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Recrystallization of ice during bulk storage of ice cream
In/. Dairy Journal 6 (1996) 1209- 1221 Copyright 0 1996 Elsevier Science Limted
PII:
SO958-6946(96)00030-l
Printed in Ireland. All rights reserved 0958-6946/96/$15.00 + 0.00
ELSEVIER
Recrystallization
of Ice During Bulk Storage of Ice Cream
Daniel P. Donhowea ‘Department
& Richard
W. Hartef’*
“Spray-Tech, Inc., Allentown, New Jersey, NJ, USA of Food Science, University of Wisconsin, 1605 Linden Dr., Madison,
(Received
Wisconsin,
WI 53706, USA
I 1 November
1995; accepted
8 June 1996)
ABSTRACT Ice recrystallizution was studied in 1.9 L contuiners qf’ ice cream stored so thut surface temperature of ice cream wus controlled with fluctuations sf’ f l.O”C. Core and surface samples were taken at regular intervals and anal,vzedfor ice crystal size by cold-stage microscopy and image analysis. Mean ice crystal *size plotted vs. timeo.33 resulted in a straight line, with a slope equal to recrystallization rate (pm duym0.33). Storage temperatures between -15 and -5°C were studied. Recrystallization rate increased with storage temperature and extent of’ temperature fluctuations. Within a single container, meun ice crystal size grevt significantly larger in ice cream at the outside wall than near the core qf’the puckage. Recrystallization rates determined in this study were compared to those found at identical temperature conditions, but in an accelerated storage study. Recrystallization was more rapid in the accelerated study,Jtirjluctuating temperature, but slower,for constant temperature. Copyright 0 1996 Elsevier Science Limited
INTRODUCTION
Many aspects of storage stability of foods, including changes in ice crystal size distribution during frozen storage, have been recently described by glass transition theory. The importance of glass transitions in foods has been recognized fairly recently, due largely to the efforts of Levine & Slade (1986, 1988); however, its importance in frozen biological systems was studied many years before by Luyet and co-workers (for example, Luyet & Kroener, 1966; Kroener & Luyet, 1966; Luyet, 1969; Rasmussen & Luyet, 1969). The glass transition, which represents a change in state from a glassy solid to a rubbery, viscous fluid (Slade & Levine, 1991), is manifested by a change in slope *Author
to whom correspondence
should be addressed. 1209
1210
D. P. Donhowe, R. W. Hartel
of volume expansion and by a discontinuity in heat capacity. The glass transition has been characterized operationally as the state at which the viscosity of the material is lOi Pa-s (Slade & Levine, 1991). Perhaps the most important implication of glass transition theory in frozen foods, and foods in general, is that translational and rotational motion of molecules (especially larger ones) do not occur in the glassy state during a practical time-frame (Wolanczyk, 1989). Since ice crystal growth in frozen systems at low temperatures is impaired by very low mass transfer rates resulting from the high viscosity of the unfrozen solution (Fennema, 1973), ice crystal growth rates are very slow and become practically negligible in the glassy state. In the rubbery state, rates of translational diffusion increase rapidly with increases in temperature or concentration of a plasticizer (e.g., water). Since diffusional rates are inversely related to viscosity, this effect is described quantitatively by the Williams-Landel-Ferry (WLF) equation (Williams et al., 1955): log,,
- rl’pT
{ %lP&
=_ 1
G(Tc2 +(T-
p,,
1 Tg
where -q is viscosity, p is density, Cr and C2 are constants, the subscript g refers to the glass transition, Ts’ has been substituted for the conventional glass transition temperature (Ts) of polymer systems, and T is in K. The ‘universal constants’ of Ci = 17.44 and C2 = 51.6 have been shown to fit the viscosity of concentrated sucrose and fructose solutions very well (Soesanto & Williams, 1981). Levine 8z Slade (1989) found that commercial ice cream and other frozen desserts have glass transition temperatures (T,‘) ranging from -27 to -41°C. This temperature range corresponds with the glass transition temperatures of the principal water-soluble components in ice cream (Slade & Levine, 1988): sucrose (Tl = -32”Q lactose (-28°C) and glucose (-43°C). Since diffusional rates are very low below or near the glass transition temperature, it would appear that very little diffusion and hence, migratory recrystallization should occur at storage temperatures of -30 to -40°C. However, diffusional rates would be expected to be much higher during distribution of ice cream due to the very strong temperature dependence of WLF kinetics. Indeed, the WLF equation with the ‘universal constants’ predicts a change in viscosity of 5 orders of magnitude over a 20°C temperature range near the glass transition (Slade & Levine, 1991). In polymer systems, WLF kinetics are usually applicable over a 100°C temperature range above the glass transition temperature (Williams et al., 1955). However, WLF kinetics would be relevant over a much smaller temperature range in frozen foods, since the melting point is only 2540°C above the glass transition temperature and solutions typically follow Arrhenius kinetics above the melting temperature (Slade & Levine, 1991). The major complication of WLF theory as applied to frozen systems such as ice cream, however, is that the composition of the unfrozen solution changes as the temperature increases, due to melting of ice (Simatos et al., 1989). Since the glass transition temperature decreases with the inverse of the weight-average molecular weight of the solution (Levine & Slade, 1986; Slade & Levine, 1988), and water has a low molecular weight relative to other solution components, increases in water content cause a ‘Ts depressing effect’ (Slade & Levine, 1991). Thus, dilution of the unfrozen solution results in an even greater decrease in viscosity as temperature increases than predicted by the WLF equation (Simatos et al., 1989).
Recrystallization in bulk ice cream
1211
The effects of storage temperature and controlled temperature fluctuations on recrystallization rate in ice cream have been recently studied by Donhowe & Hartel (1996). Their results documented the increase in recrystallization rate that occurred when both storage temperature and amplitude of temperature fluctuations increased. However, these studies followed recrystallization in a thin film of ice cream mounted on a microscope slide under controlled conditions. Ice crystal sizes over 200 urn (based on equivalent circular diameter) were measured despite working with films that were only 100 urn thick. It is likely that the slides used to mount the ice cream sample had significant influence on recrystallization rates. Thus, these results may not correlate very well with recrystallization rates observed during bulk storage of ice cream. Other differences between storage conditions (accelerated vs. bulk storage) that might influence recrystallization rate include (1) better heat transfer on the microscope stage, (2) differences in proximity of crystals to one another due to formation of a thin film for accelerated studies, and (3) disruption of air cell structure in ice cream during formation of thin films for accelerate studies. In this work, controlled storage studies were carried out to determine the influence of temperature on ice recrystallization in ice cream stored under typical bulk storage conditions. In addition, kinetic data for ice recrystallization were analyzed in terms of either Arrhenius kinetics or WLF kinetics for temperature dependence.
