1991 - Roos, Karel - Plasticizing Effect of Water on Thermal Behavior and Crystallization of...

Plasticizing Effect of Water on Thermal Behavior and Crystallization of Amorphous Food Models I

YRJe) ROOS and MARCUS KAREL

ABSTRACT Dehydrated sugar solutions were used as models of thermal behavior of amorphous foods, and of the effect of temperature, moisture content and time on physical state of such foods. The transition temperatures determined were glass transition (T,), crystallization (T,,) and melting (T,) which all decreased with increasing moisture. T, of a sucrose/ fructose model had a slightly lower value than the empirical “sticky point,” at all moisture contents studied. Crystallization of sucrose was delayed by addition of fructose or starch. Crystallization above TB was time-dependent, and the relaxation time of this process followed the WLF equation.

INTRODUCTION THE PHYSICAL STATE of food solids has received increasing attention because of its importance to food processing and shelf life (van den Berg, 1985; Simatos and Karel, 1988; Slade et al., 1989). In various food and biological materials the solids are in an amorphous metastable state which is very sensitive to changes in temperature and moisture content. Usually the amorphous state is a result of rapid removal of water by drying or freezing (White and Cakebread, 1966; Alexander and King, 1985; Levine and Slade, 1988). The amorphous matrix may exist either as a very viscous glass or as a more liquid-like “rubbery” amorphous structure. The change from the glassy state to the rubbery state occurs at glass transition temperature (T,) which is specific for each material. However, plasticizers like water decrease the glass transition temperature.

Lactose has been shown to form a glass when milk products are dried (e.g. Troy and Sharp, 1930; Sharp and Doob, 1941; Bushill et al., 1965; Saltmarch and Labuza, 1980). However, the effects of the physical state of the amorphous compounds in dried milk and other food materials on processing and shelf life have not been well known. Kauzmann (1948) reported glass transition temperatures for glucose, sucrose, and other biologically important compounds, but the equilibrium concept of water activity has remained the main criterion for food stability, White and Cakebread (1966) discussed importance of the glassy state in various foods, and related changes in glassy state of sugars in boiled sweets, milk powder, ice cream and freeze dried products to collapse, stickiness, caking and crystallization. Recent studies have shown that both dried and frozen foods exhibit glass transitions which may greatly affect chemical and physical changes during food processing and storage (Simatos et al., 1975; Herrington and Brandfield, 1984; Karel, 1985; Levine and Slade, 1986, Roes, 1987).

Many freeze-dried materials including foods are glassy after drying, and some evidence has been reported on effects of physical state on chemical changes. Karel and Nickerson (1964) related ascorbic acid decomposition and browning in dehydrated orange juice to water content and water activity, which reportedly had a strong influence on reaction rate increase with increasing water content. Flink et al. (1974) found that rates of browning in a freeze dried model system and milk increased sharply at a critical temperature considered to depend on water activity. The browning rate reportedly depended on tempera-

Authors Roos and Karel are with the Dept. of Food Science, Rutgers Univ., Cook College, P. 0. Box 231, New Brunswick, NJ 08903.

ture, moisture content and time. Gejl-Hansen and Flink (1977) reported that amorphous food glasses encapsulated oils, and the amorphous matrix entrapping the oil inhibited oxidation only in the glassy state.

Aroma retention during drying and storage of dried materials can be related to structural changes, and especially to collapse and stickiness during freeze drying, spray drying, and storage of dried materials. The critical temperatures are collapse temperature and sticky point which, similar to the glass transition in polymers, decrease with increasing moisture content (To and Flink, 1978b). During drying and storage collapse and stickiness must be avoided but desired water plasticization can be used to control stickiness in agglomeration of dried food powders (Masters and Stolte, 1973). Both collapse and stickiness depend on temperature, moisture content and time (Tsourouflis et al., 1976; To and Flink, 1978a,b,c).

Above the glass transition temperature molecular mobility is greatly increased and many amorphous compounds crystallize. The crystallization may therefore take place as either temperature or moisture content is increased. Amorphous lactose was reported to crystallize rapidly at room temperature at about 40% relative humidity (Sharp and Doob, 1941; Bushill et al., 1965; Berlin et al., 1970; Saltmarch and Labuza, 1980). Ma- kower and Dye (1956) showed that the rate of crystallization of amorphous sucrose at room temperature depended on relative humidity.

