PECA-=NAICAL COViPRESIONI
FREEZE-DRYING
OF FOOD PRODUCTS
THROUGH
FORCE
DURING
PRODUCED
BY SPRINGS
by
SE ID-
t
OSSEIN
EIMAIvAI
B.S. Food and Nutritional Sciences,
University of California, berkeley,
1974
Submitted in Partial Fulfillment
of the Requirements
for the
egree of
Master of Science
at the
IASSACtUSETTS INSTITUTE OF TC-I'iOLOGY
January, 1976
Signature o I
1-UIULI[1O.[.
Depart 6 nt of jN
u ar-'uci:an:t
&o
_,,-I
Certified by
.
t'i
r).lA.
cience K
-.y ,
-L-.I (
Thesis Supervisor
I;,
Accepted by ..
Chairman,
............
epartmental Committe on
ARCH!
,:-S
MAR 10 1976
_
L11e
raduate
tudies
-2Wiv;:t,4A.I'ALCOl'iVTES-ZSIODRYlG A
FROilUC:Si URIi.1-
O0F
'Y:-.OU(.H- FORCE
PRODUCE
FREZE
-
.B.Y ERISG
by
Seid-Hossein
mami
Submitted to the Department of Nutrition and
ood
cience
on January 21, 1976 in partial fulfillment
of -therequirements for the degree of iaster of
cience,
ABS!TRACT
Freeze-drying has been successfully used in preparation
of high
disadvantage
quality dried foods; however, a major
is the resulting low product density.
This
study has investigated the possibility of compressing
food materials during freeze-drying to increase their
bulk density.
T'hedependence of degree of compression on compressive
force, dry sample moisture contents, and rehydration behavior
were determined for a variety of vegetable,
materials.
fruit, and meat
A simple spring driven device was used to supply
the compressive force during freeze-drying.
The results indicate that the extent of compression
depends on the physical characteristics of the food material
-3-
and increases linearly with increasing compressive
force.
The compression process does not prevent dehydration of the material to low moisture contents.
Compressed samples rehydrate readily to their
pre-compression characteristics (except blueberries)
and the extent of' rehydration does not depend on the
degree of compression (except beef cubes).
Organoleptically compressed samples are not
significantly different from non-compressed freezedried foods.
Thesis Supervisor:
Title:
ir. James Iv,. 'link
Assistant Professor
of Food Engineering
-4CK'!O'QWLED('.ELIET
Although this is an early page of this paper, but is the last
and hardest to be written, since I have never been able to express
my feelings with words.
ibonetheless, I will make an effort -to
express my thanks to all those who have made this work possible.
[¥~y
debts of gratitude to
rofessors
iarcus Karel and James
.. Flink who have made it possible for me to be where I am today,
and my sincere thanks to my thesis advisor Professor James Flink
for
his continuous guidance and advices, for without him lots
of opportunities I have today would seem very hard.
Thanks are also expressed to James Hawks for drawing the
figures and
ynthia Cole for helping throughout the experiments.
I would like to appreciate the friendly assistance of the
members of the Food Engineering group throughout the completion
of this work.
I am deeply greatful to my dearest wife, Zarrin, for without
her constant help this work may have never been completed.
This study is dedicated to my wife, and my dearest
parents.
This project was supported by the U,,
Army ?Satick
Development Center, Contract io, DAAK 03-75-C-0039.
-5OF COTEi'lNTS
TABLE
TIILE PAGE
ABSTRACT
LIS
.
.. . . . .
.
.
.
0
.
2
.
i)(EMENTiS.
.
. .
4
OF COIl'"EN'TS
.
.
5
ACKNOWL
TABLE
.
1
.
.
O FIGURES . . .
EXPLAFNATIO,:
3-7
FOR T'ABLES,
LIST OF TABLES .....
1.
INTRODUCTION .
2.
SURVEY
LITE'l'sTURE
~ ~ ~
LI~~~~·~~~···~
.
I··
I··
·
·
.
10
I
~~~~~~~~~~~S 12
.
.
.
.
.
.
.
13
.
.
.
.
.
.
.
14
.
.
.
.
11
.
.
18
2.1
Freeze-Drying.
18
2.2
The Frozen State and the Ice Front . , .
20
Evidence for a Diffuse Sublimation
22.1
2.2.2
. . . . . . . .
.
24
State of Water and of Solutes in Frozen,
and
2.4
21
Evidence for a Sharp Sublimation
Front
2.3
. . . . . . . .
.
Front
. . . . . . . .
Systems
Freeze-Dried
27
Heat
iethods of Compressing Dehydrated Foodstuff .
2.4.1
2.4.2
2.4.3
.
.
.
.
iioisture
.
.
Content.
Treatment.
.
.
.
.
.
. .
.
.
.
. . .
30
.
.
.
.
. . .
39
. .
.
.
.
.
29
Hinders and Other Plasticizing
agents.
2.4.4
.
.
blanching
.
. . . . . . . . . . .
41
. . . . .
46
(Scalding)
.
-l-
D ;e
2,.i.
ffect of F'reezingRates on
Compression of Freeze-lried
Foods
2,4,6
.
Continuous
Compression.
2.4.7
3.
3.1
Di 'v '
Equipment
3,1.1
S.
3
3,1
. .
4,
RESUL'
4
.
.
53
.
. . . . .
Sample
older .
Procedure
.
. . . . .
Sample Preparation .
3.2.2
Freezin .
3.2,3
Loading of the
3,2,4
Compression
3.2.5
?IoistureContent,
Rehydration.
3.2.?
Organoleptic
3.2.2
Storage
Presentation
4. 2
Vegetables
of
.
. 63
'· 63
. . . .
otal Solid
. . . . . .
Test .
. . . .
.
. . . .
.
DICUSSIO
57
66
. . . . . . . .
Stability
Treatment
4.1
.
,.5
. , 57
ell.
Ratio .
3.2.6
. .
. . . . . .
3.2,1
A
4Q
3
Design of a Static Loading We1 1. . 53
Experimental
Data
. . .
. . .
Determination
3 3
. . . .
. . . . . . .
Freeze-Drier
l,1.2
3,2
53
*
.........
reeze-,ryin- and
Limited Freeze-Drying, .
'A'ERIALS
47
. . . . . . . .
. . . . . .
67
'
'
68
68
'
'
69
. . . 70
Results
. . . . . . . .
. . . . . . . . . . .
70
. . . 70
.
Beans
4.2.1
-7-
lage
. . . .
. . . . .
4.2.1.1
Compression Ratio
· 72
4,2.1.2
i.oistureContent.
72
4,2.1,3
Rehydration
. . . .
. . . . . . . . .
Carrots .
4.2.2
· 79
4.2.2.2
ioisture Content,
· 79
4,.2.2.3
Rehydration
4.2.2,4
Organolertic Lvaluat ion
. . . .
5
4,2.3.2
Content.
lv:oisture
· 8i7
4 2,3.3
Rehydration
4.23.4
Organoleptic
. . . .
.
.
.
.
.
valuat ion * 90
.
.
.
.
.
.
.
. . . . . . .
.
' 92
'92
4.3.1.1
'ompression Ratio
'92
4.3.1.2
ivoistureContent.
94
.
.
. . . . . .
.
Cherries
.
.
Organoleptic Evalua.t ion
4.3.1.4
4,4.1
'89
' 90
4.3.1.3 Rehydration .
.
85
Compression Ratio
blueberries
4,3.2
X
4.2.3.1
Corn.
4.3,1
· 82
. . . . .
-eas .
ruits .
i eat .
' 79
Compression Ratio
4,24
L4, 4
· 74
4,2.2.1
4.2,3
4.3
* 70
.
.
.
-,round Beef.
96
96
'99
100
.
...
.
100
-
4.4.)
teak. .
Bottom Round
. .
4.4.2.1
Compression Ratio. .
4.4.2,2
ioisture Content .
4.4.2.3
Rehydration
.
4.4.2.4 0ranoleptic
4.
General
(
.
Discussion
.
6
FCOREU'URL
Thicknes
R:.o,.
.
.
04
115
107
117
Ratio on
117
. . . . . .
Lffect of Compression during
6,3
_ffect
Layer
reeze-
Drying
on rying
Rate
. , ...
117
of S'ample 'emnerature (rozen
and ry Layer) on omnpression
Behavior
.
.
. .
.
.
.
.
.
.
.
.
.
Dependence of Compression Ratio on
Sample Loadin~.
.
.....
115
6.5 use of iTdnders and Flasticizin
6.6
10o
107
.....
6.2
6,t4
.
valuation. 105
. ..
.1
Dependence of Compression
Sample
.
101
.. . .
.CO'CuLU;
i,, .I.O.
...
*.
6; .3UihSIiKo
101
Desi.-n of a Sample Holder
Removal" of Compressed
Agr-ents .119
for "'asy
ars. . . .
.
120
6.7 Freeze-Drying/Compression with Other
Food
}Iaterials
BIBLIOGRAPHY.
APPEi!DIC IES:
.
'ests
Appendix
.
.
.
.
.
. ...
Appendix
121
. . . . .
'l'abulated Results of
A:
.120
'2eydration
and Organoleptic Evaluations,
Veetables.
-1:
Beans
.
.
.
.
.
.
.
.
.
.
128
131
131
.
.132
-9-
Page
Ap-nendix
A-2:
.
Carrots
/Avjendjix A-3:
.
Peas
.
.
.
.
.
.
.
.
. 138,
...
.. . .146
A.pendix
C:
.
.
Appendix
Appendix r:
A-4:
Corn
.
.
.
.
'ruits.
Aweoend
ix S.-1: blue berr ie s .
L;eat
.
.
.
.
.
.
.
. . . .154
.
.
.
. .16o
Arendix
C-I: Giround. Beef.
.
.
'Aeendix
C-2:
..
. . .. 162
jeef Cubes .
Explanation of Rehydration Tests .
xplanation of Organolentic
. .
Evaluation
*...
.
. .
.
162
129
.
130
-10LIST OF FIGURES
o0,
ritle
Eage
1,
Loadin
2.
jample
Holder ....
.. . .. .
3.
Frinciple
ment.
b.,
Cel
.
teps for each
.
.
.
.
ConfiFuration
~-older ...
5.
. . . . . .
.
.
.
.
.
54
58
xperi.
.
.
.
59
of beansin Sample
.
61
Effect of Pressure on Compression
Ratio
of
eans
.
. . . . . . .
73
Rehydration of Compressed and
Non-Comnressed eans at Differ-
ent Temperatures.
7.
Effect
of Vacuum on
.
eans .
of
. .
775
.
ehydration
.
.
.
.
.
.
.
P
Effect of Compression Ratio on
Rehydration of Beass..
, .
9.
Effect of Pressure on Comnression
of Carrots .
.1..
10
iffect
of Compression
Rehydration of
11,
13.
Ratio on
arrots. , .,
,
of Peas
84
. . . . . . . . . .
Effect of Compression Ratio on
Rehydration of eas .
..
Effect of
-atio
14e
.
1
Effect of Pressure on Compression
Ratio
12,
76
of
91
ressure on Comcression
lueberries.
.
. . . .
L-.iffect
of Compression Ratio on
k-eahydration
of lueberries, . ..
9
97
-110oI
15.
16.
itle
Effect of Pressure on Compression
Ratio of beef Cubes.
...
.
ffect of Compression Ratio on
Rehydration of BeefCubes . . . .
Pase
103
106
-12-
Explanation for Tables 3-7
Notation
Rehyd.
Rehydration
Comp. Ratio
Compression Ratio
M.C.
Moisture Content
T.S.
Total Solids
Ave.
Average
wt.
Weight
Ht.
Height
Comp.
Compressed
N.Comp.
Non-Compressed
-13LIST; O? Y
:\
0.
1
'ALL~
Title
P
FIao e
of
prings
used .
.
ronerties
(LU
25
iaterials
3.
Dearns,(Experimental
4.
Carrots, (Experimental Results)
.
5:
(as measured)
.
.
.
esults)
71
.
80
.
.
.
86
Results)
93
Beef Cubes, (Experimental Results).
102
Lehydration Results for Gireen L eans
132
Aehydration Results for Carrots . .
138
10.
Orpanolertic Evaluation of
145
11.
Rehydration Results for Feas .
12
Or>anoleptic
13
Rehydration Results for Corn .
14.
Rehydration Results for Blueberries
Peas,
(Experimental
Results)
Blueberries, (xperimental
7,
7.
9.
1 .
arrots.
.
.
146
valuation of E1eas
Or anoleptic
151
.
.
152
valuation of
.
Elueberries
.
.
.
.
.
.
.
159
1 6.
Rehydration Results for Uround Beef
I 1
1'7,
Reh-vdration t•esultsfor Beef Cubes.
162
1.
Oralanoleptic valuation of ieef
Cubes
.
.
.
.
.
.
.
.
.
.
.
.
.
.
167
-141.
INT RODUC 'IOIN
Freeze-drying has been recognized as the method of
dehydration giving dried products having the best quality
and retained nutritional value.
This technique is now
begining to emerge as an important method of dehydration.
However, when foods undergo freeze-drying very
little change in the physical shape of the material occurs.
This results in an open and porous structure which yields
a very light and fragile product of low packaging
density.
The disadvantages related to low packaging density can be
summarized as:
-The oen structure is very susceptible to
oxidation and moisture pickup; and thus
special packaging is often required.
-Large ackage volumes are required
close small product weights.
to
en-
-Readily friable products produce fines
during handling and transportation which
is essentially wastage of the products.
-Large stora:e space is required.
A more efficient food system will result by eliminating or minimizing the above mentioned effects.
this can
be accomplished by compressing the food to remove -thevoid
volumes which have been produced by sublimation of ice
crystals during the drying process.
-15To accomplish efficient compression, it is necessary
to have the food solids in a plastic state, so when high
compression pressures are applied to the product fragmentation can be avoided.
This has been achieved either by
thermaly plasticizing the dried food, which requires that
the food should contain a relatively high sugar content,
or by controlled humidification of the dry product to a
uniform moisture content in the range of 5-20 percent.
Depending on physical characteristics of the food there
is an optimum moisture content range for best compression
behavior., When the controlled humidification process is
used, the added water has to be removed from the compressed
wafer or bar to
ive long-term storage stability.
If it should be possible to remove the sample from
the freeze-dryer when a uniform moisture con-tent of the
desired level is reached, the initial overdrying and humidification steps of the controlled humidification process
can be eliminated.
However, with ordinary freeze-drying
methods this is not possible since the remaining water is
concentrated in the center of the freeze-dried sample as
an ice core.
By allowing
the ice core to melt for redis-
tribution in the sample the region of the core will have an
air-dried character and may be hard and discolored. Furthermore different pieces dry at different rates which makes
-16it hard to reach the optimum
pieces at the same time.
moisture
con-tent in all
(Hoge and Filsworth, 1973)
Recent studies indicate that with controlled
freeze-drying or modified freeze-dryers, it is possible
to reach a predetermined uniform moisture content throughout -thesamples.