MATERIALS
AND METHODS
Controlled bulk storage boxes
Two boxes, made of 5 cm thick Styrofoam insulation, were custom built for the controlled bulk storage experiments, as described by Donhowe (1993). To control temperature, heaters were used to counter-balance the cooling provided by the refrigeration system of the walk-in freezer in which the boxes were contained. Two fans provided continuous air flow through each box. The air entering a box first passed through a gravel bed, which acted as a heat sink to dampen any fluctuations in freezer temperature. The air was then heated to the desired temperature by the coil heater. Perforated boards at the top and bottom helped provide even, downward air flow and uniform temperature (within O.lC) throughout the sample chamber. Ice cream samples were placed on test tube racks, which had minimal thermal contact with the containers and did not significantly obstruct air flow. Each storage box also contained a ‘constant-temperature chamber’, which was jacketed to maintain air temperature fluctuations within O.l”C. The jacket contained a 50% ethylene glycol solution to provide a thermal heat sink. The constant temperature boxes were large enough to hold three 1.9 L ice cream containers. In this way, both fluctuating and constant temperature conditions could be studied simultaneously. Temperature in each storage box was controlled with a thermistor mounted near the top of the sample chamber. The thermistor signal was sent to a data acquisition board (DT-2805, Data Translation Inc., Marlboro, MA) connected to a microcomputer (ZDE-1217-BO, Zenith, St. Joseph, MI). Control was implemented by monitoring the power input to the coil heater. A 32-channel multi-
1212
plexer facilitated tures throughout locations within tal size analysis,
D. P. Donhowe, R. W. Hartel
additional temperature measurements, including air temperathe freezer and storage boxes. Temperatures at several product the 1.9 L container, corresponding to sample locations for cryswere also taken.
Mix preparation and freezing A single batch of 40% total solids ice cream mix (12% fat, 11% serum solids, 16.5% sucrose, 0.3% 250 Bloom gelatin (Vyse Gelatin Co., Schiller Park, IL), 0.1% emulsifier blend of 80% glycerol monostearate and 20% Polysorbate 20 (Drewmulse 700, PVO International, Inc., Boonton, NJ) was pasteurized at 82.2”C for 16 s, and homogenized at 13.8/6.9 MPa (second stage). Cooled mix was stored at 1.7”C for 16 h (Donhowe, 1993). Ice cream mix was frozen in a continuous scraped-surface freezer (Model 3N-15 APV Crepaco, Chicago, IL) at 80% overrun with a draw temperature of -5.9”C. Thirty-two 1.9 L cylindrical containers (12.5 cm diameter x 15.5 cm height) were filled with ice cream, hardened in a hardening room at -25°C for 24 h and stored in Styrofoam containers at -27 to -30°C until needed for the storage tests. Four 0.5 L samples of ice cream were also taken at draw for immediate analysis of ice crystal size. Finally, two 1.9 L ice cream containers were analyzed for ice crystal size after hardening. Storage treatments Ice cream containers were allocated randomly among the various storage treatments. Containers were subjected to fluctuating or constant temperature storage at four mean temperatures: -5, -7, -10 and - 15°C. Two ‘control’ containers were stored in a Styrofoam box at freezer temperature for the duration of the experiments. Experiments at -5 and -7°C were carried out over 8 days. These experiments were replicated once. Samples stored at -10 or -15°C were held for 32 days. An experiment began by equilibrating each box at the desired mean temperature and calibrating the control thermistors with a standardized thermometer. The samples for constant-temperature treatment were placed in the chambers, after equilibration for 4 days. The remaining samples were then transferred to the constanttemperature boxes, and temperature set to continuous, 2 h, sinusoidal oscillations about the mean storage temperature. The amplitude of the temperature oscillations was adjusted so that the surface temperature of the ice cream containers fluctuated by fl”C. This resulted in four temperature protocols for air temperature for the fluctuating temperature treatments: -5f7”C, -7&6X!, -1Of5.5”C and - 15f5”C. Microscopy and image analysis Samples were analyzed for ice crystal size distribution after draw, hardening and various periods of storage. The 0.5 L samples of ice cream taken at draw were analyzed immediately in a refrigerated glove box which housed an optical microscope (Donhowe et al., 1991), set to the draw temperature (-5.9”(Z). For hardened and stored ice cream, samples were taken from duplicate 1.9 L containers at various radial locations at the mid-point of the height of the
Recrystallization in bulk ice cream
1213
container. Interior samples were obtained with a cork borer. To take surface samples (at a radius of r/R = 1, where R is the radius of the container), a screwdriver was used to make a small hole in the container wall at the bisection plane, and a small amount of ice cream was removed. Surface samples were taken at 0, 0.5, 1, 24 and 8 d for ice cream stored at -5 or -7°C and at 0, 1, 2,4, 8, 16 and 32 d for products stored at -10 or - 15°C. Interior samples at r/R = l/3 were taken less frequently during the experiments, and more complete positional sampling (r/R = 0, l/3, 2/3 and 1) was done only for initial (0 days) and final samples (8 or 32 days). Samples were placed into vials, transferred in a Styrofoam container, on cold packs of frozen brine at about -lO”C, to the refrigerated glovebox, and equilibrated at -14°C for several hours. An optical microscope housed within the refrigerated glove box was used to take photomicrographs of ice crystals in each ice cream sample. A few mg of each sample placed on a microscope slide were diluted with a few drops of n-butanol and spread into a thin film by pressing a second microscope slide onto the sample. The n-butanol was used as dispersant to provide adequate dispersion of ice crystals while maintaining initial crystal integrity. Photomicrographs of ice crystals were taken within 15 min of sample preparation, so that no change in crystal size occurred over the time period of measurement. Negatives were enlarged and analyzed by image analysis according to procedures described by Donhowe (1993). One representative 20 x 25 cm photomicrograph was analyzed for each sample. Differential scanning calorimetry A differential scanning calorimeter (DSC Series 7, Perkin-Elmer, Norwalk, CT) was used to determine the apparent maximally freeze-concentrated glass transition temperature (7’s’,) of the ice cream. Triplicate samples of melted ice cream (approximately 10 mg each) were cooled at 20°C min-’ to -60°C and then heated at 5°C min-’ to 10°C. Derivative thermograms were used to identify a transition temperature in the manner described by Levine & Slade (1988). This transition temperature was probably not the true Tg’, yet provides a simple and consistent reference point for comparison purposes. In this paper, the temperature associated with this transition is called Tia, an apparent glass transition temperature. For the ice cream formulation used here, a value of -34.5”C was obtained.
