
closely studied in order to alleviate deterioration in textaral
quality of frozen and thawed blueberries.

APPENDIX

47

.Michigan State university - Fruits Laboratory (Finmness test)

Directions: Please analyze sample provided for firmness by pression
each fruit between the thumb and fingers until it ruptures.

 

 

 

 

 

 

Name: Plate No. Date
code Code Code Code
Extremely Ektrenedy' Extremely Ihdnrmedy'
firmn firm) firmn finm
very very very very
firm. firm. firm. finn
moderately Mbderately .Moderately Moderately
firm. finm finm firm
Slightly Slightly Slightly Slightly
firm. firm. firm. firm
Neither firm. Neither firm. Neither firm. Neither firm
nor soft nor soft nor soft nor soft
Slightly Slightly Slightly Slightly
soft soft soft soft
.Moderately :Moderately .Mcderately .Moderately
soft soft soft soft
very very very very
soft soft soft soft
Eamrenedy’ Extremely Extremely Indotnedy

soft soft soft soft

(Imments Cknments Ctmments Cknments

 

 

 

 

 

 

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