However, this will have a significant
effect on the rate of freeze-drying and the final drying
after compression is still required.
Previous investigations of moisture profiles during
freeze drying indicate that over a narrow thickness of
the dry layer adjacent to the ice interface, high moisture contents exist,
(Aguilera and Flink, 1974)
This study is investigating an
approach in which
it is assumed that the moisture gradients present; during
the freeze-drying will
ive sufficient plasticity to allow
compression during "normal" freeze drying.
This method for production of compressed freeze-dried
foods should be the most efficient, since it does not require either overdrying and humidification or operation
at much reduced dry layer temperatures.
1.
Investigating the feasibility of simultaneously transmitting compressive force
while allowing vapor transport.
2.
Determining the effect of compression
during freeze drying on the final moisture content.
-17-
3.
Determinin7 the relationship between compressive force and compression ratio.
4,
Determining rehydration behavior of compressed samples (versus non-compressed
controls) in terms of time dependence
and percent of rehydration in reference to
the ratio of:
iH20
Total Solids (Rehydrated)
:'r
5.
gr H20
/
" Total
olids
r(resh)
valuations of the acceptability of reconstituted compressed samples (versus noncompressed freeze-dried controls and fresh
samples.)
-182,
L '.i.'1
E:fAr'TL`
TSSURVEY
.ost studies orn compression of dehydrated foods to
reduce bulk volume
ave been conducted with freeze-dried
r,roduct. :Sincethis study investi:gatedcontinuous compression of food durin-: freeze-dryin- some basic aspects of
the freeze-dehydration technique will be presented before
concentratin.-on a discussion of'methods of food compression.
2.1
Preeze-Drin:
'Vacuum freeze-drying on a lar.?erthan laboratory
scale was first developnedfor the preservationof blood
plasma and later penicillin durin1949).
orld i,"arII (-'losdor,
oday this process is widely used for the preser-
vation of a variety of substances including:;antibiotics,
vaccines, virus cultures, pharmaceuticals, and on the
largest scale, food-stuffs.
'eneral descriptions of freeze-drying have been d:iver
by many authors
and
('owe, 1964; Rey, 1964; Va-rArsdel, 1964;
otson and "mith, 1962).ihree comrehensive
of freeze-dryinz.research have beer published.
and I"apel, 1957;
reviews
(arper
ruke and Decareau, 1964; King, 1971).
Kin7 (1971) especially concentrates on -thepertinent factors
which interact to -Tovernrates of freeze-dryinrT.
1"e p)riincipleof freeze-dryin.-r is to reduce -t'.e
temperature
-19-
of the product to below its eutectic proint, a temperature
low enour:h to esure complete solidification of all
constituen-ts,
and then -to lowEer-the surrondin-
vapor ressure to below the saturated vaor
water
ressure at
-this temperature.
Under these conditions water vapor
will tend to be trans-orted from the food to the "surrondins",
'io accomrlish tihis sublimation
of sublimation
(about 1200 '/lb
from a heat source.
of ice the heat
of -'20) must be supplied
i?'iis can be achieved
by radiation
the outer dry layer of a food bein~irfreeze-dried,
to
by con-
duction trouh
'tlhe frozen layer or by internal eat r-eneration (microwaves). It is also necessary to remove tnhe
water varor enerated by sublimation by havin' a moisture
sink (f'inni, 1971).
-dvanta-es of t,-is technique include -tie low processinq:
tem-erature, te relative absence of liquid water and the
rapid
t ransition
of anrvlocalre--ior
of the material
from
a fully 'hydrated to a nearly completely dehydrated
state.
"this rapid -transitionminimizes the extent of various
dative reactions
lwhich
often occur durinn- dryin?.
tem,?eratures involved also
deyrai'he low
elp to minimize
thesereactions
and to reduce mass trans ort rates which cortrol
of volatile flavor and aroma secies,
the
loss
';heabsence of the
liluid state hel*psto minimize de rada.ive
reactions
ad
-20-
prevents the transport of soluble species from one
region to another within the substance being dried,
(Kingf,
(i.e. it inhibits surface concentration).
1971)
Despite all these advantages, freeze-drying has some
drawbacks which include:
high initial capital invest-
ment, high processing costs,
slicing,
special
etc...) of some products
(cutting,
treatment
prior to drying to
improve dehydration, and producing products of very low
density.
22 2
(P. Lon-more, 1973)
he Frozen State and the Ice Front:
'Themost important change that occurs during freez-
ing is the appearance
of at least one new phase
conversion of water into ice).
1.
2.
3.
4.
5.
6.
7.
(i.e.
Other changes include:
Dehydration of the insoluble solid material.
Coalescence of droplets of immiscible liquids.
Crystallization of some solutes.
Formation of amorphous precipitates of other
solutes.
Exclusion of dissolved gases.
Formation of concentrated solute glasses.
Distruption of molecular complexes (lipoprotein),
(ivlacKenzie,1965).
Thus the ideal frozen state would show a sharp and
discrete dividing surface between a region which consists
of ice crystals and a region which consists of concentrated
amorphous solution.
During freeze-drying the ice region retreats inwards
as the drying proceeds leaving the dried matrix behind.
-21l';ost
analysis of freeze-dryin- rates have been made with
the
os-tulatiorof a sharp-and discrete dividingrsurface
between a region which is fully hydrated and frozen,
and a region which is nearly completely dry.
However,
there is a considerable amount of contention in recent
years relnardin7the de:-reeof sharpness of this surface
when it is retreatin. as a frozen front during freezedryin.B, Kin. (1971) reviews the evidence for a sharp
front versus evidence for a diffuse sublimation front.
He explains that a very sharp front would result if
sublimation occured from a very thin zone near the surface of the frozen re,,ion, and if the sublimation
re-
moved. essentially all of the initial moisture from the
remaininc,solid material immediately after the passage
of the frozen front from that re,?ion, This is clearly
an idealization but has been assumed by most authors
to be a
ood description of the mechanism of freeze
drvi-.r r
2.2.1
Evidence for a Diffuse
ublimation
.,;"effert
(1963) measured temperature
ront:
rofiles durin;g
freeze-drying.of rutabas7a. From the temperature and
thermal conductivities inferred from them he concluded
that some 30 percent to 40 percent of the initial
water
content was left behind by the retreating ice front.
-22'hiisremaininr ':
2 0 had to be removed by secondary drying
or desorption.
irajnikov et al (1969) indicated that the results
of some experiments demonstrated the rresence of a diffuse
sublimation front.
In one experiment he took color photo-
,:raphsof cut specimens of beef which had been freeze-dried
once, rehydrated with a solution of
freeze-dried aain.
charing
oC12 and then partially
ivalent cobalt has the property of
color from pink to blue dependin: on water activity
of its surroundings.
indicated the
L;rainikov's experiments
with Co12
resence of a -transition zone; however, Kin,
(1971) as no-tedthat the cobalt salt can lower the freezin:7
point which
could cause the presence of some liquid at the
sublimation front, even at -200C.
Luikov (1969) observed the sublimation behavior of
pure ice spheres (0
mm diameter) which were sublimed in
a vacuum chamber at -33°C.
Luikov observed that ice crys-
tallites of various shapes and len, ths (from 0.1 to several
mm) formed at the sublimation front near surface irregularities.
TI"heir rowth was observed visually and it was
found that crystallites could oscillate and rotate rapidly
at speeds up to 70rpm existin.n for 20 to 30 revolutions.
These
oscillations reflect molecular pressure from the
vapor,
escapin:r-
Often the crystallite would break from
-23the ice surface and be entrained
in the vapor at velo-
cities of .70 to .34 meter per second.
and others have
Luikov (1969)
oirted out that the very .:reat and
sudden expansion.of water from the solid to the highly
expanded vapor state upon vacuum sublimation can cause
stronr forces and unusual flow patterns.
Local uneven-
ness of temperature along;:
the sublimation front can also
cause mechanical stresses which can cause the crystallite
to break off and be entrained.
Luikov sugests
that this
entrainment mechanism can have an important effect on
the observed rate of sublimation of ice, since this would
amount to the removal of moisture without the transfer of
an ecuivalent
amount of heat to the sublimation
front.
Luikov also found that sublimation and ablimation (desublimation from vapor state to solid state) occur simultaneously
at the sublimation front of an ice sphere.
;.heoccurance
of ablimation was confirmed by Luikov in experiments involv-
ingvacuumsublimation of water vapor from spherical ice
crystals made from water containing red fucin dye. Crystallites
formed directly
from the solid
,phase apeared
but -those formed by ablimation were transparent
re(T,
and color-
less.
iThetendency for ablima.ion to occur depends uPon
the degree of saturation of surroundin- :as phase with water
vapor. Ablimation occurs to a .reater extent as the surround-
-24-
inc--'asbecame more nearly saturated in water vapor.
Ablimation
can also occur
in a supersaturated
zone
removed from the ice surface in the vapor phase.
King
(1971) states that the studies made by Luikov can account
for a transition zone with a thickness of perhaps a mm,
but not for a zone with a thickness of an inch or more.
E-vidence or a
2.2.2
,,any
ront:
harr,Sublimation
investircatorshave reported cuttin- open partially
freeze-dried specimens and observin, a distinct and relativelyv sharp demarkation between the still frozen zone and
dry zone of uniform
coloration,
'hese
include .-ardin (1965)
who studied freeze-dried beef, Ivararritis
who studied
studied
reeze-dryini turkey meat and
and
eke
inc: (1949)
(1969) who
freeze-dryin.. of pork.
Additional evidence was obtained by
atcher (1964) who
used g-ammaray attenuation measurements to monitor the
profile of moisture content across relatively thick (2 inches)
slabs of beef as a function
of -time during. freeze-dryin.
-atcher reports that visual inspection of partially dried
specimens
revealed
the ice front to be planner.
,zammaray beam diameter was -
W;ince his
inch he concluded that the
ice front was sharrer than could be detected by the beam.
He also found that the residual moisture in the dry zone
was so low that it did not affect the gamma ray count rate.
-2.5-
However, non-uniform retreat of a still sharp
ice front has been found by J;arcaritis and
in-
and
:Tatcher (1964) also has reported some preferential drying
from -the edge of his samples,
i'.acKenzie (1966) constructed
a "freeze-dryinp?
microscope" in which he could observe freeze-dryin
as
it occured in various transparent frozen liquid systems.
An. extremely sharp sublimation front was observed, althou.:h
the front, while sharp, did not necessarily remain planner.
There should be a finite partial pressure of water vapor
in the
as within te
dry layer, since the water vapor
:enerated by sublimation must escape across the dry layer.
One would. therefore expect a finite amount of sorbed water
to be present
in the dry layer corresnondin.: at least
to the amount that would be predicted by equilibrium
sorotion isotherm.
-andall
(1966) made calculations for
-tyrical conditions of freeze-dryin.
usin- the sorption isotherms
was measured by
outer
dry
'Kin
for
et al (968).
the
of turkey breastmeat
same material that
For heatin,n from the
surface he condluded that equilibrium between
the dry layer and the
escapin7 water vapor would
avera.e moisture content within the dry
percent of the initial
content
layer
ive an
equal to 3
water, or about 1,5percent moisture
on a solid basis,
"lowever, this
surprisingly
low
-26-
value results from the increasinn temperature across the
dry layer toward the outer surface as well as from the
decreasing water vacor partial pressure,
(King, 1971)
.owever, one would expect that the deQ-reeof the sharpness
of the ice front durinr; freeze-dryinr mainly reflect the
condition(s) of freeze-drying[. Important parameters which
seem directly to effect broadening:of the front are
pressure and, -temperature difference between local temperature and ice front temperature.
iralsford (1967) postulated
the existence of a "diffusion zone" broadening which was
due to solute migration which
melted as the zone recedes
more
:ivesand more ice being
into ice core.
Hie felt -that
at higher chamber pressures more liquid water is present
in the ice core which results in some liquid diffusion into
the dry layer.
Ai.ruilera
and
link (1974) calculated moisture
profiles from temperature measurements durin- freeze-dryin,.,
'Theircalculation.was based on -therelationship between
bound water content ()
and temperature difference
(
D)
between any position in the dry layer (desorption temperature)
and the temperature of ice (sublimation temperature) which
was ori.inally postulated by &entzler and Schmidt (1973)
T.ihe
mathematical expression is based on hig;her desorption
temperatures for bound water as compared with the sublimation
temperature for frozen water and can be expressed as:
-27,,(= e (a
+ b)
where a and b are constants dependent on each particular
product.
A,;uileraand
link (1974) based on their experi-
mental results reported that due to above relationship
h.ipher moisture
contents
are to be expected
at a
distance from the ice front as drying preceeds.
iven
Their
results show that at a heater temperature of 128°0C. and
a surface temperature of 560, the diffusion zone was about
ice front had retreated ' mm (after 2.5 hrs),
2 mm when te
and when the ice had retreated about 15 mm the diffusion
zone increased to about 5 mm (after 9 hrs).
Another factor which seems to effect the degree of
broadenin"
of the diffusion
is discussed
2I
zone is freezinc
in the following
tate of ?Waterand of
Dried Systems:
'..:estate
method.
'This
section.
olutes in Frozen and Freeze-
of water bound to polymers has been investi-
,ated using various physical and physicochemical methods
of analysis,
Nuclear
Karel and Flink (1973) explain that recent
affneticResonance studies and infrared work indicate
thatwaterboundto
olymers is less "free" than water in
solution, and more "free" than water in ice crystals.
'.he
thermodynamic confirmation of the
reater mobility
of the -non-freezablewater in muscle derived foods compared -to
-2Pthe water
in ice as stated by Karel
and Flink
(1973)
is shown by 'iedel who studied the apparent specific heat
of frozen beef.
'he specific heat for bound water (unfree-
zable water) calculated by
iedel was intermediate between
that of ice and pure unfrozen water.
Since specific heat
reflects molecular mobility, "bound" water is evidently
more mobile than water in ice.
A.P. ?iacKenzieand
.J. Luyet (1967) sowed
that
water binding properties of ox-muscle, expressed in the
form of water adsorption isotherms, were markedly dependent
u.on the manner of initial freezing and at the same time
practically independent of the freeze-drying procedure.
T'hey concluded that the quantity of water contained
in the
monolayer is not affected by the manner of freezing or by
the freeze-dryin- treatment; however, the energy with which
-themonolayer is bound is greatly increased when the initial
freezing velocity is raised while not affected by freeze-drying
procedure.
They also showed that at a given constant relative
humidity quickly frozen, dried sample holds more water than
slowly frozen dried samples.
Karel and Flink (1973) explained that in slow freezing,
the resultant high salt concentration acts on muscle protein.
long: enou;?h to cause dislocation
and a-re-cation.
The -total
number of accessible hydro-?hylic sites does not change, hence
-29there is no effect of rate of freezing on monolayer value.