RESULTS AND DISCUSSION ControIIed bulk storage The controlled bulk storage experiments were designed to simulate typical storage conditions, yet still facilitate comparisons with previously reported accelerated recrystallization experiments (Donhowe & Hartel, 1996). Air temperature cycles of 2 h were similar to the cycle time of a typical cabinet freezer. However, air temperature fluctuations were set to attain product temperature fluctuations of f 1°C at the surface of the 1.9 L containers for comparison to previous results (Donhowe & Hartel, 1996). The amplitudes of the air temperature fluctuations (-5f7”C, -7f6”C, -1Of5.5”C and -15f5”C) were therefore much higher than
1214
D. P. Donhowe, R. W. Hartel
typically encountered under commercial conditions. It was necessary to increase the amplitude of the air temperature oscillations as mean temperature increased, since at higher temperatures the ice cream had a higher effective thermal mass (more unfrozen water and more latent heat changes during a temperature cycle) and lower effective thermal diffusivity. Unless otherwise stated, temperatures discussed below refer to product temperatures rather than air temperatures. Change in ice crystal morphology Ice crystals after hardening (0 days storage) were smaller at the surface (r/R = 1) than in the center (r/R = 0) of the containers. After 1 day storage at -5fl”C (2 h cycle; surface temperature), ice crystals at the surface became rounded, but also somewhat angular. The number of fused crystals reflected the importance of accretion. After 8 days storage, crystals became very rounded, but had not developed into oblong crystals observed during accelerated recrystallization at -5fl”C with 2 h cycle time (Donhowe & Hartel, 1996). Ice crystals in the interior (r/R = l/3) of ice cream containers held at -5fO.Ol”C after 8 days were very similar to crystals at the surface of the containers. From the shape of ice crystals, bulk storage at -5&0.2X! (the interior temperature oscillation) gave results that were indistinguishable from those for storage at constant temperature (-S~tto.OlC). This is evidence for a ‘threshold’ amplitude of temperature oscillations, below which melt-refreeze recrystallization is unimportant, as noted previously for accelerated recrystallization at -10°C (Donhowe & Hartel, 1996). Crystals remaining after 8 days at -5f0.2”C or -5fO.Ol”C were more rounded and smaller than crystals present after 8 days at -5fl”C. Ice crystals at the surface of containers after 8 days storage at -1Ofl”C were very similar in shape to crystals after 8 days at - lOfO.Ol”C, with rounding and accretion being prevalent for both conditions. However, crystals were smaller and much less rounded than crystals held for 8 days at -5fl”C. This increase in isomass recrystallization at higher temperatures is probably due to a decrease in the viscosity of the unfrozen solution. Change in ice crystal size The ice crystal size distributions in ice cream containers subjected to oscillatingtemperature storage at -5°C are shown in Fig. 1. Ice crystals increased in size between draw and hardening (0 days storage). However, the ice crystals at the surface (r/R = 1) were slightly smaller after hardening than the crystals in the interior (r/R = l/3). This is due to the slower rate of cooling and hence, more rapid recrystallization in the interior of the container relative to the surface during hardening. This difference between surface and interior, although not great, was significant (P = 0.05) in virtually all containers tested after hardening. After 4 or 8 days storage under oscillating-temperature conditions, crystals at the surface of the container were larger than crystals in the interior (Fig. la). This effect was due to the larger temperature fluctuations at the surface (fl”C) than in the interior (f0.2”C at r/R = l/3) of the ice cream containers. Mean ice crystal size increased and the range of crystal sizes became larger during storage. For the constant-temperature experiments at -5”C, the initial difference in size distributions between interior and surface ice crystals became negligible during storage (Fig. lb). Crystals did not become quite as large and size distributions
Recrystallization in bulk ice cream
1215
a) Oscillating Air Temperature (-6 f 7 “C, 2 h Cycle)
90 80 d 3
70
1
6o
-Draw
--C+O~dR=lM +o~d,l +1d,rm=1
BB 6o
+24rflw
3
-44rfR=m +44d,nk1
CO E 30 Q)
~8d,rklRl --e-Sd,r/Jkl
@I 20
0
50
100
160
200
250
Cry&al Size (pm) b) Constant Air Temperature t-5 f 0.1 “C) lOO90. 80$ a
70-
d c
Draw +O&r/R=l/3
60-
-o(lril2=1 +ld,r/R=l -O-22,rlklB
!I ‘tl
so- . 40.
-2d,m=1 -44rnm
& 30@ pI
+8d,r/R=lI3 --e-Sd,r/R=l
20-
100
160
Crystal Size (pm) Fig.1. Ice crystal size distributions
in 1.9 L containers of ice cream during storage at -5°C: (a) Oscillating air temperature (-5f7”C, 2-h cycle); (b) constant air temperature (-5fO.l”C). Error bars represent standard errors based on two replicates per data point.
1216
D. P. Donhowe, R. W. Hartel
were somewhat narrower as compared to storage at oscillating temperature conditions at -5°C (Fig. la). Ice crystal size distributions after 8 days storage at all storage temperatures are shown in Fig. 2. As expected, oscillating-temperature storage (filled symbols) resulted in greater recrystallization than constant-temperature storage (open symbols), while recrystallization increased with temperature within each set of experiments (constant or oscillating-temperature storage). The data for mean ice crystal size at the surface of the containers fit t1’3kinetics well, as seen in Fig. 3. Higher regression coefficients were obtained when the earliest data (less than 1 day of storage) were excluded from the regressions, since there appeared to be an initial lag in recrystallization rate. This lag in time before recrystallization fit the t ‘I3 kinetic model seemed reasonable, since it took about 1 day for the ice cream to equilibrate to the storage box temperature (containers were initially at a temperature of -25 to -27°C). Recrystallization in ice cream at the surface of containers stored at oscillatingtemperature conditions (such that the surface temperature fluctuated Itl“C) increased with increasing mean storage temperature (Fig. 3a). Fairly good agreement was obtained between samples for any given experiment. Slightly slower recrystallization was observed at the surface of containers held at constant temperature (Fig. 3b). Constant-temperature experiments at -5 and -7°C were repeated and gave reproducible results. 100 90’ 80’
70-5 f 0.01 “C, Trial 1 -5 f 0.01 “C, Trial 2 -5 f 1 “C, 2 h Cycle -7 f 0.01 “C, Trial 1 -7 f 0.01 “C, Trial 2 -7 f 1 OC,2 h Cycle -10 f 0.01 “C -10 f 1 “C, 2 h Cycle -15 f 0.01 “C
6050’ 40’ 3020-
-15 f 1 “C, 2 h Cycle
10’ O+0
50
100
150
200
250
Crystal Size (pm) Fig. 2. Ice crystal size distributions at the surface of 1.9 L containers of ice cream after 8 days controlled bulk storage at constant or oscillating temperature. Error bars represent standard errors based on two replicates per data point.
Recrystallization in bulk ice cream
Oscillating Product Tempmature (k 1 “C, 2 h Cyde)
a) MO. 140:8
;
Q
--o-5 “C, Sample 1 +-5%,Sampie2 + -7 “C, Sample 1
.
+ -a+
lOOi,
--e *
120;
1217
0
-7 -10OC, “C,Sample Sample2 2 1 “C, Sample -16 1 -16 “C, Sample 2
iz 1
8o:
u
60-
A
f z
.
0.0
0.5
1.0
1.5 t*
2.-o
2.i
3.-o
(dl’$
b) Constant Product Temperature (i 0.01 “C) 140 7 -o-0--o+ l20- + ~-o+ z &:I__ e-15 -w-
-5 “C, Trial -5 OC,Trial -5 “C, Trial -5 VI, Trial -7 Trial
1, Sample 1 1, Sample 2 2, Sample 1 2, Sample 2 1
e
-7 “C, OC,!h+ Trial 2, 1, Sample 2 1 I/ i -;ooFdyal2. Sample 2 . ,
P
_- _ oc: s
w -15 “C, Sample 2 An
20~.-..1----‘-...‘....‘-..-‘....’..-.’ 1.0 0.5 0.0
1.5 tlB
Fig. 3. Effect of storage temperature
2.0
2.5
3.0
3.5
(d1’3)
on mean ice crystal size at the surface of 1.9 L containers of ice cream: (a) Oscillating product temperature (il’C, 2 h cycle); (b) constant product temperature (kO.Ol“C).
1218
D. P. Donhowe, R. W. Hartel
Kinetic models for ice recrystallization in ice cream Recrystallization rates were determined from the slopes of plots of mean ice crystal size vs. t’13 and were found to increase rapidly as the temperature approached the melting point of about -2.9”C, as shown in Fig. 4. Oscillating temperatures (* 1“C) generally enhanced recrystallization relative to storage at constant temperature. Data for accelerated recrystallization of ice cream in a thin layer on a microscope stage from Donhowe & Hartel(1996) are included in Fig. 4 for comparison to data under bulk storage conditions from this study. Temperature fluctuations (3~1°C) on the cold stage resulted in greater recrystallization than in the bulk storage boxes. The main reason for the difference in results may be the physical limit on growth in the vertical direction for the accelerated recrystallization samples, which consequently results in greater extent of growth in the horizontal directions. At constant temperature, recrystallization rates were higher for the controlled bulk storage experiments than for the accelerated recrystallization experiments. This difference could be due to closer proximity of crystals and more crystal-crystal contacts (and hence, increased accretion) in the bulk storage experiments, since ice crystals prepared on the cold stage were dispersed into a thin layer. In addition, microstructural differences may have been important, since the air cells were disrupted in the accelerated recrystallization experiments. These data were fitted to the Arrhenius kinetic equation, and a reasonable tit obtained. However, it must be stressed that it may be fairly easy to tit Arrhenius 140 -
120 -
0
Accelerated Recrystallization: 10 min. f 1 “C Oscillations
X A
Accelerated Recrystallization: 2 h, f 1 “C Oscillations Accelerated Recrystallization: Constant Temperature Controlled Bulk Storage: 2 h, f 1 “C Oscillations
A
Controlled.Bulk Storage: Constant Temperature
0
% h
100 -
i J
80 -
B !!