The rate of crystallization above T, is related to viscosity which decreases rapidly at temperatures above Tg. The rate of crystallization is also dependent on composition of the amorphous matrix. To and Flink (1978c) found that addition of acetone decreased the crystallization temperature of amorphous sucrose. Iglesias and Chirife (1978) reported delayed crystallization of amorphous sucrose when polymeric compounds were added. In some studies the crystallization of amorphous sugars was related to rate of browning (Saltmarch et al., 1981), stickiness and caking (Moreyra and Peleg, 1981).

Increase of moisture content of food materials may affect rate of deteriorative reactions similar to increase of temperature (van den Berg, 1985). This plasticization of amorphous structure leads to increased molecular mobility, decreased viscosity and increased reaction rates. Typical thermal transitions which characterize crystallizable amorphous materials are glass transition, crystallization and melting. The purpose of our study was to determine the moisture dependence of the phase transitions of dried amorphous food models, to determine time dependence of the crystallization of amorphous sugars, and to study the effect of composition on phase transitions in models used.

MATERIALS & METHODS Amorphous food models

Food models were prepared using sucrose (Sigma Chemical Com- panv, grade II), a-lactose (Fisher Scientific, certified A.C.S.), B- b( 1 j-fructose [Sigma Chemkal Company), and Amioca (a high irn$ looectin starch (1.8% amvlose\. National Starch and Chemical Cor- pdration). Sucro$e and cw-lktosg’were dissolved in distilled water (10% solution). Amioca was gelatinized in distilled water with sucrose (sucrose/Amioca 4:1, 10% solution). Fructose was dissolved in distilled water with sucrose (sucrose/fructose 7:1, 10% solution). All solutions after preparation in weighing bottles containing log solution were

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frozen immediately, at - 20°C for 24 hr. After freezing samples were tempered over dry ice 3 hr, and subsequently freeze-dried at below 0.1 mbar using a laboratory freeze-drier (Virtis, Benchtop 3L). After freeze-dlying the vacuum was broken with dry nitrogen, the weighing bottles were transferred into vacuum desiccators, and dried over P,05 (Fisher Scientific, Certified A.C.S.) for at least 7 days for completely dried materials. Since drying over PZOs is used as a standard method to obtain complete dryness, moisture content of dried samples was not determined. However, the glass transition temperature of amorphous sugars is very sensitive to moisture, and dryness of the materials was confirmed by determining their glass transition temperatures.

Differential scanning calorimetry

Differential scanning calorimetry (DSC) was used to determine glass transition, crystallization and melting of amorphous models, and to determine the effect of moisture on phase transitions, and time to isothermal crystallization above glass transition temperature. The DSC used was a Perkin Elmer DSC-2 equipped with Intracooler II and Perkin-Elmer 3600 Data Station. Dynamic measurements were made using Perkin-Elmer TADS standard software, and isothermal measurements used the isothermal software. Hermetically sealed ‘2Op,L aluminum sample pans (Perkin-Elmer) were used in all measurements. An empty aluminum pan was used as reference sample. The DSC was calibrated for temperature and heat flow using distilled water (distilled several times, mp O.O”C, A H, 3335/g), Gallium (mp 29.8”C, Aldrich Chemical Co.) and Indium (mp 156&Z, A H, 28.45 J/g, Perkin- Elmer standard). The dly box of the DSC was dried using desiccant, and flushed with dry nitrogen. The sample head was purged with dry nitrogen flow (20ml/min) to avoid condensation of moisture from air.

Sample preparation for differential scanning calorimetry

The amorphous materials dried over P205 were extremely hygroscopic, and samples were placed in DSC pans (2-3mg) in dry nitrogen in a dry box, and hermetically sealed before weighing. Samples of varying water contents were prepared by weighing 2-3mg of dried materials into DSC pans. The open pans were equilibrated over saturated salt solutions in vacuum desiccators 24 hr. The salts used were LiCI, CHjCOOK, MgCI, and K&O3 (Fisher Scientific, Certified A.C.S.). The respective water activities, and equilibrated moisture contents of the samples are shown in Table 1. After equilibration the pans were hermetically sealed, and reweighed. Water content of the samples was determined from weight gain after equilibration (Roos, 1987).