,owever, the access to these sites is more difficult and
this is thermodynamically equivalent -to reducing tle energy
of water bindinr( site and it similarly explai.ns the requirement for a hi-her relative humidity for a -iven amount of
water bound by the protein.
,.he above explanation
l.ac'enzie
and the results
obtained by
and Luyet (1967) could indicate
that freez ing
rate may influence moisture radierts at the ice interface
durinc freeze-dryin.
2,.4
Ietlods of
ompressin~g Dehydrated
oodstuff:
'he technique of compression was introduced to the
industry in the late 30s, T2hehritish Mvinistry of AFriculture, isheries and lF"ood,through -their experimental factory
in Aberdeen experimented with the compression of veetables,
especially
carrots,
cabba,.e and fruit
bars,
(ii.i
.amdy,
1960).
1Juring
1942-1943, f....
carried
out a research proFram
on the compression of dehydrated -foods.
:esearc'hAdministration of te
Ti
he A'ricultural
U.,.,.A.,conducted a food
compression project in 1943 and 194'4,and the
efense
,esearca I.edical Laboratories in Canada workred on the
development of meat and fruit bars (L.±.
-ftuer
194L-L
te
-lamdy,1960).
interest in compression of foods faded until
-301960 when the
;,
'
Army Quartermaster ,vorps initiated
new research projects in compressing dehydrated foods,
then there has been an increasin'
'ince
interest
in
compressed foods, with most of the research being sponArmy and reported by Rhaman, brockman,
sored by the U.3.
"rennin'~, 7'estcott, Schafer and others.
In all
methodsemployedthe most important
factor
necessary for successful compression is that the foodhas
dehydrated foods with low
state·. viost
to be in a plastic
,ile and fra-menteven under
moisture contents are very fra,,
compressive pressures.
li.::hzt
present
are
to
cast
and
developments of methods for compression of foods
examined.
the
In this section the
'he
reconditionin.
section is Feerally
subdivided according
techniques that have been employed
prior to compression.
2,4.1
oisture
ontent:
',oisture content has a ronounced effect on the texture
It is known-that most low moisture foods
of food materials.
(generally less than 5 percent) are brittle and fra.?ile.
Accordin.gly they do not yield to compression without destroyin-:the characterof food product.
floods of low soluble
solid (sugar) contents, are fragile when below 3 percent
moisture. Ver-etables of all kinds (shredded, diced, etc...)
were found to frarnment easily at low moisture. Pulverized
-31and powdery dehydrated foods; however, are expected
by compression
to be less affected
content (amdy, 1940).
at low moisture
Nevertheless, moisture content
in the ran?,e of 5-20 percent
has a lar-;e plasticizing
effect on most foods.
KaDsalis
et al (1970a) in a study of mechanical
properties of low and intermediate moisture foods found
important chan::esof the textural properties of precooked,
of aw, 0.15-0.3, which
freeze-dried beef in the ran.~!e
corresponds to a moisture content ran;Teof 5-10% moisture
contee nt.
In another study of precooked, freeze-dried beef
Kapsalis
et al (1970b)
reported
that-s-he mechanical
properties of hardness and cohesiveness (as defined by
Szczeniak, 1963 ) increased with increase in relative
humidity from 0-66
called te
,
whereas, the mechanical proer-Lty
crushability index (as defined by Drake,!966)
and -thesensory property "ease of bitin"
decreased,
It
is expected that over that moisture content ranrgesinificant textural charr:es to occur with other foods as well.
1Durin,::
the latter part of wV'orld ar II at the Aberdeen,
Scotland Factory, dehydrated cabbage and carrots were
beinr.:
compressed after conditionin
-to
percent moisture
by inlectin..steam in a heated air stream (amdy,
1960).
-32oWciers
In the Uriited s:tates, otato
and e.
aft er
content
adjustin:- the moisture
1960),
were
to 15 percent
comressed
(amdy,
In.a study on compression of dehydrated food (U.L,..,
194 ° ) it was reported
that,"usually
the surface of the food
nieces are at considerably lower moisture levels than the
centers and consequently the material will not compress satisUniform distribution of moisture can be obtained
factorily.
by storag7eat room temperature for several days, and satisfactory homonenity can be brought about in a relatively short
-thematerial in recirculated air under care-timeby heatirf'l,
fully controlled condition of temperature and humidity."
'Theybasically used both moisture and heat treatment (described
below) as the means of conditioninc
compression.
the product before
ihey report, further, "moisture content often
critically affects the compression characteristics of a
material.
At lower moisture levels, lower bulk densities are
usually obtained at a
iven pressure, and cohesive quality
may be considerably lower.
The temperature required for
satisfactory compression generally decreases as the moisture
levels rises.
Temperature conditions, especially as they
influence breaka-e and production of fines, become more important as the moisture level falls,
At very low moisture
contents, many dehydrated foods do not ,ive
at any temperature."
However,
coherent blocks
in the majority
of cases the
-33Procedures adopted in this experimental study were applied
to secific
TT.
comodities at moisture levels conformin
to
Quartermaster Corps specifications in force at the
time -the tests were conducted,
content (ie.
Vegetables with low sucar
beets, cabbai-e, carrots, onion, and ruta-
ba-.as)were coml-cressed
satisfactorily at moisture levels
of 3.5-5.5;, with the temperatures ran.-ingfrom 120 to 160°F.
and pressure rancir- from 200 to 5500 psi.
ved witl tese
Problems invol-
comodities were usually relaxation of the
product after compression, weakness of the bars and difficulty
with rehydration.
Fruits such as peaches, apricots, and
prunes were compressed at moirsture levels of 10.7-18.7,:.
at room temperature with relatively low pressures (100-900 psi).
SApple nu,-.retsat moisture
and 100-400
1500 F. at
in? on suar
si.
contents
Cranberries
ressures ran-in
content,
of 1,3-2,1:
at moisture
from 400-2000
at moisture
at 120-130
r'.
level of 5.5. at
si.
Potatos, depend-
levels of 9-15, at 140-1601 ',
with pressures rancin'- from 200-2000 psi.
And finally e-:;
powder at moisture
at temperature
content of 2.0 and 5.0,
of 65-90°F'.with 600-1500 psi.
E.- powder did not compress
well at 2>:-moisture; however, at 5 eercent, satisfactory
bars wereobtained which were redried to 2 percent to meet the
specifications.
.I'he
increase in density of compressed
products in -these experiments ran:e from 15-7
fold,
."amdy
-34enerally used low moisture content foods for
(1961)
compression except blanched precooked carrots which
'ad moisture levels of 5.45-6,6
of 3000-4500
achieved
.At pressures
si a compression ratio of about 4 was
¥_e also used heat and sormetimesrlasticizin.:
a:-ents in his experiments.
compressed
percent.
products.
Results indicated acceptable
In 1962
amdy reported that accept-
able compressed freeze-dried spinach could be obtained by
increasin>.the plasticizin. moisture content to 9 percent,
Ishler (1962) found that successful compressed food can
be achieved by sprayin- freeze-dried cellular food with
water -to attain -13 ercent moisture, compression and
to less than 3 percent moisture.
redryin7
Lampi has indi-
cated that the moisture level of the food rior to compressior. may affect rehydration.
(Rahman, et al, 1969)
et al, (1969) used lived steam or water sprays
Ra'nhman
after freeze-dryin- for preconditionin:-a variety of vepe-
table
for
cor-
'.hedesired equilibrium moisture content
roducts.
was 12 .percentbefore compression.
All compressed
products were redried in a vacuum oven to moisture levels
of
aproximately
psi was used.
2
ercent
Compression pressure of 500-2500
Compression ratios obtained were:
peas, 4:1;
corn,4:l; sliced onions, 5:1. spinach, 11:1; carrots, 14:1
and ·.
'reen beans 16:1.
-35-
Rahman, et al, (1971) studied non-reversible compression
a
of intermediate moisture fruit bars where the compressed
product is to be eaten "out of hand" rather than rehydrated.
Yo successfully produce compressed fruit bars the
moisture content of the fruit in.redients such as dates
and fi-s, maraschino cherries and others was reduced to
approximately
8 percent
to be applicable),
(a ranrle of ?-1 4 '? was reported
Compressing these fruits with their
-.,)
ori-iinalmoisture (ranin?- from 15-35
due to excessive extrusion of pul.
were unsuccessful
Approximately 2 per-
cent lecithin was incorporated to the bar to enhance the
texture .acKenzie
and Luyet (1969)studied the effect of
water on freeze-dried foods, compressed after moisturizing
to various relative humidities.
i!hey found that freeze-
dried carrots and Deas compressed after equillibration at
60 percent relative humidity were restored durinr. rehydration to their
re-compression character,
vackenzie and Luyet (1971) conducted another experimental study where twelve freeze-dried foods, alone and in
combinations were brou-ht to certain predetermined water
con-tertsby exposure, via the vapor phase, to water at
controlled activities-, .;'he
moist freeze-dried materials
were subjected to compression and to further dryin'g,after
which they were rehydrated
h.'ey reported that
rocessin-
-36-
conditions isurin.
best rehydration could be defined
in terms of water activities to which the foods were adjsusted
prior to compression.
They also found that composite foods
were more likely to respond well to compression where component items wre selected on the bais of compatible water
activity dependent behavior.
Their experimental data indicates
activities
that the hi--l water
(0. -0. ), which. corresponds to about 15-35 per-
cent moisture, most food-?,either collapse (flow) under compression, or compress readily but have very little or no recovery
upon rehydration.
Also, at low water activity
(dependin
on
surar content) products fra,-mentor crumble upon compression.
Fruits which are
enerally hi;gh in su/.arcontent showed best
restoration when compressed after resorption to relatively
lower water
activities.
compression
are aples
Examples of conditions rivin- :ood
at a water activity. close to 0.3
(?' moisture), peaches at water activity of 0.3 (4.5'.)
and inatcles at water activity
of 0325-0,3 (5%) Vecretables,
which nave lower sugar content and 'hiher fiber content than
fruits, required hin-herwater activites.
For example,
froodcompression was achieved with carrots at a water
activity of 0.55,
cabbage at a water activity of 0.35, and
potatos at a water activity
of 0.6.
They reported that this difference is mainly due to
-3?the su-ar con-tent.
Ihe freeze-dried
plant tissue
viewed as a two-phase, two component sstem
cellulose ad su-ar,
amorpnous
after
cellulose.
comrosed of
uOnexposure to water the su,-:ar phase,
freeze-dryinr
dive-rn
is
sorbs more wa-ter than the
hlbih enou?h water activity,
the
su-ar phase softens and flows under the compression force
while the cellulosic phase deforms simultaneously presumably
with
more
only
above its
water
resistance.
lass
o ince the su ar rich
transition tem-erature, the suar:
as to be controlled
ratio
-')Iase
flows
so tat
the the a-lass
transition temperature falls near but below the temperature
selected for compression,
lfhere
he system is moisturized
further,the sucar-rich phase may flow too readily, the
various internal surfaces beinm-annihilated
yihe cellulosic
in the process.
component may moreover, lose its ability
retain a strained form at hih water activities,
sul-ar and the excess water plasticizin-
to
with the
an irreversible
d eo rmati ion,
Chicken muscle beh'aved best whenthe water activity
was
increased
corresonds
to about
0.6 (a ran.'e of 0O5-0.7)
which
to a moisture content of about 1i,5,prior to
com-rression.
Tuna showedoo" compression behavior a-t moisture levels
of 11-22";.idepende-.t of the way in which -the water contents
-38were achieved.
ituomy (1971) moistened dehydrated products by spraying
-them with
the correct wei,:rhtof water to :-ive the amount
needed for compression.
equilibration was obtained by
holdirny: the moistened products under 27 inches of vacuum
for 27 hours,
He used "The Ldisonian Approach" of trial
watercontent
and error in his recipes for best results. nhe
of the recipes
ran-ed
from 1 -35i; before compression.
Pilsworth Jr. and :-oge (1973)
compressed
freeze-dried
beef to form bars by plasticizin:-with water transferred as
a vapor.
They retorted that "freeze-dried beef can be plasti-
cized for compression by partial rehydration in which water
is transferred to the beef in the form of vapor to an equilibrated level of 11-12 percent moisture content for satisfactory
compression with compression pressures of about 3000 psi."
tIheyalso madea test of plasticizing- thefreeze-dried
material by spraying. with water until a uniform moisture
level of 11.5-12 percent by wei:h.t was obtained.
dified products formed satisfactory
compressionpressure of 5000 psi.
the rehumi-
compressed bars under
'Iinner,-ardt
and Sherman
(1975) used rehumidification by sprayinnr with water to moisture
levels of 9.6-12.9';'Jfor compressinto freeze-dried foods, such
as chili can carne, beef hash, beef with vegetables, chicken
with vegetables, chicken with brown rice, beans and franks.
-39Rahman, et al, (1971) studied the effect of level of
b
moisture content of peas on compression behavior.
They
sprayed freeze-dried peas with water to achieve moisture
contents of
, 12, 16, 1,
and 20,,w,
and compressed them
after equilibration in an over for 10 minutes at 200°F.
'.hreecompression ressures (1000, 1500,
were used to form bars.
and 2000 psi)
i'heirresults indicate
that
the
amount of moisture added to peas prior to compression
significantly affected the rehydrated product.
he rehyd-
ration ratio decreased whereas the resistance to shear
became
reater as the conditioning moisture increased.
Pressure did not have an effect on rehydration ratio.
I-thas been shown that
ence as a
lasticizin-
water has a significant
a,-ent on the compressibility
food, its texture, reversibility
stability of the compressed
ifluof
upon relhydration, and
product.
However, it is
noted
that in almost all cases, the products which have been
rehumidified prior to compression have been redried to
low moisture content so ta-t the microbial and storage
stability
2,4.2
properties of the dehydrated food are not affected.
'.eat reatment:
Heat treatment alone or to,-etherwith moisture content
also have been used for plasticizing- foods prior to compresseion.
As mentioned before (2.4.1) the effect of heat is
-40most evident
in hi7h
as the
moisture content decreases, especially
sugar content foods which are sensitive to heat
treatment.
Some combination examples of moisture content
and thermal -treatment are
iven in the previous
section,
;.:Thermal
lasticity at low moisture contents is primarily
possible with high su?7arcontent foods such as most fruits.
problems associated with -thermal plasticization are
discoloration, burninr, and loss of nutritional value of the
product,
Rahman, et al, (1970) successfully compressed freezedried red tart
than 2
heat
itted cherries (RTP) and blueberries at less
ercent moisture content by subjecting7them to dry
in an oven at 200°r', for approximately
10 minutes,
followed by compression at pressures of 100-1500 psi.
resultsindicated
a significant
ratio and compressive
correlation
ressure.