60 -
3
!!I
40 -. 20 -
0,.
-20
I’
I.
-18
-16
-14
-12
-10
Mean Temperature
-6
-6
-4
(“Cl
Fig. 4. Temperature dependence of ice recrystallization rates in ice cream. Accelerated recrystallization data from Donhowe & Hartel(l996).
Recrystallization in bulk ice cream
1219
kinetics over the relatively small temperature range of the experiments (- 10 or - 15°C). The ‘apparent’ activation energies of recrystallization were fairly similyr for all data sets, with apparent activation energies from 112 to 126 kJ mol- . These values agree with the value of 115 kJ mol- obtained by Kingery (1960) for ice sintering, and the value of 116 kJ mol-’ obtained by Martin0 & Zaritzky (1987, 1989) for ice recrystallization in NaCl solutions. Recrystallization rate data for accelerated storage with temperature cycles of fl.O”C were fitted to WLF kinetics based on two different sets of constants (eqn 1). The recrystallization rate at the apparent glass transition temperature was not known, although the rate of diffusion-limited processes should be ap roximately zero at this point. For this work, an arbitrary value of 25 pm day -lp3 at -10°C was used as a point of reference for the model due to the lack of more exact data. The data fit the WLF equation reasonably well with ‘universal constants’ (Slade & Levine, 1991) C, = 17.44 and Cz = 51.4. Although extensive statistical tests for fitting this model were not carried out, a better lit was obtained for values of C, = 20.4 and C2 = 154.8, which are coefficients found to apply to highly symmetrical, readily crystallizable polymers in which the difference between the melting point and the glass transition temperature is much greater than 100°C (Slade & Levine, 1991). Since neither of these criteria apply to ice cream, good fit of these values for Ci and C2 to the data is probably coincidental. Thus, the recrystallization rate data can be empirically fitted to WLF kinetics based on the 7’ia, but the ‘universal coefficients’ did not appear to apply. Recrystallization rate data were also correlated with the change in amount of water frozen, as suggested by Bradley (1984), and shown in Fig. 5. Changes in percent frozen water in the 40% total solids ice cream during temperature fluctuations of fl”C were determined from calculated freezing point depression curves (Arbuckle, 1986; Bradley, 1984). As expected, recrystallization rates increased with the amount of water that melted and refroze (assuming ice phase equilibrium) during each f 1“C temperature cycle. Recrystallization rates were greater for il”C oscillations under accelerated storage conditions (Donhowe & Hartel, 1996) than in the storage box. Although recrystallization rates correlated fairly well with changes in percent frozen water, there are limitations to this approach. In particular, recrystallization rates at constant temperature (&O.Ol’C) should be negligible, since there should be no change in amount of water frozen, which clearly does not agree with the results shown here.
CONCLUSIONS Recrystallization in ice cream stored in bulk containers increased with mean storage temperature for both constant (~O.OlC) and oscillating (fl”C) temperatures. Recrystallization rates generally increased with amplitude of temperature oscillations, but there appeared to be a threshold amplitude (about 0.25”C) below which oscillations had no effect. An upper limit on the effect of amplitude may also be present, since results at f2”C were not significantly different from those at f 1°C. Longer cycle times (2 h) of the temperature oscillations resulted in a lag in recrystallization, but recrystallization thereafter was rapid. Mean crystal size increased linearly with time0.33. The temperature dependence of recrystallization rate fitted Arrhenius kinetics well, with apparent activation
D. P. Donhowe. R. W. Hartel
1220
-oX -og
120-
h v
100 -
1 3
80 -
Ii 3
60-
a! 0
Accelerated Beerystallization:
10 min, f 1 “C!Oscillations
Accelerated Recrystallization:
2 h, f 1 “C Oscillations
Controlled Bulk Storage: 2 h, f 1 “C Oscillations
cf --.-I.‘..,*
3
5
10
..t-.
.
15
.
I
20
-.
.
.I
25
Change in Percent F’rozen Water due to f 1 “C Oscillations (96) Fig. 5. Correlation of ice recrystallization rate with calculated change in frozen water content of ice cream during 3~1°C temperature oscillations. Error bars represent standard errors based on two replicates per data point. Accelerated recrystallization data from Donhowe & Hartel(1996).
energies between 112 and 124 kJ mol-‘. Recrystallization rate data also appeared to fit WLF kinetics reasonably well, although statistical evaluation for best fit was not carried out. Recrystallization rate increased with the change in amount of frozen water for fluctuating temperature conditions. However, this index was unable to predict recrystallization rates under constant temperature conditions. The data obtained in this study, with ice cream stored in bulk containers, were compared to previous data for accelerated recrystallization on a microscope stage (Donhowe & Hartel, 1996). Relative to bulk storage, recrystallization was more rapid under accelerated conditions for oscillating temperature, but slower for constant temperature. Both of these effects could be attributed to geometrical and/or microstructural factors, since samples were spread into a thin layer for the accelerated storage study, but kept in the original state in the bulk storage experiments.
ACKNOWLEDGEMENTS This work was supported through the College of Agricultural and Life Sciences at the University of Wisconsin-Madison.
Recrystallization in bulk ice cream
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REFERENCES Arbuckle, W.S. (1986). Ice Cream, 4th edn, Van Nostrand Reinhold, New York. Bradley, R.L.Jr. (1984). Protecting ice cream from heat shock. Dairy Ret, 85(10), 120-122. Donhowe, D.P. (1993). Ice Recrystallization in Ice Cream and Ice Milk. PhD Thesis, University of Wisconsin-Madison, 148 pp. Donhowe, D., Hartel, R.W. & Bradley, R.C.Jr (1991). Ice crystallization processes during manufacture and storage of ice cream. J. Dairy Sci., 74, 3334-3344. Donhowe, D.P. & Hartel, R.W. (1996). Recrystallization of ice in ice cream during controlled accelerated storage studies. Znt. Dairy J., 6(1 l/12), 119 I-1208. Fennema, 0. (1973) Nature of the freezing process. In Low-Temperature Preservation of Foocis and Living Matter. eds O.R. Fennema, W.D. Powrie & E.H. Marth. Marcel Dekker, New York, p.151. Kingery, W.D. (1960). Regelation, surface diffusion and ice sintering. J. Appl. Phys, 315, 833-838.