Tl.he
between compression
Volume reductions of P and
7 fold, respectively, for cherries and blueberries were
obtained; when the compressed volume was compared with loose
frozen product,
obtained.
a compression
ratio of 12 and 13 to 1 was
Pies prepared from compressed products were equi-
valent in flavor, texture and appearance -tothose prepared
from the non-compressed products.
'he extent of rehydration
was about 30-50 percent of the fresh samples,
In another study
o, et al, (1975) successfully compressed
freeze-dried sour cherries with
less than 3
ercent moisture
-41content
eati-n; for 1 minute
by
a
200° .'': under an
Comressive
infrared lamp nrior to compression.
of 100-400
psi were used.
ressures
I'heirresults showed -that the
texture of compressed sour cherries became softer as the
comnressive
h'Inerehydration
ressure was increased,
was less for lower compressive
ressure.
J.he
time
freeze-dried
cherries were sli:h-tly wrinkled prior to compression.
overall
uality declined durin. stora
e.
Donnell.,, (1947) developed another
foods ir. which'-.e refri.erated
to acroximately
ve:etables
'he
flaked,
me-thod of debulkin.:
unseasoned mixed
2001°.and foods of relatively
. and then comressed
hiz-h fat contents to about 0 to -200°
a-t
ressures
around
1000-1l00
si,
-:e reported that depend-
in.- on fat content and/or su.-ar coten-t, the
oisture level
vrior to comression ca:n vary from 2-4 ercent with hirh
fat conten-t foodsand to 6-7 percet with foods of low fat
content and withn relatively hih su-ar conten-'t.
2. 43
;inders and other Plasticizin- A:ernts:
A variety of binders ad plasticizin.
a ents (other
-- th.anY
moisture a,nd heat) have been used (up to permitted
levels)
in foods with low moisture content (3-4 percent)
-to imrrove compressibility,
of
texture,and reversibility
the compressed products.
oodin., et al, (19-:) obtained a better quality of
compressed dehydrated cabba-e when they scalded the
cabba.e before dehydration in a solution containin-i
1-1.
percentsu-ar;however,the final
product was
rot acceptable because of sweetness and browrin.,
( Tamdy, 1960)
.err' (194"'),
percent
in compressin;-cereals, added 0.5-3
of ethylene
claimed imroved
or propylene
fclycol, which
he
the firmness of'the compressed
roduct,
(Y-amd,,190 )
$,loodin-:
(1952) srayed
of
lycerol
solution
but found tat
per
120 ml of a 5 -ercent solution
ound of steam-blanched
cabba,e,
brownirn. occured more readily,
at hir:hstora.,-e
temperature.
especially
--.
e also obtained the same
results
wen.
leate
F'ats have been used as binders in compressin- wheat
wafer
he used
meat bars,
percent solution of .'lycerolmono-
and muttom blocks.
All methods recommend usinan imal
fats
clusion o
or
ard-shortenin.s
(amdy,
1960)
i:.fi-meltin<- fa-ts suc
as
in order to decreaseex-
fat durin. compression,
Durst (1967) demonstrated -the effectiveness of a
bland, hi,Fhcaloric bindin7 material to
rovide a desi-
rable level of cohesion in bars prepared from a lar[-e
variety of dehydrated comporen-ts. It
was rroven earlier
that i-ncorroration of redetermined amounts of this
-43-
binding material provides a basis for comressin a
variety of foods into bars of' uniform size and equal
content,
caloric
a related study mlorethan thirty
I
sauces, spreads and relishes in the form of flexible
in
shee-ts suitable for direct consur-,.tion
conjunction
with a bland carrier were prepared.
!e describes te
binders
( a-trices
carriers
which are easy -to handle,
and 6.3
values (4.9
calorie per
) as
kA and I-*
-J2
caloric
have hi;-'h
ram, resnectively)
stability, mild flavor and good rehydration pro?c-ood
k.'atrix
-certies,
j
2
also
matrices
A1-,which hias bet-ter bindin<-:Dower than
as a h.iner
, and
23.4
16.P
2
su-,arcontent.
Ihe formula for
are:
20.0
.1
47. 7
31.6
32._5
2.2
1.1
0.
'J.he rotein beinpc sodium casinate, the carbohydrates;
sucrose, dextrose, starch and lactose and te
$00 oil.
fat Durkee
Diurstused these binders in a wide variety of
food recipe with accer-tableresults,
'Healso used sprayed
water in most of his experiments, in combination with the
binders mentioned, to achieve moisture of about 4 percent.
Ishler (1962) reported that sprayin. the dehydrated
lycerine or propylene ,Tlycolbefore compression
food with -.
-44produced bars with excellent rehydration characteristics
as stated by Rahman,
et al, (1969).
Rahman, et al, (1971) reported that incorporation
of 2 percent lecithin enhanced the texture of the intermediate moisture food bars.
Konigsbacher, (1974) investigated the applicability
of various plasticizing agents and techniques for compression.
Use of vanishing plasticizers such as FDA
approved freons alone or in combination with oil, or
liquefied gases such as carbon dioxide; however, was
not successful.
He also proposed a process of split stream
treatment where fresh food is added to dehydrated food
to increase the moisture content of the dried food to give
a material in the plastic range.
He reported that this
process is much more efficient than the classical method
of rehumidification by vapor phase equilibration or water
spray.
He used propylen glycol, glycerol alone or in
combination with water, vegetable oil, or combination
thereof.
He retorted that, "the use of propylene glycol or
glycerol as plasticizing agents in combination with a brief
steam
treatment immediately before compression yielded
compressed foods of best Quality."
H{ealso reported the
use of binders to prevent expansion after compression.
-45-
r-'he
results
indicated tha-t:
-?7inimumexpansion was achieved with systems
contaiin.:- .elatin
as the binder,
-est results were obtained when a mixture of
equal arts propylene I,_lycol
and steam were
applied at the 10 percent level as the plasti-
cizin aent.
Ranadive,
et al, (1974) used abou-t 2
binders in combination wi-th moisterin
(be-tween10-12
different
with water
ercent) for comr.ressin.meat balls and
pork sausa.res. "l'he
birders were ~-enerallystarch, modified starch, corn meal, e.--albumin, wheat
luten, soy
protein, or locust bean grum,alone or in combination.
'ihey reported
rehydration
that the product quality (which includes
roperties,
flavor, and -texture) depended
mainly on the type or types of binders
used.
'i'he best
binder for sausar:esw-,ascomposed of modified starch
(~':atioral
:'o,.
10),
wheat
(:ational o., 71),
luten and preelatinized
starch
and for meat balls modified cross
linked starch (iational '.o.10) and wheat 7luten.
I:avey (1975) in an study of techniques for controlling
flavor
intensity
in compressed foods used both remoistenin'rs
and a conditionin/birdir..--
aent
which minimized the redry-
inr after compression,. Ihis aent
was
water and crlycerine,
and then addinthe
elatin to swell for
minutes.
repared by mixin.-
elatin
and allowin-.
'i'his
was heated urtil
-46i-tmelted and then mixed with freeze-dried food products
prior to compression.
It was reported
that
this
plasti-
cized the meat for compression purposes and allowed the
other ingredients to adhere -to the
etted
; chafer, et al, (1975) developed
compacted and dehydrated
meat
surfaces.
a me-tsod to produce
food bars which may be easily
bitten or chewedwithout orior rehydration or rapidly
rehydrated in bar form in cold water, by incorporatingr
potato
than
articles,
which have been freeze-dried
to less
4 percent and thereafter equilibrated with water to
a moisture content of from about 5-15 percent.
food
in the
reparation mixture a proportion of about 10-20
nercent Potato -to about 90-80
formin.of
inrredients was used.
comr-essed at a pressure
ercen-tnon-potato food bar
I'hismixture can be
from about
00-100
si and
-thenredried to a moisture content below 1. percent.
Flavoring a,c-ents such as lemon powder are used to mask
potato flavor,
"'he otato
articles act to allow break-
a -e of tlhe food bar during? chewin-: without prior rehyd-
ration or result in the quick rehydration (10 minutes
in
cold water).
2,4,4
,ilanchin,-(:caldin?-):
.tudy of the effect of blanchin'-on the final quality
of the compressed
foods was done by
oodin-; (195;).
Hre
-47concluded -h'a-t steam blanchin.: yielded a better
qualitT
than the quality
of dehydrated and compressed cabbag-;e
obtained from water scalded samples.
explained that te
:oreover, he
leaching effect of blanching with
water caused increased frajmentation. To overcome this
effect, e su,-'cested a 1-1.5 percent solution of soluble
solids for water blanchin;
however, brownin:.- was observed
with samples treated in this manner. ('-amdy,1960)
2,4.5
~n of
_ e oom'oession
ffect
of Freezing,
_ried Foods:
·..acenzie
of
ates o
re
rreeze-
and Luyet (1971) who tested four different
freezing-rates prior to freeze-dryin. of cooked beef, chicken
and carrots, rerorted that there was no s
i:nificant
differ-
ence in the products after compression and restoration.
However, they
indicated that some trends
were evident in
the results:
-rih f-reezingrates were more deleterious for
compression of beef, though no effect on rehydrat-ionwas observed,
-arrots showed less resistance to comoression
when hit':freezin- rates were used. Also their
recovery was low.
-Chicken when frozen with high freezing rates
showed less recovery when rehydrated,
'ley explained that the reason for the observed
behavior was mainly due to small ice crystal formation,
which presumably decreases the routes available for the
-48re-entry of water,
2,L.. Continuous
Freeze- rin?
and Com pression:
_oni.-sbacher, (1974) conducted
a -test were
a constant
laced. on a sample of diced chicken (no plastici-
load was
d.urin.-freeze-dryin....'He
zers were used)
indicated
-that
the diced chicken .ad been compressed to about half its
ori.inal size,
';.i'e
samples rehydrated to their ori.irnal
shape wit' no chan.es in their texture or appearance when
compared with non-compressedcicken,
thait, "this
findinr- oper.s a new aproach
freeze-dryin" and compression.
represents
because
re-or-ed
-ie further
owever,
to simultarn.eous
this a-croach
a concept that needs to be investiated
as a one-step , process."
of its potential
We did not further
essentially
further
investi:rate -thismet-iod whic
the aroach
is
undertaken in this study.
24, 7 Limi-ted reeze-.Dryrin:
-[.ost
develoumertal work to date for compressed freezedried foods has utilized
full freeze-dryingf to low moisture
contents, followed 'byremoisteninr,to attain the desired
mositure content for compression.
conditions
it is
ossible
Under certain controlled
-to carry out- te
freeze-acryir.
-rocess such tat- the ma-terial is left wit. a desired uni-
form moisture content at the "end" of dryin-.,thereby avoidin.:te
remoistenin,- step., '.?heterm "Limited
reeze-bryin2-
-49w:
Pir-st az- ied b,r
a-d Luret, 19;9)
out freeze-drrir-^
z.acle-zie
-t this
:iherincile
rocess
G'-.ac enziee
of -'the method is 'to carrv
i- a-, enviriorment such ta-t
the relative
t
e surace a.d wi-thin the :roduct does not
lali beiwo somre reetermnec_, value, wici is hi-h enou'h
hurnidit
a
`Io e-'.sure thaite
dror, belowl. thie desired
lised
will not
m.oisture cor;-eren in an? retion
value
by see-arate contro
duryicn,
durin'
of te
: 'his
is accownr)-
samp-le-chambertemperature
and. con-de-)s r temn..erat,ure each w.ithir. arrow limits
-'ic Iq causes t .e fr eeze-dryin:
to rocee
(+ ,
),
b simultaneous
subimation. of ice and limited cdesor-ntionof unfrozen water,
t
' nile
thre subllimatio-n prroceecs to comnletion, te desorption
nrocess
stos
wen
vaor
;e e
-cressu e of te
.water in-rthle
,-,roduc- is redu ed..to thaL, of water vanor in te
cham.ber .....e:!uilirium
with ice on t;e condenser,
s.m--le c.a -rr-conenser
srraller
sammle
.reater
emera-t-uredifferences result in
ul tima-te water activities
.-reaoer deso-.-tior from te
i
samp-les.
-tesamp,,lechzamber; hence
.enerally,
t"he selection
of sam--le c amber temmeratures in the ran-e -10 to -40 ° ,
-rermits the
,--ram
100
roduction of ma-terials containin- about 25 to 5
0
dry solids
dram
res pectively (,acl:enzie and Luyet, 171).
rAccompishin.?limited freeze-dryinr. trouh.lh control of
the -nvironmertal relative humidity as described above requires
tha-tte
-temperaturedifference and water-vapor partial
pressure difference drivin- forces for heat and mass
transfer be less than in ordinary commercial freezedriers which necessarily leads to lon..-er
dryinc times.
Kinf, et al, (1o7
) noted
that
lacTlenzieand Luyet
reor--tedthat dryin'-times of 25-100
for 1 cm cubes of beef and various
T
owever,
inr,
freeze-dryin
1.
et al, (197
) reportedtat t'lerate of
can be accelerated if:
:he heat -ransfer
'..e
were
wours required
ve?-etable products,
coefficient
source to the surface of the
increased.
2,
ad
from the heat
roduct is
temp,erature of -the heat source
is raised
durin thleearly part of dryir:" so that temperature of the roduct surface rises toward
the point where -the surface itself experiences
the prescribed relative humidity.
3, Control of environmental relative
accomplished in some other way,
ihey.--rovosed two alternative
methods to accomplish
a more efficient limited freeze-dryinmethocd te
humidity is
rocess.
In one
t
heatinr -.aten -tem-rerature is controlled
at
-thedesired final surface temnerature, instead of a -the
samrle-chamber
emperature which was ori.inally
MacKenzie and Luyet (1969, 1971).
They indicated
used by
tat
the iece-surface temperature rises as dryin;: proceeds
from a initial value close to -the frozen-core temperature
-51-o a fin-al
value
approaclin,- the
artial pressure within
and similarly, the water-vapor
t-'e dryi'
laten temperature
cehamber
'
may
a
be held a-t the desired value,
v.wit-theknowledge that -thewater-vapor partial
at the
ressure
iece surface will approximate that value and be
no lower,
Under these conditions, -therelative
at the iece
surf ace will be
value and will be at a hi:her
sta,:es of the drying,
umidity
o lower than -the rescribed
value durin.
A major
rocess.
-.
the early
drawback
of
this
method is that small fluctuations of the laten temperature will result
in a -:reat chanre in.final
mois-ture
cortent by affect in- the local relative humidity. !(in-,
et al, (1975 ) h'he
expected dryin? times usin,- tis
cedure are about 3 times lon-.erthan
freeze-dryingr to low moisture
`-ilhe
other method
pro-
for conventional
contents,
is a self-re-rulatin- control
enviornmental relative humidity by utilizin:uptake medium which maintains a
of the
a water-
articular, prede-termined
relative ihumidit independent of temperature or amount of
moisture taken up, '.hisrelative
same as tat
humidity
would be the
the surface of the food pieces, provided -that
food pieces and water-uptake medium are at -thesame -temperature,
in.-,
--
et al, (1975 )
In a layered bed with alternatin
layers of food and
-52-
hydrating salt desiccant, a circulating low pressure
gas is used to transfer heat and mass between the food
and desiccant.
Since the latent heat of absorption for
most desiccants is about equal to the latent heat of
sublimation for the same amount of water, the gas will
be cooled as it passes through the food layer and then
will be dried and reheated as it passes through the desiccant layer.
configuration
The good mass transfer possible with this
ermits the use of inert gases at higher
total pressures, which reduces the drying time.
King,
(Jones and
197.5)
The hydrating salt desiccantsare good since for any
hydrate they have the unique property of maintaining a
constant relative humidity over a range of water contents.
They also have the additional advantage of maintaining a
relative humidity that is independent of small changes in
temperature (Jones and King, 1975).
With this method the
drying times are even shorter than the ones described before;
however, they may exceed by about a factor of 2 the drying
times of conventional freeze-drying to low moisture contents.
Although limited freeze-drying has good promise in food
compression processes, it requires longer drying time and
the compressed food still must be dried to low moisture
contents
o preserve stability.
-533.
3.1
DiATERIALS AljD i'LTHODS
Equipment:
3.1.1
Freeze-Drier:
A virtis laboratory freeze-drier (Uni-trap iodel
10-100) with a custom drying chamber, was used.
It
was operated with no heat input (except the ambient
leakage) chamber pressure, less than 100 microns, and
a condenser temperature of-60°C.
3.1.2
Design
of a
tatic Loading
Cell:
As the size of the chamber of the Virtis freezedrier used is relatively small (30 cm height, 25 cm
diameter), there is not enough space for a large static
load (such as lead) having appreciable compressive force
to be placed on the sample during freeze-drying.
However,
it is possible to use the resultant force from extension
of springs to attain compressive pressures hic:h enough to
compress the food.
Figure 1 shows the cell loading system.
This system is not truly, a static system since the length
of the extended springs decrease as the height of the food
bed is reduced due to being compressed.
This causes the
force produced by the springs to decrease as the food is
compressed.
This change in force during compression can
I
Base
Plate
2
Top
Plate
3
Matched Set of Springs
4
Threaded Guide Rods
5
6
7
8
Nuts for Raising Top Plate
Sample Cell Stand
Sample Holder
Compression Plate
Fig. . Load Cell.
(Extension Type)
-55-
be minimized by selecting springs which have a large
initial tension (force required to initiate extension)
and a relatively small relationship of modulus to initial
tension (i.e. A F due to
A
L is small compared to F)
so that the main part of the force is due to the initial
tension and only a small fraction of the force is due to
extension,
Ideally this means that after the force re-
quired to initiate extension is achieved no more force
is necessary to further extend the springs.
is not possible,
that
A
it is desirable
Since this
to use long springs
so
L/L is small for a given sample thickness.
Different sets were used in order to widen the range
of compressive force.
All springs were tested for linear
response of force with extension.
The extension of the
spring versus force was graphed for all springs of each
set.
T'able1 represents the properties of the springs
selected for this study.
Cell configurations using either
2, 4, or 6 springs have been constructed, allowing three
levels of compressive force from each spring set.
3.1.3
Sample Holder:
The sample holder is constructed of aluminum and
stainless steel mesh to allow application of the compressive force without sinificantly
of vapor from the drying surface.
impending the flow
Its dimensions are
0)
U0
U,
o
r4
o
0
o
.-4
4-4
0
4J
P,
0)
P.
r.
E-
0
0
V)
(n
0sk Q
du
o
a
r4 E
(I
o
4
bn
· to
H
-I
la
4i
to
0)
P4
a) P
4!
) Q
r.0E
'HZ'
'-4
(1
*,LX
Un
C'
0C Cl
00
o
G~
c0
C
-"q
cr
Cs
Co
v
o
x.D
c
cS
H,
d
o
r
O
00
cV
o0
,-I
H O.o q o O
-C
Cl
K
\D
C'
--
00
-4
u.
a,%
0
¢N
C
oO
rH
00
cc
CN
-
pH
cn
ao
H
*
C
U
(2
,
0
CJ
Jr0
a
va
o
-Ka
K4C
X
ed e
k
1-
a
b0
0
4-0
v
,~~~
to
JJ
U
o
0
0,
a
0o
a)
P
*1
ut
92
O~
(12
-i
-57-
approximately 2x3x2 inches.
Figure 2 shows the detail
of the sample holder.
In practice, the cell is assembled without the
sample holder.
A small scissors jack is used to raise
the top plate (2, in Figure 1) and extend the springs.
The nuts (5) are raised until they are just under the
top plate.
T'he jack is removed so that the frozen
material in the sample holder (7) can be placed on the
loading cell stand (6) while the nuts (5) maintain the
springs under tension.
The compression plate (8) is
inserted and the nuts (5) are lowered permitting the
top plate (2) to contact the compression plate which
is on the bed of food.
The nuts are lowered sufficiently
so that the top plate (2) can move down the guide rods
(4) without contacting the nuts before -the end of
compression.
3.2
Experimental Procedure:
Fiure
3 shows the principle steps used for each
experiment.
3.2.1
Sample Preparation:
Table 2 lists te
food materials used.
All samples except for cherries and blueberries
were kept in a freezer at -200C.
0 Compression Plate
©
©
Sample Holder (wire mesh)
Green Beans (sample)
Fig. 2.
Sample Holder.
-59-
Packin~? of sample
in sample holder
Freezino~
Insertion. of frozen sample
in its
holder in the loading cell with
springs under -tension.
lowering the nuts to allow the
compressive force to be placed
on the frozen material.
freeze-drying/compression
Unloading the dried, compressed sample
Stora:
,e4-compress ion ratio--Rehydration
determination
behavior
;inal moisture content and
percent total solid
measurements
Orpanoleptic
evaluations
Fig. 3.
Principle
steps for each experiment.
-greenbeans
Vegetables
'*carrots
-- peas
Fruits
Comments
er ial
Lieat
system
- **blueberries
.- * black
,eat
cherries
"-~,[round beef
''bottom round
steak
cut frozen packed
diced frozen packed,
cubes approximately
6 mm on each side
frozen packed
frozen acked, kernels
fresh
fresh, pitted and
halved
fresh, 1
percent
fat
fresh
-:Purchasedfrom a local supermarket as frozen packed,
in large quantities (5-6 lar-e packagces)to minimize
variability within -thesamples.
'"Furchased as mixed frozen peas and diced carrots,
they were separated by hnd and stored.
purchased fresh from a local supermarket (cherries
and blueberries were purchased in quantities sufficient for one or two experiments since the quality
and texture would change upon storage.)
-L
2
Materials
used.
-61-
The frozen vegetables were first thawed by placing
them in water at room temperature for several seconds.
TLhe excess water was then removed by an absorbent paper
towel.
'he reason for this procedure is that the frozen
vegetable usually had a shell of ice around it which
would decrease the measured total solid percent of the
material.
All samples were hand packed in the sample holder
with minimum void volume between pieces.
When the frozen
sample was initially subjected to the force (before freezedrying) some breakage was produced in the frozen sample.
To reduce this cracking layers of cheese cloth were placed
in the sample holder under and over the bed of food to
provide some cushioning.
Uniform sized cut green beans were selected for
compression.
They were aligned uniformly next to each
other as rows and layers in the sample holder (Figure 4),
with the force perpendicular to the grain.
I
a.
Fig. 4.
I
I
top view
Configuration
b. side view
of beans in the sample holder.
-62-
Diced carrots were placed into the sample holder
such that all cubes formed an orderly array of rows and
layers.
::o effort was made to separate dices according
to presence or absence of carrot core material.
blue-
berries, peas and corn were placed in the sample holder
fully packed.
Cherries were pitted first and ther put in-tothe
sample holder either whole, or halved. ,(round beef was
g7ently "packed" into the sample holder so that void volume
was avoided while "pre-compression" on the fresh sample
was minimized.
The steak was trimmed of fat and cut into
cubes approximately 1 cm on a side.
he cubes were then
packed as rows and layers into the sample holder with
:Grain perpendicular to the direction of force.
?.few tests were conducted with the
(as beans)
rain parallel to the
direction of force.
All samples were weighted and final weights were
obtained by difference of the sample holder with and without the sample.
In all experiments
one or two samples
were freeze-dried without compression as control.
One
control, approximately the same weight as the sample being
compressed, was freeze-dried in a 250 ml glass beaker
randomly filled.
Trams sample
'Theother control was usually a 10-12
in a 50 ml beaker,
.he first control
was
-63-
used for moisture, rehydration and organoleptic comparisons with the compressed sample while the second
control gave a second value of percent moisture content
and percent total solids of the raw material.
3.2,2
Freezin:
In most cases the samples in the holder were frozen
in a freezer at -20°C.
Prior to the experiment they were
further cooled down by a stream of liquid nitrogen.
(In
some cases when there was not time for slow freezing the
thawed samples were quickly frozen by the aid of liquid
nitrogen.)
3.2.3
Loadin.gof the Cell:
After freezing, the sample holder was quickly placed
into the loading cell with the springs extended.
The
force was placed on the frozen sample by lowering the
nuts.
The length of the springs were recorded, and then
the system was immediately placed in the freeze-drier and
the process started,
The lengths of the springs were recorded again after
freeze-dryin./compression so that the force on the sample
during compression could be calculated.
3.2.4
Compression Ratio:
The term compression ratio is defined as the ratio
of the initial volume of the sample prior to compression
-64to the final sample volume following compression and
drying .
owever the numerical value of the compression
ratio can vary depending on the method of measuring or
defining volume.
In this study compression ratios are
given by three different methods.
a-
Compression ratio
by displacement:
the lowest numerical value of compression ratio
is obtained by taking the initial and final density
(on a dry basis).
This can be done by a displacement
technique for determining volume using lead shots similar
to that used for bread volume measurements using rapeseed.
In this measure, no void volume for the bulk sample is
included, hence it approaches closest to the "real"
volume changes of the sample.
Lead shots were used due
to the fact that their diameter was much smaller than
rapeseeds.
Also, since small quantities of sample were
used, lead shots which have a very high
much better to small displacements.
density respond
To calculate the
density (dry basis) the following equation is used,
wt.(cont.+L.S.)- wt.(s+cont.+L..)
PL.S.
vol. _ 1
t.s.
where:
vol.
t.s.
gr t.s.
p
volume
total solids
density of the sample
-65PL.S.
density of the lead shots
wt.
cont.
L.S.
s
weight
container
lead sots
sample
It was not feasible to measure the volume of the
wet, initial sample since lead shots would stick to
surface.
In this case the ratio of the densities of
the freeze-dried non-compressed to the freeze-dried
compressed samples was used assumin.?that the change
in volume of the freeze-dried samples (non-compressed)
is not sirrnificant.
b-
Compact compression ratio (by height) and
random compression ratio:
Compact compression ratio is obtained by taking
the volume of the food as occupied in the sample holder
before and after compression.
ince the compression area
remains unchanged and that compression is uni-directional
(i,e. only the thickness decreases),measuring!the bed
heirht before and after the compression can be
ood
indication of compression ratio.
This compression ratio
ives lower values than
normally reported in literature, since these values use
a random fill of the initial material as base rather -than
the ali.nMedfill used in this study.
compression
ratio
reported
T'o make values of
here comparable to those in
-66-
t'heliterature a volume ratio of random to aligned
packin..was determined for each sample.
This is
done by measuring the volume that each food with
the given piece size occupies randomly, and measuring
the corresponding.weiht
of the sample.
The same
quantity of food occupies a much smaller volume if
packed orderly.
(as measured in the sample holder).
The ratio of these two different volumes
ives a
correction factor for each food product with a given
piece size,
By multiplying the correction factor to
the compression ratio by height, a
ood measure of
random compression ratio for each sample can be calculated, as long as they have a relatively uniform
piece size,
3.2_i5
;loistureContent, Total Solid Determination:
Immediately after freeze-drying the weights of
the compressed and non-compressed samples were recorded
and a random portion of each used to determine moisture
content.
From this the percent total solid in the fresh
(or frozen) samples was calculated.
The moisture content was determined on 1-2 gram of
sample by drying under the IR lamp to a constant weight.
The percent moisture content left after freeze-drying is
used to calculate the grams of water left in the dried
-67sample, and subtracting this from to-tal dried weight,
the grams of total solid is measured.
grams
'_.o.'=
32 .6
Then:
total solid x 100
initial wet weifht
Rehydrat ion:
Rehydration behavior of the samples was examined
at different water temperature, namely cold (about, 40°1.),
0 F),
warm (about, 90-100°
and hot (about, 130-160°F.).
Individual pieces were rehydrated and their weights
measured
at 1, 2, 5, 15, 30 minutes
or 2, 7, 15, and 30
minutes after -the initial immersion.
In most cases 30
minutes or less was enough for full rehydration.
In a
few cases rehydration was carried out for up to 1 hour.
In most rehydration tests the samples were immersed
under water and held submerged with a screen; except
when weights recorded.
To test for air entrapment in
the void volumes produced by sublimation of ice (specially
in non-compressed samples) vacuum was used initially in
order to release any entrapped air.
In one test the
samples were evacuated for 5 minutes, -thevacuum was
released with air and then the samples were rehydrated.
In other method the vacuum was released with water instead
of air and the rehydration was carried out.
The results of rehydration tests were calculated as,
-68-grams
20 up-take
Crams total solid
the,
tram H20 in fresh (frozen sample)
was used as a reference
gram total solid
ir order to calculate the percentage of the rehydration.
3.2.7
Organoleptic Test:
In a few cases rehydrated cooked or fried (steak
only) samples were evaluated organoleptically.
In each
'testthree samples were compared: compressed, non-compressed,
and fresh (frozen), all prepared the same way.
panel of 8-15 people used for each test.
A taste
wo preference
evaluations were conducted simultaneously for each test,
one based on a 1-9 (dislike extremely- like extremely)
hedonic scale, and a ranking -test. (Larmond, 1970)
3,2.8
Storage Stablit:
All samples were stored in darkness in desiccators
with no vacuum.
Rehydration tests were conducted on a
few samples in order to see if there was any change in
rehydration behavior after storage, and also to see if
there was any relaxation of the compressed sample with
time.
Carrot samples stored showed a sizable disappearance of the color pigments (carotinoids).
-ortions of
-69-
compressed and non-compressed samples from the same
experiment were stored in dark under vacuum over desiccant and in dark over desiccant but without vacuum.
3.3
Data Treatment:
Fior each food, the compression ratio as a function of
pressure used during compression was determined.
he
rehydration behavior of freeze-dried compressed and
non-compressed samples examined in relation to the fresh
(or frozen) foods.
The rehydration behavior of compressed
samples was also examined with respect to compression ratio.
Or;ganoleptictests were used to investigate acceptability
of compressed samples with respect to non-compressed
and fresh samples.