Kroener, C. & Luyet, B. (1966). Discontinuous change in expansion coefficient at the glass transition temperature in aqueous solutions of glycerol. Biodynamics, 10, 4145. Levine, H. & Slade, L. (1986). A polymer physico-chemical approach to the study of commercial starch hydrolysate products (SHP’s). Carbohydr. Pofym, 6, 213-244. Levine, H. & Slade, L. (1988). Principles of ‘cryostabilization’ technology from structure/ property relationships of carbohydrate/water systems - a review. Cryo-Lett, 9, 21-63. Levine, H. & Slade, L. (1989). A food polymer science approach to the practice of cryostabilization technology. Comments Agric. Food Chem, 1, 315-396. Luyet, B. (1969). On the amount of water remaining amorphous in frozen aqueous solutions. Biodynamics, 10, 278-291. Luyet, B. & Kroener, C. (1966). The temperature of the ‘Glass Transition’ in aqueous solutions of glycerol and ethylene glycol. Biodynamics, 10, 3340. Martino, M.N. & Zaritzky, N.E. (1987). Effects of temperature on recrystallization in polycrystalline ice. Sci. Aliments, 71, 147-166. Martino, M.N. & Zaritzky, N.E. (1989). Ice recrystallization in a model system and in frozen muscle tissue. Cryobiology, 26, 138-148. Rasmussen, D. & Luyet, B. (1969). Complementary study of some nonequilibrium phase transitions in frozen solutions of glycerol, ethylene glycol, glucose and sucrose. Biodynamics, 10, 3 199332. Simatos, D., Blond, G. & Le Meste, M. (1989). Relation between glass transition and stability of a frozen product. Cryo-Lett, 10, 77-84. Slade, L. & Levine, H. (1988). Non-equlibrium behavior of small carbohydrate-water systems. Pure and Appl. Chem, 60(12), 1841-1864. Slade, L. & Levine, H. (1991). Beyond water activity: recent advances based on an alternative approach to the assessment of food quality and safety. Crit. Rev. Food Sci. Nutr, 30, 11S-360. Soesanto, T. & Williams, M.C. (1981). Volumetric interpretation of viscosity for concentrated and dilute sugar solutions. J. Phys. Chem, 85, 3338-3341. Williams, M.L., Landel, R.F. & Ferry, J.D. (19.55). The temperature dependence of relaxation mechanisms in amorphous polymers and other glass-forming liquids. J. Amer. Chem. Sot, 77, 3701-3707.
Wolanczyk,
J.P. (1989). Differential
Cryo-Lett, 10, 73-76.
scanning
calorimetry
analysis of glass transitions.
PII:
SO958-6946(96)00030-l
Printed in Ireland. All rights reserved 0958-6946/96/$15.00 + 0.00
ELSEVIER
Recrystallization
of Ice During Bulk Storage of Ice Cream
Daniel P. Donhowea ‘Department
& Richard
W. Hartef’*
“Spray-Tech, Inc., Allentown, New Jersey, NJ, USA of Food Science, University of Wisconsin, 1605 Linden Dr., Madison,
(Received
Wisconsin,
WI 53706, USA
I 1 November
1995; accepted
8 June 1996)
ABSTRACT Ice recrystallizution was studied in 1.9 L contuiners qf’ ice cream stored so thut surface temperature of ice cream wus controlled with fluctuations sf’ f l.O”C. Core and surface samples were taken at regular intervals and anal,vzedfor ice crystal size by cold-stage microscopy and image analysis. Mean ice crystal *size plotted vs. timeo.33 resulted in a straight line, with a slope equal to recrystallization rate (pm duym0.33). Storage temperatures between -15 and -5°C were studied. Recrystallization rate increased with storage temperature and extent of’ temperature fluctuations. Within a single container, meun ice crystal size grevt significantly larger in ice cream at the outside wall than near the core qf’the puckage. Recrystallization rates determined in this study were compared to those found at identical temperature conditions, but in an accelerated storage study. Recrystallization was more rapid in the accelerated study,Jtirjluctuating temperature, but slower,for constant temperature. Copyright 0 1996 Elsevier Science Limited
INTRODUCTION
Many aspects of storage stability of foods, including changes in ice crystal size distribution during frozen storage, have been recently described by glass transition theory. The importance of glass transitions in foods has been recognized fairly recently, due largely to the efforts of Levine & Slade (1986, 1988); however, its importance in frozen biological systems was studied many years before by Luyet and co-workers (for example, Luyet & Kroener, 1966; Kroener & Luyet, 1966; Luyet, 1969; Rasmussen & Luyet, 1969). The glass transition, which represents a change in state from a glassy solid to a rubbery, viscous fluid (Slade & Levine, 1991), is manifested by a change in slope *Author
to whom correspondence
should be addressed. 1209
1210
D. P. Donhowe, R. W. Hartel
of volume expansion and by a discontinuity in heat capacity. The glass transition has been characterized operationally as the state at which the viscosity of the material is lOi Pa-s (Slade & Levine, 1991). Perhaps the most important implication of glass transition theory in frozen foods, and foods in general, is that translational and rotational motion of molecules (especially larger ones) do not occur in the glassy state during a practical time-frame (Wolanczyk, 1989). Since ice crystal growth in frozen systems at low temperatures is impaired by very low mass transfer rates resulting from the high viscosity of the unfrozen solution (Fennema, 1973), ice crystal growth rates are very slow and become practically negligible in the glassy state. In the rubbery state, rates of translational diffusion increase rapidly with increases in temperature or concentration of a plasticizer (e.g., water). Since diffusional rates are inversely related to viscosity, this effect is described quantitatively by the Williams-Landel-Ferry (WLF) equation (Williams et al., 1955): log,,
- rl’pT
{ %lP&
=_ 1
G(Tc2 +(T-
p,,
1 Tg
where -q is viscosity, p is density, Cr and C2 are constants, the subscript g refers to the glass transition, Ts’ has been substituted for the conventional glass transition temperature (Ts) of polymer systems, and T is in K. The ‘universal constants’ of Ci = 17.44 and C2 = 51.6 have been shown to fit the viscosity of concentrated sucrose and fructose solutions very well (Soesanto & Williams, 1981). Levine 8z Slade (1989) found that commercial ice cream and other frozen desserts have glass transition temperatures (T,‘) ranging from -27 to -41°C. This temperature range corresponds with the glass transition temperatures of the principal water-soluble components in ice cream (Slade & Levine, 1988): sucrose (Tl = -32”Q lactose (-28°C) and glucose (-43°C). Since diffusional rates are very low below or near the glass transition temperature, it would appear that very little diffusion and hence, migratory recrystallization should occur at storage temperatures of -30 to -40°C. However, diffusional rates would be expected to be much higher during distribution of ice cream due to the very strong temperature dependence of WLF kinetics. Indeed, the WLF equation with the ‘universal constants’ predicts a change in viscosity of 5 orders of magnitude over a 20°C temperature range near the glass transition (Slade & Levine, 1991). In polymer systems, WLF kinetics are usually applicable over a 100°C temperature range above the glass transition temperature (Williams et al., 1955). However, WLF kinetics would be relevant over a much smaller temperature range in frozen foods, since the melting point is only 2540°C above the glass transition temperature and solutions typically follow Arrhenius kinetics above the melting temperature (Slade & Levine, 1991). The major complication of WLF theory as applied to frozen systems such as ice cream, however, is that the composition of the unfrozen solution changes as the temperature increases, due to melting of ice (Simatos et al., 1989). Since the glass transition temperature decreases with the inverse of the weight-average molecular weight of the solution (Levine & Slade, 1986; Slade & Levine, 1988), and water has a low molecular weight relative to other solution components, increases in water content cause a ‘Ts depressing effect’ (Slade & Levine, 1991). Thus, dilution of the unfrozen solution results in an even greater decrease in viscosity as temperature increases than predicted by the WLF equation (Simatos et al., 1989).
Recrystallization in bulk ice cream
1211
The effects of storage temperature and controlled temperature fluctuations on recrystallization rate in ice cream have been recently studied by Donhowe & Hartel (1996). Their results documented the increase in recrystallization rate that occurred when both storage temperature and amplitude of temperature fluctuations increased. However, these studies followed recrystallization in a thin film of ice cream mounted on a microscope slide under controlled conditions. Ice crystal sizes over 200 urn (based on equivalent circular diameter) were measured despite working with films that were only 100 urn thick. It is likely that the slides used to mount the ice cream sample had significant influence on recrystallization rates. Thus, these results may not correlate very well with recrystallization rates observed during bulk storage of ice cream. Other differences between storage conditions (accelerated vs. bulk storage) that might influence recrystallization rate include (1) better heat transfer on the microscope stage, (2) differences in proximity of crystals to one another due to formation of a thin film for accelerated studies, and (3) disruption of air cell structure in ice cream during formation of thin films for accelerate studies. In this work, controlled storage studies were carried out to determine the influence of temperature on ice recrystallization in ice cream stored under typical bulk storage conditions. In addition, kinetic data for ice recrystallization were analyzed in terms of either Arrhenius kinetics or WLF kinetics for temperature dependence.
MATERIALS
AND METHODS
Controlled bulk storage boxes
Two boxes, made of 5 cm thick Styrofoam insulation, were custom built for the controlled bulk storage experiments, as described by Donhowe (1993). To control temperature, heaters were used to counter-balance the cooling provided by the refrigeration system of the walk-in freezer in which the boxes were contained. Two fans provided continuous air flow through each box. The air entering a box first passed through a gravel bed, which acted as a heat sink to dampen any fluctuations in freezer temperature. The air was then heated to the desired temperature by the coil heater. Perforated boards at the top and bottom helped provide even, downward air flow and uniform temperature (within O.lC) throughout the sample chamber. Ice cream samples were placed on test tube racks, which had minimal thermal contact with the containers and did not significantly obstruct air flow. Each storage box also contained a ‘constant-temperature chamber’, which was jacketed to maintain air temperature fluctuations within O.l”C. The jacket contained a 50% ethylene glycol solution to provide a thermal heat sink. The constant temperature boxes were large enough to hold three 1.9 L ice cream containers. In this way, both fluctuating and constant temperature conditions could be studied simultaneously. Temperature in each storage box was controlled with a thermistor mounted near the top of the sample chamber. The thermistor signal was sent to a data acquisition board (DT-2805, Data Translation Inc., Marlboro, MA) connected to a microcomputer (ZDE-1217-BO, Zenith, St. Joseph, MI). Control was implemented by monitoring the power input to the coil heater. A 32-channel multi-
1212
plexer facilitated tures throughout locations within tal size analysis,
D. P. Donhowe, R. W. Hartel
additional temperature measurements, including air temperathe freezer and storage boxes. Temperatures at several product the 1.9 L container, corresponding to sample locations for cryswere also taken.
Mix preparation and freezing A single batch of 40% total solids ice cream mix (12% fat, 11% serum solids, 16.5% sucrose, 0.3% 250 Bloom gelatin (Vyse Gelatin Co., Schiller Park, IL), 0.1% emulsifier blend of 80% glycerol monostearate and 20% Polysorbate 20 (Drewmulse 700, PVO International, Inc., Boonton, NJ) was pasteurized at 82.2”C for 16 s, and homogenized at 13.8/6.9 MPa (second stage). Cooled mix was stored at 1.7”C for 16 h (Donhowe, 1993). Ice cream mix was frozen in a continuous scraped-surface freezer (Model 3N-15 APV Crepaco, Chicago, IL) at 80% overrun with a draw temperature of -5.9”C. Thirty-two 1.9 L cylindrical containers (12.5 cm diameter x 15.5 cm height) were filled with ice cream, hardened in a hardening room at -25°C for 24 h and stored in Styrofoam containers at -27 to -30°C until needed for the storage tests. Four 0.5 L samples of ice cream were also taken at draw for immediate analysis of ice crystal size. Finally, two 1.9 L ice cream containers were analyzed for ice crystal size after hardening. Storage treatments Ice cream containers were allocated randomly among the various storage treatments. Containers were subjected to fluctuating or constant temperature storage at four mean temperatures: -5, -7, -10 and - 15°C. Two ‘control’ containers were stored in a Styrofoam box at freezer temperature for the duration of the experiments. Experiments at -5 and -7°C were carried out over 8 days. These experiments were replicated once. Samples stored at -10 or -15°C were held for 32 days. An experiment began by equilibrating each box at the desired mean temperature and calibrating the control thermistors with a standardized thermometer. The samples for constant-temperature treatment were placed in the chambers, after equilibration for 4 days. The remaining samples were then transferred to the constanttemperature boxes, and temperature set to continuous, 2 h, sinusoidal oscillations about the mean storage temperature. The amplitude of the temperature oscillations was adjusted so that the surface temperature of the ice cream containers fluctuated by fl”C. This resulted in four temperature protocols for air temperature for the fluctuating temperature treatments: -5f7”C, -7&6X!, -1Of5.5”C and - 15f5”C. Microscopy and image analysis Samples were analyzed for ice crystal size distribution after draw, hardening and various periods of storage. The 0.5 L samples of ice cream taken at draw were analyzed immediately in a refrigerated glove box which housed an optical microscope (Donhowe et al., 1991), set to the draw temperature (-5.9”(Z). For hardened and stored ice cream, samples were taken from duplicate 1.9 L containers at various radial locations at the mid-point of the height of the
Recrystallization in bulk ice cream
1213
container. Interior samples were obtained with a cork borer. To take surface samples (at a radius of r/R = 1, where R is the radius of the container), a screwdriver was used to make a small hole in the container wall at the bisection plane, and a small amount of ice cream was removed. Surface samples were taken at 0, 0.5, 1, 24 and 8 d for ice cream stored at -5 or -7°C and at 0, 1, 2,4, 8, 16 and 32 d for products stored at -10 or - 15°C. Interior samples at r/R = l/3 were taken less frequently during the experiments, and more complete positional sampling (r/R = 0, l/3, 2/3 and 1) was done only for initial (0 days) and final samples (8 or 32 days). Samples were placed into vials, transferred in a Styrofoam container, on cold packs of frozen brine at about -lO”C, to the refrigerated glovebox, and equilibrated at -14°C for several hours. An optical microscope housed within the refrigerated glove box was used to take photomicrographs of ice crystals in each ice cream sample. A few mg of each sample placed on a microscope slide were diluted with a few drops of n-butanol and spread into a thin film by pressing a second microscope slide onto the sample. The n-butanol was used as dispersant to provide adequate dispersion of ice crystals while maintaining initial crystal integrity. Photomicrographs of ice crystals were taken within 15 min of sample preparation, so that no change in crystal size occurred over the time period of measurement. Negatives were enlarged and analyzed by image analysis according to procedures described by Donhowe (1993). One representative 20 x 25 cm photomicrograph was analyzed for each sample. Differential scanning calorimetry A differential scanning calorimeter (DSC Series 7, Perkin-Elmer, Norwalk, CT) was used to determine the apparent maximally freeze-concentrated glass transition temperature (7’s’,) of the ice cream. Triplicate samples of melted ice cream (approximately 10 mg each) were cooled at 20°C min-’ to -60°C and then heated at 5°C min-’ to 10°C. Derivative thermograms were used to identify a transition temperature in the manner described by Levine & Slade (1988). This transition temperature was probably not the true Tg’, yet provides a simple and consistent reference point for comparison purposes. In this paper, the temperature associated with this transition is called Tia, an apparent glass transition temperature. For the ice cream formulation used here, a value of -34.5”C was obtained.