-70-
4,
RESUL'.TS
A1' DISCUSSION
T'heprimary aim of this study was to investigate
feasibility of compressing-foods during freeze-dryin-:
to produce easily reversible compressed foods or food
bars with hirthcompression ratios that are highly
acceptable, upon rehydration.
4,1
Presentation of Results:
The experimental results have been divided into
3 major sections:
1,
2.
Veg.etables
Frui-ts
3.
£:eart
Subdivisions of these sections cover experimental
results obtained for each particular type of food.
Detailed experimental results for each subdivision are
tabulated. in Appendicies.
a
At the end of this section
eneral discussion coverin-gall food materials tested
is presented.
4,2
Ve.etables:
The data for this section is tabulated in Appendix
A.
4.2.1
Beans:
Table 3 shows the summarized results for green beans.
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4.2.1,1
Compress ion Ratio:
Beans did not produced bars upon compression;
however, they
ave the highest compression ratios
compared to other samples.
As seen in table 3 the
ranre of pressures used was from 15 psi-130 psi which
,gaverandom compression ratio ran.in- from 3:1 to 12:1.
T!he increase in compression ratios with pressure was
linear over the range experimented.
('iure
5)
T 'he correction
factor -to calculate the random compression
ratio is 1.7.
Over 12 months the compressed beans did not
seem to relax and increase their volume.
4.2.1.2
voisture Content:
The final moisture content of freeze-dried compressed
beans were always similar to the non-compressed products
which were dried at the same time
('iable 3).
This
indi-
cates that compression during freeze-drying~does not produce a surface layer of lower permeability sufficient to
prevent complete dryin.
after 4
The percent moisture range obtained
hours dryinc was between 1.35-4.,7;. The percentage
of total solid calculated were very close and areed
well
with values in the Table of Composition of Foods, U.L.D.A,
Wandbook number R.
The observed ran-e was from 6.4 to ll.y%"
solids, while the literature value is about
percent.
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-74Rehydration:
Initial studies of rehydration were conducted at 3
temperatures,
Figure 6 shows the time dependence of
rehydration for a typical experiment.
It was noted in
these studies that rehydration was most complete when
cold water was used, and that compressed beans rehydrated more completely than non-compressed,
In later
experiments only cold.water was used for rehydration.
The rehydration essentially reached equilibrium after
15 minutes under conditions where beans were placed in
the water and held below the surface with a screen.
Tlhe
original sample volume was recovered after rehydration.
To investigate if the difference of rehydration between
compressed and non-compressed freeze-dried samples was
due to air entrapped in the non-compressed samples,
vacuum was used prior to rehydration, (see section 32.5).
Fi:ure 7 is a typical example,
Little effect was noted for
compressed beans, while the non-compressed samples showed
a marked increase in rehydration when vacuum was used,
Vacuum rehydration increased the extent of rehydration of
non-com-ressed
beans to that of compressed.,
his sug~gests
that there has beer.some chanfes in bean structure during
compression which affects the ability of air to escape
from the non-compressed cells,
'.l.he
compression ratios
Fig. 6. Rehydration of compressed and non-compressed
beans at different temperatures.
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-77attained did not appear to affect the extent of rehydration of green beans.
(Figure 5)
iore detailed
results of rehydration of beans are given in Appendix
A-l.When the rehydration of compressed and non-compressed
freeze-dried beans stored for 2 months was tested, no
change in rehydration behavior was observed, with the
same
eneral pattern of rehydration being obtained.
(Appendix A-l)
?Jhenno protective cheese cloth was used in the
sample holder the surface of beans that were in contact
with the mesh (the sample holder) showed some destruction
of the tissue when compressed at high pressures ( >90
psi).
Upon rehydration the texture of these beans was in general
soft and mushy, and in some cases they fractured extensively
after rehydration.
hiowever,when layers of cheese cloth
were used under and over the bed of sample, the destruction was
reatly reduced, due apparently to a cushioning
effect of the cheese cloth.
The texture of the compressed
rehydrated samples (as felt by hand)showed no difference
from that of non-compressed,
Compression (with cheese
cloth) did not affect the physical appearance of the rehydrated sample and the compressed beans showed a more
desirable green color than non-compressed, which were pale.
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4.2 2
Carrots
:
ta>le 4 shows the summarized results for carrot dice.
4.2.2,.1
Comression
Ratio:
Carrot dices were compressed at pressures from 65-132
psi.
i.ure
9 shows that all three co-mression
ratios
(disFlaceme-n(t,
height, and random increase linearly with
increasin
tested,
compression
ressure, over the ra-:--:e
of
ressures
'.'he
random compression ratios for the pressure range
tested varied from 2.7- 7.77
The correction factor -tocal-
culate random compression ratio of diced carrots was 1.76.
1,o visual expansion of volume of compressed carrots was
observed
4,2.2,2.
durin?
7 months of storag-e.
Li oisture
ontent:
As seen from '..'able
4 in many cases the moisture content
of non-comressed
ins moisture
an indication
process.
samples is much hither than the correspond-
con-tent of compressed
samples.
`This could
be
of a faster rate of dryinl/ with the compression
os
sibl
sam,le holder.
due t1o better heat transfer
in -the acked
It should be noted that the comparisons in
tables are made with non-comnressed controls which weiphed
arproximately the same as the compressed sample, while the
small quantity non--compressedcontrol (10-12 -rams) dried to
final moisture contents which were equal to or lower than
the compressed samples. V'ith the exception of two experiments
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-82where non-compressed samples showed a very high moisture
content after drying which might have been due to accidental exposure to high humidity or incomplete drying, the
rankleof moisture contents left after drying is between
1.5-7.7 percent,
(Table 4)
Drying times were between
Percent total solid based on measured mois-
4P-60 hours,
ture contents were in the range of 9-2
which aain
closely
ag:reewith the literature value of 11,5',.
4.2,2.3
Rehydration:
Tests on the effect of water temperature, rehydration time, and sample evacuation on the carrots rehydration were similar to those of beans.
When samples rehyd-
rated without prior evacuation, compressed samples showed
higher extent of rehydration in cold water ( 700 F.) than
hot water (1400 F.) or warm water ( 90°F.).
Only one
experiment was conducted at 3 different temperatures and
thereafter cold water was used in later rehydration tests.
Hydration was rapid generally reaching near to equilibrium
in 15 minutes.
Vacuum rehydration had a little effect on
compressed samples whereas non-compressed samples showed
an increase in rehydration to a level equal -tothat of
compressed.
(Appendix A-2)
The effect of vacuum on rehyd-
ration of carrots suggests some structural chanres during
compression which effects the amount of air entrapped in
_.Z3-
the cells or ease of its removal with water influx.
iirure 10
shows that there is little effect of compression ratio
(random) on extent of rehydration.
Rehydrated compressed
carrots did not differ from non-compressed samples in
physical appearnace.
They returned to their orip:inal
volume upon reconstitution.
By hand feel the texture of
compressed rehydrated samples were slightly softer than
that of non-compressed.
A -testof rehydration behavior of stored
samples
showed no changes (Appendix A-2); however, when storing
compressed and non-compressed samples
it
became
from earlier studies
appparent -thatair-storage at 0
absence of light
(i.e. in a cabinet)
in~.of the pi:ments in carrot dices
R
even in
in a bleach-
resulted
and loss
of
desirable
flavor (as indicated from organoleptic tests).
An experiment
was conducted
where compressed and non-
compressed carrots were stored in the absence and presence
of air in the absence
of
light.
After 2 months,
which were stored in presence of air
carrots
were completely bleached
and became almost pale yellow in color; the samples stored
in absence of air did not visually seem to have lost any
pigments.
It is concluded that bleaching and the loss of
flavor detected in org--anoleptictests is dueto air oxidation of dried carrots. i,;ore detailed
results of rehydration
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-85-
tests for carrots are presented in Appendis A-2.
4.2.2.4
Organoleptic Evaluation:
Two organolertic tests were conducted to evaluate
acceptability of compressed carrots versus non-compressed
and frozen.
testing.
Carrots were boiled for 5 minutes prior to
In the first test frozen carrots were rated
significantly better than compressed and non-compressed,
while compressed was rated slightly higher than non-compressed.
Comments indicated a large loss of color and
flavor in both dried samples.
The carrots used in the
above organoleptic test had been stored for over a month
in an unevacuated light free desiccator.
A second organo-
leptic test was conducted on dried samples which were not
stored.
This significantly improved the organoleptic
evaluations of the dried samples.
Although, frozen
carrots still were rated best there was no signigicant
difference observed in taste and texture.
The compressed
ranked second while the non-compressed remained last.
Results of or,2,anoleptic
tests for carrots are given in
Appendix A-2,
4.2.3
Peas:
Table 5 gives the summarized results for peas,
4.2.3
1.
Compression
Ratio:
Compression pressures used for peas were between
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-87si s-iving random compression
39-97
ratios of 2,9 to 5.5.
Figure 11 shows the different compression ratios as a
function of compression pressure.
It is seen that compre-
ssion ratio increases linearly with increasing pressure,
A correction factor of 1.33 was determined to obtain
random compression ratios from the measured compression
Y.ochange in volume of compressed
ratios based on sheiht.
peas was observed over 12 months of storage.
4,2.3.2
Mioisture Content:
Dryingrtimes of 4-60
seen from table
hours were used for peas.
As
5, final moisture content of compressed
samples are very close to those of non-compressed (control
number
1).
This suggests that the process of compression during
freeze-drying of peas does not impede the flow of vapor to
the extent that drying to low moisture contents would not
be possible.
It can be noted that the final moisture
content of control number 2 which was freeze-dried in
smaller quantities were always lower or equal to that of
the compressed samples.
Tlherange of final moisture contents
in the compressed samples were from 0.9-7,0'.. T'"hepercent
total solids calculated from percent moisture content was
between 19.R-22,7 which is quite close to reported literature values of 19.3 (U.S.D.A.,
andbook
o.
).
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-89-
4 . 23.3 Rehydration:
Rehydration of dried compressed peas was quick and
went to equilibrium in about 15 minutes in cold water.
The original volume was recovered.
In general compressed
peas rehydrated better than non-compressed, which is
similar to beans and carrots.
Vacuum rehydration with
vacuum release with water improved rehydration of peas
to a large extent.
However, it was still noted that
some non-compressed peas rehydrated very slowly and to
low extents. (Appendix A-3)
Further investigation showed that most compressed
peas had a hole or crack in their outer skin which
apparently was produced during compression.
On the
other hand the skin of many non-compressed peas ( 40 percent) remained intact.
The unbroken skin slows down -the
extent of rehydration to a
reat degree.
The effect of
vacuum on rehydration of non-compressed peas and the
influence of the skin are noted in Appendix A-3.
seen that vacuum and broken skin faciliate
It is
the extent
of rehydration in the case of non-compressed peas.
Little
influence of vacuum was observed for compressed peas.
In
later experiments, only non-compressed peas which were
cracked were tested for rehydration.
The color and texture
of rehydrated compressed peas were very good and did not
-90seem to have been affected by compression,
i;i-:!ure
12
shows the effect of compression ratio on the extent of
rehydration after 30 minutes.
It is seen that compression
does not affect the rehydration behavior of the dried sample.
Appendix A-3 .ives more detailed experimental results of
rehydration behavior of neas.
4
r-:anoleptic
2.2_4
Evaluation:
Frozen, non-compressed rehydrated and compressed
rehydrated samples were boiled for 5 minutes and served
with a touch of salt and butter.
Although
all samples
showed insignificant statistical difference, the compressed
and fresh were rated very close in terms of their texture
and taste.
Ranking test showed aain
that compressed
and frozen were very close, with the frozen rated first,
compressed next, and non-compressed rated last.
The
results of organoleptic evaluations for peas are
Appendix
4.2.4
iven in
A- 3.
Corn:
Only 2 experiments were conducted on cut corn kernels.
At pressures of 43 and 47.5 psi, the correspondinr random
compression ratios were 2.
and 2.9, respectively.
The
final moisture content of both compressed and non-compressed
dried products was very low, being between 1.5-2.9',, indicatin, that the compression process did not affec-L;
r-in'-
0
Q.
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-92to low moisture contents.
upture or fracture of the outer
kernel skin of compressed products produced some fines.
Upornrehydration the whole ernels re-a-ttained their ori:- inal volume,
Vacuumdid not seem to affect
rehydration
and
both compressed and non-compressed rehydrated to nearly
the same extent
(ur to 70>).
Appendix
A-4
-ives
more
detailed results of rehydration with timie.
4.3
Fruits:
The data for this section is riven in Appendix B.
4.3.1
Blueberries:
Table 6 shows the summarized results.
4.3.1.1
Compression Ratio:
Fresh blueberries were slowly frozen and compressed/
freeze-dried at pressures from 3.9-30 psi.
ments, 5 were classified
product and 2 as havin-'
Out of 19 experi-
as havin,,' sir'nificantly
collapsed
completely collapsed product. (A
collapsed sample was identified by exuded juice which had
run out throuoxh the sample holder mesh and which usually
were puffed durin:-dryinE.)
Collapse phenomena was probably
due to hi-'h su,-.ar coltent of blueberries.
Also due to the
short season tat fresh blueberries are available, the
textural variation in the raw material is relatively 'hi:chr.
It was noted th'at toward tine end of the season the texture
of blueberries
became softer
and they were more prorne to
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-94-
collapse,
The experimental data -:ivenfor blueberries
is based on the
12 exreriments where little or no collapse
was observed.
iiure 13 shows the relationship between
compression ratios and pressure,
correlation
is not very -:ood which
variation i
It is seen that the
is probably
due to the
the texture of dif:feren,-t
blueberry batches
used., A correction factor of 1,3 was calculated to convert compression ratios by hei:ht -to random compression
ratio,
Compressive forces required to co-mpress
blue-
berries were much lower
for
:iven
compression
than
that
ratios.
the skin was usually
needed for vegetables
.or the compressed.samples
broken or cracked; however, the berry
itself remaired whole.
i.o relaxation of compressed samples
was observed over time.
shranksomewhat after
;,on-compresseddried controls
freeze-dryin.
and their skin became
quite wrinkled,
4,
3.1,2
',:oisture
ContUent:
Final moisture contents for blueberries which were
successfully compressed durin. freeze-dryin?'were g.enerally
between 2.0-L.A
bo 60 hours,
i· hours.
ercent.
since
This
The drying cycle was len~thened
the samples were not completely dry in
is probably
due to the touch
cause the dryi-'. rate to be reduced.
solids
was obtaired
skin which
A rarn.:efor -total
(13.6-1/.4), which corr:mares
well with
OIIV8I NOISS3dWO03
U)
N
o
0.
C'
a.
w
1
n-
-96the re-orted value of
4. 3 1 3
1
'<.(T.,>.o.A,
:iandbook
.io.
).
Rehydration:
h'leextent of rehydration
of both compressed and non-
compressed blueberries was ::enerally poor.
Rehydration at
different water temperature indicates that blueberries
rehydrate
best in hot water. (ppendix 'L-l) Vacuumprior
to rehydration
generally
:?ave some improvement for both
com-ressedand non-compressed
comparison
of the
extent
(Appendix
-1)
samples.
A
of rehydration of compressed and
non-compressed samples showed that the non-compressed samples
reh,.ydrated
about 2 times that of coTmressed samples.
The
oriwinal volume was not recovered after rehydration and
compressed samples remained lat, even thourh there was some
water pickup and the product softened somewhat.
rated samples were not as soft as fresh.
'iherehyd-
Some leachin-
of
the color pi-ments was also noted during rehydration of
compressed samples,
,Forcompressed samples, there did not
seem to be any particular relationship between cormression
ratio and rehydration ('irure 14).
L4. 1.4
Oran.olernticEvaluation:
A number of or.:anoleptictests were conducted with
blueberry products, one with blueberry
three with blueberry muffins.
utherland,
u:
et al,
(1973).
ie fillin?-and
Recipes were taken from
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-98The Hedonic preference -testwith pie filling showed
significant differences between the texture of fresh or
non-compressed samples with compressed,
No significant
difference was observed between the texture of fresh and
non-compressed.
No significant difference in taste was
observed between any samples.
In the ranking test fresh
and non-compressed blueberries were rated the same and
significantly-higher than the compressed sample.
Pie fillin
The
made from compressed sample was too "thin"
since compressed blueberries rehydrate very poorly and
thus there was too much water in the pie filling.
In later experiments with muffins, the dried blueberries (compressed and non-compressed) were either held
over steam until they were soft, or only very little
water was added to just soften the samples.
.['The
panelists
were asked to evaluate the taste and texture of only the
blueberries in the muffin and not the muffin itself.
were to be ranked based.only on the blueberries.
They
In the
first test both the texture and taste of compressed blueberries (in muffins) were preferred over fresh (not significant) and non-compressed, (significantly different). In
the second test fresh and compressed samples were
rated
equal in terms of taste and texture, with non-compressed
rated last;
however, no significant
difference was observed
-99-
between all samples.
In the third experiment all samples
were rated very close to each other and no significant
difference was observed.
Appendix -i
:ives the results
of or-anoleptic evaluation for blueberries.
4,3.2
Cherries:
PFreshcherries were pitted and used as whole or
halves,
Almost all dryin,:experiments conducted were
unsuccessful, in that the product underwent collapse.
Compresseion
collapse.
pressures
as low as 11 csi resulted
in
'wo experiments ,raveproduct that seemed to
be uncollapsed, thouh
the physical appearance was
oor.
;'hesample had browned and was cracked and wrinkled.
I ey showed compression ratios of 2.9 and 3.0 (random)
with compression
ressure of 41 and 43
si, respectively.
'i'he
moisture contents were about 4 percent.
,iion-compres-
sed cherries which also had a poor appearance (browned
and wrinkled) had final moisture contents of 2.5-3 percent.
In another experiment in which the cherries appeared to be
uncoll]ase
In tis
t'e
+
final moisture content was 2,4 percent.
case the compression
compression ratio of 14,3.
ressure of 17 psi Cave a
ihe percent total solids
calculated from the final moisture content rangfedfrom
17,1-20.,5!,
which arees
with the reported value of 20.6,
-100(U.S..D,A.,
andbook :o,
),
'he dried samples had
texture.
ood taste but a rather
chewy
Uxponreconstitution the com-ressed samples
fractured extensively and browned further, while the
non-compressed samples, though cracked, remained whole,
and became browner.
yhe results with cherries
do riot seem to be reliable
and it seems that cherries are not very suitable for
compression durinp-freeze-dryings,at least with the loading
cell used in these experiments.
It should be noted that
the drying apparatus was always operated at chamber
Pressures below 100 millitorr.
L, 4
?';eat:
'hedetailed experimental data for this section is
?iven in Appendix 3.
44,1 Ground Beef:
'.'wo
experiments were carried out with 1 ~,' fat plround
Avera'-ecompression pressures of 44.7 and 66.3
beef
si
were applied durin? freeze dryi-n,y
:-,iivin,,r compression ratios
(hei:iht)
1.2 and 1.2, respectively.
The final moisture
content of both compressed and non-compressed samples were
relatively
low in the ran{le of 1.7-2.5}5 after 4
hours of
dryinre.
!''The
percent total solids calculated from the final
-101moisture cortert were between 32.6-34. ,.-:,
Pehviydrat io
of
both compressed and nor-compressed
samples in cold water was very fast,
enerally reaching
more than PO percent relative to fresh after 2 minutes
and more than 90 percent after 30 minutes.
Vacuum did
not seem to affect rehydration. (krppendi
x C-1)
4.4.2
Bottom iound Steak:
Table
4.4.2.1
7 shows the summarized
:omression
results.
Ratio:
Fresh bottom round steak trimmed of fat was used
as cubes approximately
were
-racked
force would
in te
be
were conducted
1 cm on each side.
Frozen cubes
sample holder so that th e direction
to the ,?rain, A few
erpendicular
with' ,rain
of
tests
arallel to the d irection of
compression , :,'heinfluence of compression pressure on
compression
ratio
ratio based
on beirlht is c-iven since the ra ndom compression
is sown
in
1i-ure 1.
ratio will derTendon the size of te
1 c
beef c ubes.,
beef cubes, the correction factor
randoom comression
,he compression
ratio frorm the hei-h
',ith
to c alculate the
va lues
is aroxi-
mately 2, It car be noted from 'i?-ure 15 t hat the decree
of compression is linearly related to the c ornrress ion.
pressure; however, comnression is much less resuonsive to
increase in ?rressurethan the fruits and ve'-etables tested.
'-I
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-104'Thecomrression ratios of the samples havin
the ,rain
perpendicular to the direction of force were not different
from those having the g.rain parallel,
i.aximum compression
ratio (eirTht) obtained for beef cubes was 2.9 at a
pressure of 15
psi.
;o expansion of volume of compressed
beef was observed over 8 months of storage.
4.4.2.2
Iioisture Content:
Final moisture contents of both compressed and noncompressed dried beef cubes were quite low, in the range
of 0.4-2,60;. A
seernfrom table 7 after 48 hours of freeze-
dryinf the final moisture contents of compressed samples
are very close to non-compressed, and in many cases less.
':Thisindicates that compression of beef during freezedryin.-does not prevent achievin,glow moisture contents.
Percent total solids in fresh samples were calculated
from values of final moisture content and raned
25-29.5' which compares well with
from
the reported literature
value of 27.3' (U.£.D.A., 'andbook Fo. 2) for lean, raw
round beef.
4.4.2.3
Rehydration:
Examination of rehydration behavior of freeze-dried
beef cubes showed that in all cases non-compressed samples
rehydrated better than compressed (Appendix C-2).
There
was little influence of prior evacuation on rehydration
-10.5-
of either compressed or non-compressed samples (Appendix
-2).
In one test on the effect of water temperature
on the extent of rehydration, it was noted that warm
water ( 100°i;.)gave better rehydration than either cold
or hot water (Appendix C-2).
Col'dwater rehydration
ave
the best color, since warm or hot water rehydration
resulted in samples with a darkened, greyish--brown.
color,
and a cooked appearance.
l'herate of water uptake by
compressed samples was much slower than -thatof noncompressed.
Rehydration of non-compressed samples was
fast and went to equilibrium
in about
2
inutes.
In most
cases compressed samples still had a hard core after 30
minutes of rehydration and more than one hour was required
for complete penetration of water (Appendix
-2).
Compression
samples recovered almost full volume after rehydration.
The influence of compression ratio on rehydration is
shown in Fi--ure 16.
Some scatter
of the data poi.nts is
noted, perhaps due in part to the narrow range of compression
ratios (compared to other products tested).
straight
line to the data
Fittinr a
does indicate a trend toward
decrease in rehydration with increasing compression.
.4,4,2.4Or.anoleptic Evaluation:
:resh, non-compressed rehydrated and compressed
rehydrated beef cubes were fried in butter and served
a
a,
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-107immediately for or.anoleptic evaluation.
I'The
results
showed that both compressed and non-compressed samples
were very close in taste and texture.
Fresh samples
were rated first, non-compressed next, and compressed
last.
-lowever,no sifnificant difference was noticed
betweer all samples at 1 percent level of sig.nificance.
A prpendix -2 :rives the results of or-anole-tic
test
evaluation of fried beef cubes.
4.5
General Discussion:
All samples tested showed good compressibility,
except for cherries and in some cases, blueberries.
These products underwent collapse at the pressures applied.
It is likely, that the collapse which occured with these
products is mainly due to mass transfer limitations
which give rise
in sample temperature and the high sugar
contents.
The packed configuration in the mesh container and
the thick skin of these samples can contribute significant
mass transfer resistances.
Also, compression of the dry
layer during freeze-drying reduces the channals and pathways by which vapor escapes.
These effects can increase
the temperature of the frozen layer.
On the other hand as
mentioned in section 2.4 i.iacKenzie
and Luyet (1971) explained
that increase in sugar content and/or water:
sugar ratio
-108can significantly decrease glass-transition temperature
of sugars.
When the temperature of the boundary layer
passes this glass-transition temperature, the sugar phase
then flows under pressure with simultaneous and irreversible
deformation of cellulosic phase which seems to be the case
with blueberries and cherries.
It should be noted that
examination of non-compressed samples which had been freezedried at pressures well below 100 microns with only ambient
leakage heat input showed poor physical appearance, and in
some instances even
artial collapse,
Compressed samples
handled carefully.
enerally did not form bars unless
This was due to the difficulties in
releasing the compressed sample from the holder.
This problem
should be eliminated by design of the sample holder with
sufficient lubrication and possibly by use of binders in
the product.
Examination of products before completion of compression/
freeze-drying showed that compression proceeded with the
retreat of the ice front which indicates that compression
took place at the ice-dry layer interface.
This supports
the concept that at the ice-dry layer interface, the high
moisture content present gives a material having plastic
properties.
The compressive pressures used in this study (4-185 psi)
-109are significantly lower than those required for compression
of foods with the other methods currently employed (100-5000
psi) to obtain corresponding compression ratios.
Besides
differences in plasticity this may be due to the difference
in the thicknesses of the samples to be compressed.
In general a bed height of several centimeters is used
for techniques involving compression after complete dehydration.
Studies on moisture profiles during freeze-drying
have shown that the "diffusion zone" at the interface where
high moisture contents exist, has a thickness of a few millimeters (Aguilera, and Flink, 1974).
As mentioned above the
only place which is continuously compressed during freezedrying is this narrow layer where enough plasticity exists
due to the high moisture content present.
Therefore, the
pressures to compress this narrow layer should be much less
than the pressures required to compress samples with a thickness of several centimeters.
Compression ratios obtained were linearly related to
compressive pressure and increased with increasing pressure
over the range tested.
It is expected that the compression
ratio would reach a maximum value at some compressive pressure
so that no increase in compression occurs with further increase
in pressure.
It should be noted that during compression process
two types of void volumes are being eliminated.
One is the
-110void volume between individual
ieces in the sample
holder and th.eother is the voids that are produced
by sublimation of ice, which reflect the true change
in the volume of individual pieces.
This reduction
in volume is directly related to the initial water
content of the fresh material and its maximum value
approximately equals the volume originally occupied
by water in the fresh sample.
ments with beef,
when fresh.
This is clear in experi-
Lean beef has approximately 65;
water
Tihemaximum values of compression ratios
obtained in this study or reported elsewhere for this
product is approximately 3 which corresponds to about
657 reduction in volume.
Pressure was
increased
Also that, when the compressive
from 175; to 1_85 si (able
increase in compression ratio was observed.
7) no
T'his could
probably indicate the maximum compression ratio obtainable
by this technique is attained at pressures around lQO psi.
If food is considered
as a plastic material
with a
constant resistance to deformation, when stress is applied
there will be a minimal (residual) force required to initiate
compression (yield stress).
This was estimated to be (not
shown in graphs) relatively high for peas, carrots, and
beef cubes (around 50 psi), but very low for blueberries
and beans (<10
psi) which indicates that the force required
-111to initiate compression depends on the particular food.
If this resistance remains constant during compression
process then, the net force which causes deformation
can be expressed
as:
F1 = F2 =
F
=
;.a
d2 s
,,2
at
ds
d( )
or:
AF
dt
[
dt
= d(d)
=
MFdt
By integrating the above equation:
ds
-_
dt It -t,
then:
ds =
where: to = 0
Ftdt
Second integration results:
s - -
F
It:
'
t
2
2
If the drying rate is constant
t2
2MD
K1
and:
s = K 1 A F'
since:
F 2 = Constant
then:
s = K1 (F 1 ) -
then:
-112where:
F1
F2
M
t
s
force applied (springs)
resistance force (food)
mass of element being compressed
time element is plastic enough
distance that element moves in.time = t
From the above equation it can be seen that the extent
of deformation which is related to the degree of compression
is directly related to the force applied, inversely, to the
mass at the interface (which is related to the thickness of
diffusion zone), physical characteristics of the material
and time.
As mentioned before the bigger the diffusion zone
the higher force will be required for compression or in other
words with constant compressive pressure the degree of compression of the boundary layer depends on the thickness of the
boundary layer and increase in thickness of this layer results
in decrease in degree of compression.
Final moisture contents of both compressed and noncompressed samples were always similar and in most cases
about 1.5-45'.
TL'his
shows that compression process during
freeze-drying does not prevent the dehydration of the samples
to low moisture contents.
There were some indications that
the heat transfer might be improved due to compression; however, further investigations of drying rates during compression
are necessary to evaluate drying times.
-113Rehydration of samples was quick and in most cases
went to equilibrium within 15 minutes.
All samples except
blueberries re-attained their pre-compressed volume after
rehydration.
Non-compressed vegetables rehydrated to
lesser extents than compressed.
Vacuum rehydration reveal-
ed that the cause for lower uptake of water by non-compressed
vegetables was due to air entrapped in the cells.
Higher
rehydration of compressed samples than non-compressed (when
no vacuum was used) indicates some structural changes (distruptions) caused by compression either faciliate water
uptake or removal of entrapped air.
It is known that cell
distruption as produced by slow freezing (Logan, 1973)
cooking (Rahman, 1972) faciliates water uptake.
or
Curry,
et al, (1976) showed that soaking pre-cooked carrots in
water or low concentrations of "NaCl (0,1-0.2 Piolar) before
freeze-drying markedly improved rehydration.
They suggested
that soaking in water for relatively long times (24 hrs)
causes cells to become turgid with maximum water uptake
which in turn cause more distruption of cells during freezedrying resulting in better rehydration.
NaCl solutions do the same thing; however, it has also
been suggested that after freeze-drying, NaC1 crystals
deposited on the cells may attract water molecules (polar
effect) to faciliate flow inside the cells which causes even
-114higher rehydration than soaking in plain water.