RESULTS AND DISCUSSION ControIIed bulk storage The controlled bulk storage experiments were designed to simulate typical storage conditions, yet still facilitate comparisons with previously reported accelerated recrystallization experiments (Donhowe & Hartel, 1996). Air temperature cycles of 2 h were similar to the cycle time of a typical cabinet freezer. However, air temperature fluctuations were set to attain product temperature fluctuations of f 1°C at the surface of the 1.9 L containers for comparison to previous results (Donhowe & Hartel, 1996). The amplitudes of the air temperature fluctuations (-5f7”C, -7f6”C, -1Of5.5”C and -15f5”C) were therefore much higher than
1214
D. P. Donhowe, R. W. Hartel
typically encountered under commercial conditions. It was necessary to increase the amplitude of the air temperature oscillations as mean temperature increased, since at higher temperatures the ice cream had a higher effective thermal mass (more unfrozen water and more latent heat changes during a temperature cycle) and lower effective thermal diffusivity. Unless otherwise stated, temperatures discussed below refer to product temperatures rather than air temperatures. Change in ice crystal morphology Ice crystals after hardening (0 days storage) were smaller at the surface (r/R = 1) than in the center (r/R = 0) of the containers. After 1 day storage at -5fl”C (2 h cycle; surface temperature), ice crystals at the surface became rounded, but also somewhat angular. The number of fused crystals reflected the importance of accretion. After 8 days storage, crystals became very rounded, but had not developed into oblong crystals observed during accelerated recrystallization at -5fl”C with 2 h cycle time (Donhowe & Hartel, 1996). Ice crystals in the interior (r/R = l/3) of ice cream containers held at -5fO.Ol”C after 8 days were very similar to crystals at the surface of the containers. From the shape of ice crystals, bulk storage at -5&0.2X! (the interior temperature oscillation) gave results that were indistinguishable from those for storage at constant temperature (-S~tto.OlC). This is evidence for a ‘threshold’ amplitude of temperature oscillations, below which melt-refreeze recrystallization is unimportant, as noted previously for accelerated recrystallization at -10°C (Donhowe & Hartel, 1996). Crystals remaining after 8 days at -5f0.2”C or -5fO.Ol”C were more rounded and smaller than crystals present after 8 days at -5fl”C. Ice crystals at the surface of containers after 8 days storage at -1Ofl”C were very similar in shape to crystals after 8 days at - lOfO.Ol”C, with rounding and accretion being prevalent for both conditions. However, crystals were smaller and much less rounded than crystals held for 8 days at -5fl”C. This increase in isomass recrystallization at higher temperatures is probably due to a decrease in the viscosity of the unfrozen solution. Change in ice crystal size The ice crystal size distributions in ice cream containers subjected to oscillatingtemperature storage at -5°C are shown in Fig. 1. Ice crystals increased in size between draw and hardening (0 days storage). However, the ice crystals at the surface (r/R = 1) were slightly smaller after hardening than the crystals in the interior (r/R = l/3). This is due to the slower rate of cooling and hence, more rapid recrystallization in the interior of the container relative to the surface during hardening. This difference between surface and interior, although not great, was significant (P = 0.05) in virtually all containers tested after hardening. After 4 or 8 days storage under oscillating-temperature conditions, crystals at the surface of the container were larger than crystals in the interior (Fig. la). This effect was due to the larger temperature fluctuations at the surface (fl”C) than in the interior (f0.2”C at r/R = l/3) of the ice cream containers. Mean ice crystal size increased and the range of crystal sizes became larger during storage. For the constant-temperature experiments at -5”C, the initial difference in size distributions between interior and surface ice crystals became negligible during storage (Fig. lb). Crystals did not become quite as large and size distributions
Recrystallization in bulk ice cream
1215
a) Oscillating Air Temperature (-6 f 7 “C, 2 h Cycle)
90 80 d 3
70
1
6o
-Draw
--C+O~dR=lM +o~d,l +1d,rm=1
BB 6o
+24rflw
3
-44rfR=m +44d,nk1
CO E 30 Q)
~8d,rklRl --e-Sd,r/Jkl
@I 20
0
50
100
160
200
250
Cry&al Size (pm) b) Constant Air Temperature t-5 f 0.1 “C) lOO90. 80$ a
70-
d c
Draw +O&r/R=l/3
60-
-o(lril2=1 +ld,r/R=l -O-22,rlklB
!I ‘tl
so- . 40.
-2d,m=1 -44rnm
& 30@ pI
+8d,r/R=lI3 --e-Sd,r/R=l
20-
100
160
Crystal Size (pm) Fig.1. Ice crystal size distributions
in 1.9 L containers of ice cream during storage at -5°C: (a) Oscillating air temperature (-5f7”C, 2-h cycle); (b) constant air temperature (-5fO.l”C). Error bars represent standard errors based on two replicates per data point.
1216
D. P. Donhowe, R. W. Hartel
were somewhat narrower as compared to storage at oscillating temperature conditions at -5°C (Fig. la). Ice crystal size distributions after 8 days storage at all storage temperatures are shown in Fig. 2. As expected, oscillating-temperature storage (filled symbols) resulted in greater recrystallization than constant-temperature storage (open symbols), while recrystallization increased with temperature within each set of experiments (constant or oscillating-temperature storage). The data for mean ice crystal size at the surface of the containers fit t1’3kinetics well, as seen in Fig. 3. Higher regression coefficients were obtained when the earliest data (less than 1 day of storage) were excluded from the regressions, since there appeared to be an initial lag in recrystallization rate. This lag in time before recrystallization fit the t ‘I3 kinetic model seemed reasonable, since it took about 1 day for the ice cream to equilibrate to the storage box temperature (containers were initially at a temperature of -25 to -27°C). Recrystallization in ice cream at the surface of containers stored at oscillatingtemperature conditions (such that the surface temperature fluctuated Itl“C) increased with increasing mean storage temperature (Fig. 3a). Fairly good agreement was obtained between samples for any given experiment. Slightly slower recrystallization was observed at the surface of containers held at constant temperature (Fig. 3b). Constant-temperature experiments at -5 and -7°C were repeated and gave reproducible results. 100 90’ 80’
70-5 f 0.01 “C, Trial 1 -5 f 0.01 “C, Trial 2 -5 f 1 “C, 2 h Cycle -7 f 0.01 “C, Trial 1 -7 f 0.01 “C, Trial 2 -7 f 1 OC,2 h Cycle -10 f 0.01 “C -10 f 1 “C, 2 h Cycle -15 f 0.01 “C
6050’ 40’ 3020-
-15 f 1 “C, 2 h Cycle
10’ O+0
50
100
150
200
250
Crystal Size (pm) Fig. 2. Ice crystal size distributions at the surface of 1.9 L containers of ice cream after 8 days controlled bulk storage at constant or oscillating temperature. Error bars represent standard errors based on two replicates per data point.
Recrystallization in bulk ice cream
Oscillating Product Tempmature (k 1 “C, 2 h Cyde)
a) MO. 140:8
;
Q
--o-5 “C, Sample 1 +-5%,Sampie2 + -7 “C, Sample 1
.
+ -a+
lOOi,
--e *
120;
1217
0
-7 -10OC, “C,Sample Sample2 2 1 “C, Sample -16 1 -16 “C, Sample 2
iz 1
8o:
u
60-
A
f z
.
0.0
0.5
1.0
1.5 t*
2.-o
2.i
3.-o
(dl’$
b) Constant Product Temperature (i 0.01 “C) 140 7 -o-0--o+ l20- + ~-o+ z &:I__ e-15 -w-
-5 “C, Trial -5 OC,Trial -5 “C, Trial -5 VI, Trial -7 Trial
1, Sample 1 1, Sample 2 2, Sample 1 2, Sample 2 1
e
-7 “C, OC,!h+ Trial 2, 1, Sample 2 1 I/ i -;ooFdyal2. Sample 2 . ,
P
_- _ oc: s
w -15 “C, Sample 2 An
20~.-..1----‘-...‘....‘-..-‘....’..-.’ 1.0 0.5 0.0
1.5 tlB
Fig. 3. Effect of storage temperature
2.0
2.5
3.0
3.5
(d1’3)
on mean ice crystal size at the surface of 1.9 L containers of ice cream: (a) Oscillating product temperature (il’C, 2 h cycle); (b) constant product temperature (kO.Ol“C).
1218
D. P. Donhowe, R. W. Hartel
Kinetic models for ice recrystallization in ice cream Recrystallization rates were determined from the slopes of plots of mean ice crystal size vs. t’13 and were found to increase rapidly as the temperature approached the melting point of about -2.9”C, as shown in Fig. 4. Oscillating temperatures (* 1“C) generally enhanced recrystallization relative to storage at constant temperature. Data for accelerated recrystallization of ice cream in a thin layer on a microscope stage from Donhowe & Hartel(1996) are included in Fig. 4 for comparison to data under bulk storage conditions from this study. Temperature fluctuations (3~1°C) on the cold stage resulted in greater recrystallization than in the bulk storage boxes. The main reason for the difference in results may be the physical limit on growth in the vertical direction for the accelerated recrystallization samples, which consequently results in greater extent of growth in the horizontal directions. At constant temperature, recrystallization rates were higher for the controlled bulk storage experiments than for the accelerated recrystallization experiments. This difference could be due to closer proximity of crystals and more crystal-crystal contacts (and hence, increased accretion) in the bulk storage experiments, since ice crystals prepared on the cold stage were dispersed into a thin layer. In addition, microstructural differences may have been important, since the air cells were disrupted in the accelerated recrystallization experiments. These data were fitted to the Arrhenius kinetic equation, and a reasonable tit obtained. However, it must be stressed that it may be fairly easy to tit Arrhenius 140 -
120 -
0
Accelerated Recrystallization: 10 min. f 1 “C Oscillations
X A
Accelerated Recrystallization: 2 h, f 1 “C Oscillations Accelerated Recrystallization: Constant Temperature Controlled Bulk Storage: 2 h, f 1 “C Oscillations
A
Controlled.Bulk Storage: Constant Temperature
0
% h
100 -
i J
80 -
B !!
60 -
3
!!I
40 -. 20 -
0,.
-20
I’
I.
-18
-16
-14
-12
-10
Mean Temperature
-6
-6
-4
(“Cl
Fig. 4. Temperature dependence of ice recrystallization rates in ice cream. Accelerated recrystallization data from Donhowe & Hartel(l996).
Recrystallization in bulk ice cream
1219
kinetics over the relatively small temperature range of the experiments (- 10 or - 15°C). The ‘apparent’ activation energies of recrystallization were fairly similyr for all data sets, with apparent activation energies from 112 to 126 kJ mol- . These values agree with the value of 115 kJ mol- obtained by Kingery (1960) for ice sintering, and the value of 116 kJ mol-’ obtained by Martin0 & Zaritzky (1987, 1989) for ice recrystallization in NaCl solutions. Recrystallization rate data for accelerated storage with temperature cycles of fl.O”C were fitted to WLF kinetics based on two different sets of constants (eqn 1). The recrystallization rate at the apparent glass transition temperature was not known, although the rate of diffusion-limited processes should be ap roximately zero at this point. For this work, an arbitrary value of 25 pm day -lp3 at -10°C was used as a point of reference for the model due to the lack of more exact data. The data fit the WLF equation reasonably well with ‘universal constants’ (Slade & Levine, 1991) C, = 17.44 and Cz = 51.4. Although extensive statistical tests for fitting this model were not carried out, a better lit was obtained for values of C, = 20.4 and C2 = 154.8, which are coefficients found to apply to highly symmetrical, readily crystallizable polymers in which the difference between the melting point and the glass transition temperature is much greater than 100°C (Slade & Levine, 1991). Since neither of these criteria apply to ice cream, good fit of these values for Ci and C2 to the data is probably coincidental. Thus, the recrystallization rate data can be empirically fitted to WLF kinetics based on the 7’ia, but the ‘universal coefficients’ did not appear to apply. Recrystallization rate data were also correlated with the change in amount of water frozen, as suggested by Bradley (1984), and shown in Fig. 5. Changes in percent frozen water in the 40% total solids ice cream during temperature fluctuations of fl”C were determined from calculated freezing point depression curves (Arbuckle, 1986; Bradley, 1984). As expected, recrystallization rates increased with the amount of water that melted and refroze (assuming ice phase equilibrium) during each f 1“C temperature cycle. Recrystallization rates were greater for il”C oscillations under accelerated storage conditions (Donhowe & Hartel, 1996) than in the storage box. Although recrystallization rates correlated fairly well with changes in percent frozen water, there are limitations to this approach. In particular, recrystallization rates at constant temperature (&O.Ol’C) should be negligible, since there should be no change in amount of water frozen, which clearly does not agree with the results shown here.
CONCLUSIONS Recrystallization in ice cream stored in bulk containers increased with mean storage temperature for both constant (~O.OlC) and oscillating (fl”C) temperatures. Recrystallization rates generally increased with amplitude of temperature oscillations, but there appeared to be a threshold amplitude (about 0.25”C) below which oscillations had no effect. An upper limit on the effect of amplitude may also be present, since results at f2”C were not significantly different from those at f 1°C. Longer cycle times (2 h) of the temperature oscillations resulted in a lag in recrystallization, but recrystallization thereafter was rapid. Mean crystal size increased linearly with time0.33. The temperature dependence of recrystallization rate fitted Arrhenius kinetics well, with apparent activation
D. P. Donhowe. R. W. Hartel
1220
-oX -og
120-
h v
100 -
1 3
80 -
Ii 3
60-
a! 0
Accelerated Beerystallization:
10 min, f 1 “C!Oscillations
Accelerated Recrystallization:
2 h, f 1 “C Oscillations
Controlled Bulk Storage: 2 h, f 1 “C Oscillations
cf --.-I.‘..,*
3
5
10
..t-.
.
15
.
I
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-.
.
.I
25
Change in Percent F’rozen Water due to f 1 “C Oscillations (96) Fig. 5. Correlation of ice recrystallization rate with calculated change in frozen water content of ice cream during 3~1°C temperature oscillations. Error bars represent standard errors based on two replicates per data point. Accelerated recrystallization data from Donhowe & Hartel(1996).
energies between 112 and 124 kJ mol-‘. Recrystallization rate data also appeared to fit WLF kinetics reasonably well, although statistical evaluation for best fit was not carried out. Recrystallization rate increased with the change in amount of frozen water for fluctuating temperature conditions. However, this index was unable to predict recrystallization rates under constant temperature conditions. The data obtained in this study, with ice cream stored in bulk containers, were compared to previous data for accelerated recrystallization on a microscope stage (Donhowe & Hartel, 1996). Relative to bulk storage, recrystallization was more rapid under accelerated conditions for oscillating temperature, but slower for constant temperature. Both of these effects could be attributed to geometrical and/or microstructural factors, since samples were spread into a thin layer for the accelerated storage study, but kept in the original state in the bulk storage experiments.
ACKNOWLEDGEMENTS This work was supported through the College of Agricultural and Life Sciences at the University of Wisconsin-Madison.
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