Never-
theless compression or vacuum rehydration seem to improve
rehydration
of carrots to the same extent.
ihe percent rehydration (relative to fresh sample)
did not seem to be affected by the degree of compression
(except for beef cubes).
Organoleptic evaluations of prepared non-compressed,
compressed, and fresh samples indicated that compressed
were always rated equal to or hi-her -thanthe non-compressed.
The fresh samples usually were rated hiher
than either the
compressed and non-compressed freeze-dried product.
How-
ever in most cases the differences noted were not significant at the 1:. level.
-11 5-
5.
1.
COiCLUS
IOis
Compression during freeze-drying can be success-
fully accomplished.
2.
High compression ratios are attainable with appli-
cation of relatively low compressive forces.
3.
This process allows effective drying to low moisture
contents of compressed foods.
4.
During freeze-drying the ice-dry layer interface has
enough plasticity to allow compression.
This supports the
existance of a non-linear moisture gradient having high
values near the ice interface or a "diffusion zone," though
may be narrow at the ice interface.
5.
I'i~ost
dehydrated foods which can be plasticized for
compression through remoistening should be compressible during
freeze-drying using the procedure described.
6.
The degree of compression obtainable depends on
physical characteristics of the food material and the compressive pressure applied.
7,
Rehydration of compressed samples obtained by this
procedure is rapid and generally not affected by the extent
of compression,
8. Products prepared from compressed samples are as
acceptable as non-compressed samples.
9.
Compression during freeze-drying is advantageous
-116and more efficient than the methods currently used in
compression,
in that:
a.
It provides samples having a considerable
decrease in volume, which are below moisture
contents allowed, and which rehydrate rapidly.
b.
It does not require an initial over-drying
step to low moisture contents prior -to compression.
c.
It does not require a conditioning step prior
to compression.
d.
It does not require a final drying step after
compression.
-1176,
6.1
SUGGESTIONS FOR FUTURE RESEARCI
Dependence of Compression Ratio on Sample Thinckness:
In the course of this study the time was insuffi-
cient to perform a thorough investigation on the effect
of sample thickness on compression ratio.
However, in
some cases when the same force was applied to different
thicknesses of the same product different compression ratios
were obtained.
Therefore, is assumed that compression
ratio also is dependent on the sample thickness.
However,
further studies are required for a more precise measurement of the effect and determination of the best sample
thickness to be used for compression of particular samples.
6.2
Effect of Compression during Freeze-Drying on Drying
Rate:
There is little doubt that compression during freezedrying will affect the subsequent rate of drying since the
volume decrease is being achieved at the expense of void
volume which normally serves as pathways for vapor escape.
Reduction of the dimension of the vapor pathways will increase with increasing compression ratio, and furthermore,
as drying proceeds these reduced vapor pathways will increase in length.
From another view point compression of
the dry layer will result in improved heat transfer in the
-118dry layer.
The combination of these effects will
result in complex changes in drying behavior, and
it is necessary
to consider
them.
6.3 Effects of Sample Temperature (Frozen Layer and
Dry Layer) on Compression Behavior:
While moisture content is an important parameter
influencing plasticity, the temperature of the matrix
is also an important factor in determining the existing
plasticity.
The temperature gradients present should
interact with the resultant moisture gradients, to
produce the thickness of the layer which is capable of
being compressed.
Also, the frozen layer temperature
seems to be critical, since the higher moisture content
regions of the matrix should have a temperature very close
to that of the frozen layer.
The effect of this is most
evident when collapse occurs.
6.4
Dependence of Compression Ratio on Sample Loading:
To some degree this has been investigated with the
system used in this study.
However, there was no control
of the compression force during freeze-drying.
If compre-
ssive force is transmitted bya hydraulic pressure system,
the pressure on the sample at any time can be controlled
to produce improved final compression ratio and also
minimize the physical damage to the sample.
With the technique used in this study, it was necessary
-119to subject the frozen sample to pressure prior to
the start of freeze-drying.
This in many cases caused
cracking of the sample, since initially the frozen
sample is quite rigid and no plastic layer has yet
been produced.
This cracking effect was minimized by
use of cheese-cloth
to give a cushion
effect.
owever,
with use of a hydralic system it is possible to start
the freeze-drying first and after the plastic layer
is formed to apply the pressure for compression.
6.5
Use of Binders and Plasticizing Agents:
As mentioned earlier, binders and plasticizing
agents have been added to dehydrated food prior to
compression.
This can have a tremendous effect on
compressibility of the product and the final character
of the compressed bar.
However, for compression during
freeze-drying it is necessary to add these agents in the
wet state, that is prior to freeze-drying.
has to be evaluated.
(1972) used
The effect
For instance, when Shipman, et al,
lycerol to improve the texture of celery,
the glycerolated samples underwent low temperature evaporative drying rather than true freeze-drying as glycerol
preventsfreezing in most cases,
Nevertheless, use of
binders may improve the cohessiveness of the food pieces
and result in an improved compressed food bar.
-1206.6
Design of a Sample
Compressed Bars:
older for "Easy Removal" of
As mentioned, unless careful handling is exercized
in most cases the compressed samples separated into
individual pieces during removal.
A better design
of the sample holder with smooth and lubricated walls
should reduce this problem.
6.7
Freeze-Drying/Compression with Other Food IMaterials:
For more completeness other food materials than the
ones already tested could be considered.
-121LIBLIOC
`1)
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RA1 TY
and Flink, J.M. (1974). "echnical
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2)
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(1969). "The effect of sublimation temperature on the rate of the freeze-drying
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3)
£err,
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i.A.
"Compressed
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U..
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4)
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"Trans-
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International Institute of Refrigeration,
Commision
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Brake, R.F. and Decareau, R.V. (1964). "Advances in
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.,larke,
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,,
and
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8)
Curry,
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.CI., Burns,
j.L. (1962),
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.
"ireeze-Drying of Pood-
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,E., and
eidelbaugh,
r.D.
(1976).
"Effects of Sodium Chloride on Rehydration
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Do,
. Y., -risannam, C,, alunkhe, D,K , and Rahman,
.i.(1975;). "1reeze-Dehydrated
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I,
Product ion
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1])
iD~rake,
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ackac-in,
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-'12)
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Food Components
Contract
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Army lNatickLaboratories,
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AD-,662060.
13)
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(1949).
14)
Gentzler,
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16) Goodin.,
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"Freeze-bryin",
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"Iveat and Ve<etable Slocks",
i.oodinF,
i.'. b. (1955).
"Problems
,-,sL
and
olfe,
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Food 'iechnolo
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unclassi-
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311.
(1957).
".Some
ecent
on jDehydration in the United
,Alork
1 )
16,
fied Renort SA/IA/A/Ri/31, Iviinistry of Apgric.,,
and Food, reat
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f'isheries,
r'oodiianufac.,30,
17)
Reinhold, Iew York.
,
e~
1,
Kingdom,
302-306.
,BD,
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R ., ivlacDougall,
.C., and
Gasay, J. (1958)
"T'he tora?-e Life of
DehydratedVeetables III",
Unclassified
Report A/DA/iv;/82, i;inistry of Aric,,
reat
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''isheries, and ood,
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iamdy, V.i.. (1960). "Compression of ehydrated Foods",
Review of Literature, ontract lo,
DA19-129-Ql.-1630, Quartermaster ood
and Container Institute for the Armed
Forces, hicago, Illinois.
20)
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/iK.lVi.
(1961), "Compression of Dehydrated Fioods",
R11eport10(o. 3,
Contract
No.
A19-129-Qivl-
'ood and Container
1630, Quartermaster
Institute for the Armed Forces, Chicar o,
Ill inois.
'21)
22)
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Final Report, Contract Io. DiA19-129-QMi1999, Quartermaster Food and Container
Institute for the Armed Forces, Chicago,
Illinois.
'ardin,
, C
(1965).
:. H-.
hesis, L-eoria
Inst. of
Technology.
23)
Harper,
J.C. and ''appel,
,.L. (1957), "Advances in Food
tewart, .F., and
Research", Vol..7,
rds., Adademic Press, !"ew York.
iirake,
B;.i,,
(1964). kfv.S.Thesis, '5eor,(ia Inst. of
24) MHatcher,J.,
-25)
Technolog!y, Atlanta,
Georgia.
'innerffardtand Sherman, (1975). "Evaluation of rial
Procurement of eversibly Compressed
Freeze-L)riedFoods", 'echnical Report
75-96-FEL,
U.S. iatick Laboratories,
?Watick,
-'26) Jones, R.L.
"I'ass.
(1975). "Use of Hydratino
and Kin-, Ji.J.
Salts to Accomplish Limited 'reeze-Dryin",
For resentation at Symposium on ryin6,:
Erincirles
and iechnolo.g-y,l:ational iveetinp,
American Institute of Chemical
ngineers,
Boston, Mlass.
`2a)
Ka-salis,J,,
,lvalker,
"A
jr,
j.L., and Wolf, ii, (1970)
hysico-chemical
tudy of liechanical
properties o- Low and Intermediate ioisture
1, 464-483.
Foods". J.Texture Studies,
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B,, and Johansson, B., "Textural
Properties of Dehydrated Foods. Relationship with the Thermodynamics of Water
Vanor Sorption",
J. Texture Studies, 1,
285-308.
*28)
Karel, i. and Flink, J.ii. (1973). "Influence of Frozen
State on Freeze-Dried Foods", J. Agr. Food
CHEM. 21(1), 16-21.
*29)
fiacKenzie, A.P. and Luyet,
341.
t30)
KinF,, C.J. (1971). "reeze-Drying
B.J. (1967). J. Cryobiology,
3,
of Food". CRC Press.
*31) Kinq, C.J., Carn, R.m., and Jones, R.L. (1975).
"Processing Approaches For Limited Freeze-Drying",
For Presentation at Annual Mleeting, Institute
of Food Technologist, Chicago, Illinois.
*32)
33
Konigsbacher, K.S. (1974). "Miethodsfor Storing Shape and
structure of Compressed Dehydrated Animal
and Combination Products", Contract Nio.
iAA G17-73-C-0030, T'echnical Report 75-56-FL,
U.S. Army Natick Laboratories, Natick, ivass.
Larmond, E. (1970). "kIVethod
for Sensory valuation of Food",
CanadaDepartment of Agriculture, Publication
12,94.
'34) Lon-more, P. (1970). "Freeze-Drying" Chemistry and Industry
970.
3r9) Lo:.an, B.J, (1973), "Effect of Processing Variables on
Quality of Freeze-Dried Carrots", P.Hj.D.
Dissertation,
36)
Luikov, A.V. and Vasiliev,
Texas A and iVUniversity.
L.L. (1969).
"11
eat Transfer
Capillary Porous Bodies in a Rarefied 'as
of
Flow", In Symposium on Thermodynamic Aspects
of Freeze-Drying", International Inst. of
Refrigeration,
Switzerland.
37)
Commission X, Lausanne,
acKenzie, A.P. (1965). "Basic Principles
of Freeze-Drying
for Pharmaceuticals", Bull. Par. Drug. Assn.,
20/4,101.
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A.P. (1966). Bull Parenteral
20, 101.
*39)
ivacKenz ie,
A.P. and Luyet, B.J. (1967). 'ater Binding
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*40)
IlacKenzie, A.P. and Luyet, B.J. (1969). "Recovery of
38)
rug Assn.,
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Report FL-90, U.S. Army atick Laboratories, Natick, Miass. AD-703901.
*41)
42)
MacKenzie,
Iiargaritis,
A.P. and Luyet, B.J. (1971). "Recovery of
compressed Dehydrated Foods" Phase II,
Contract No. DAAG 17-67-C-0126, Technical
Report 72-33-FL, U.S. Army atick Laboratories,
Natick, ivass.
A. and King, C.J. (1969), Paper presented at
A.I.ch.E. MIeeting,Portland, Oregan.
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Proceeding XI International
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iunich
"44) Pavey, R.L. (1975). "Study 'ITechniques for Controlling
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Phase I, Contract
in CompressedFoods",
No.
DAAG 17-73-C-0121,
Technical Report 75-49-FEL,
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viass.
Pilsworth, Jr., M.N., and Hoge, H.J.
(1973). "The compression of Freeze-Dried beef to Form Bars;
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Vapor", Technical Report 73-56-PR,
U.OS.
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IViass.
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D.E. (1969). "Studies on Reversible
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Teclhnical Report
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U.S. Army Niatick
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Iass. AD-698457.
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Rahman, A.R., Taylor, .R., Schafer, G., and Westcott,
D.E. (1970). "Studies of Reversible
Compression of Freeze-Dried RTP cherries
and Blueberries", Technical Report
70-52-FL, U.S. Army Nlatick Laboratories.
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Rahman, A.R., Schafer, G:.,Prell, P., and estcott, D.E.
(1971). "on-Reversible Compression of
Intermediate IMioisture
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*49b)
Rahman, A.R.,, Bishov, S., and ,*Xestcott,D.E. (1971).
"Reversible Compression of Dehydrated Peas",
J. Texture Studies, 2, 240-245.
49)
Rahman, A.R. (1972). "Increased Density for ilitary
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Rey, L. (1964). "Fundamental Aspects of Lyophilization,
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Schafer, G.R. and Rahman, AR.
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*58)
59)
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*Literature reviewed
-128-
APPENDICIES
TABULATED RESULTS OF
REHYDRATION TESTS AND ORGANOLEPTIC EVALUATIONS
-129-
EXPLANATION OF REHYDRATION TESTS:
Notations
a
% Rehydration
gr H20
gr H20
gr T.S. (rehydrated)
b
Sample codes:
Comp.
N.Comp.
gr T.S. (fresh)
Compressed
Non-compressed
C
Indicates if vacuum applied prior to water content
(see section 3.2.5)
No
No vacuum
Vac.-air
Vacuum released with air
Vac.-H20
Vacuum released with water
d
Water temperature
Cold
(40-700 F.)
Warm
(90-1000 F.)
Hot
(130-1600 F.)
-130EXPLANATION OF ORGANOLEPTIC EVALUATION:
Notations:
a
Mean Score (9 point hedonic scale)
1: Extremely dislike to 9: Extremely like
b
Ranking test:
Scale of +0.85 to -0.85 used
c
Sample codes:
Comp.
N.Comp.
F.
@, #
Compressed
Non-compressed
Non-dried material
thawed)
(fresh or frozen
Indicate if there is any statistical difference.
Diffenret symbols indicate significant difference
(at 1% level).
-131-
APPENDIX A
TABULATED RESULTS OF
REHYDRATION TESTS AND ORGANOLEPTIC
EVALUATION FOR VEGETABLES
Cl)
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APPENDIX
B
TABULATED RESULTS OF
REHYDRATION TESTS AND ORGANOLEPTIC
EVALUATION FOR FRUITS
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