Dynamic measurements

The samples were heated at S”C/min from -50°C until crystallization or melting was completed. The thermograms obtained were typical of amorphous materials (Fig. l), and were analyzed for T,, (onset temperature of glass transition, determined as onset temperature of an endothermic shift in apparent specific heat), Ts2 (end temperature of glass transition, determined as end temperature of an endothermic shift in the apparent specific heat), T,, (onset temperature of crystallization, determined as the onset temperature of an exothermic peak), Tpc, (peak temperature of the crystallization exotherm), and T,,, (melting temperature, determined as the peak temperature of the melting endotherm). Latent heats of crystallization (AH,,) and melting (AH,) were obtained by integration of the respective peaks. In calculations TB1 was used as glass transition temperature, and is referred to as Ta. The Tgl and Tg2 values of the sucrose/fructose model were used to determine relationships between glass transition temperature and sticky point. The sticky point values reported by Downton et al. (1982) were used. At least four replicates were used, and the results were calculated as the mean value + standard deviation.

Isothermal measurements

Isothermal measurements were made to obtain the time required to crystallization of amorphous sucrose and lactose samples. Isothermal crystallization time was determined for sucrose samples dried over P,Os, and samples rehumidified over CH,COOK. The lactose samples used were dried over P,Os. At least four repeated measurements were made at 4-5 different temperatures 20-4o”C above the respective glass transition temperatures (T& determined using the dynamic mode. The isothermal crystallization hmes were obtained by heating samples from ambient temperature to the respective final temperature at 40”C/min,

Table l-Saturated salt solutions used for rehumidifications of freeze- dried amorphous food models, and moisture contents (g/lOllg matter + standard deviation) after rehumidification

Moisture content g/lOOg dry matter

Sucrose/ Salt a,’ ru-lactose Sucrose Sucrose/Amioca Fructose

w5 0.00 0.0 0.0 0.0 0.0 LiCl 0.11 1.2-cO.3 1.5kO.3 0.5 + 0.2 1.120.2 CH,COOK 0.23 3.5 r 0.4 3.9k1.3 l.lkO.3 4.1 20.4 Mc.Cb 0.33 6.0t1.3 b 4.4 2 0.3 5.2el.O W ’S 0.43 b b 6.5kO.7 8.1~ 1.6

a Water activity at 25°C. b Moisture content allows crystallization at room temperature.

Fig. 1 -DSC thermograms for amorphous lactose (A), sucrose (B), sucrose/fructose (7:l) (C) and sucrose/Amioca (4: 1) (D) showing glass transition, (1) crystallization; (2) onset temperature of crystallization; (3) temperature at the peak of crystallization exotherm) and (4J melting. Scanning rate was 5Wmin.

and collecting the isothermal data until a crystallization exotherm was completed. Crystallization time (0,,) was determined as time at the peak of the exotherm. The latent heat of crystallization (AH,3 was obtained by integration of the crystallization exotherm.

Time dependence of crystallization

EJcr Values were used to determine the time dependence of crystallization. Arrhenius (1) and Williams-Landel-Ferry (2) (WLF, Wil- l iams et al., 1955) equations were used:

-17.44 (T - T,) 51.6 f (T - Tg)

8, was obtained by solving Eq. (2) for all experimental points, and the average time obtained for each set of samples was used as t&. Thus, ecr to the WLF type behavior could be calculated:

loge,, = loge, + - 17.44 (T - T,) 51.6 + (T - Tg)

The constants, - 17.44 and 51.6, have been shown to apply to most materials (Williams et al., 1955), and were therefore used in calculations.

RESULTS Effect of moisture content on glass transition temperature

The materials showed typical thermal behavior of amorphous materials, and thus glass transition, crystallization and melting temperatures could be determined (Fig. 1). Ts decreased with increasing water content. The T, temperatures

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WATER PLASTICIZATION IN FOODS. . .

Table 2-Glass transition temperatures (T,,, onset of glass transition, Tgt end of glass transition) of freeze-dried amorphous food models rehumidified over saturated salt solutionsn

Glass transition temperature (“C) a-lactose Sucrose Sucrose/Fructose Sucrose/Amioca

ewb T,l T,Z TBl T,Z Tg1 Tg2 Tg1 T,Z 0.0 101.2~0.9 109.0*1.4 56.623.4 68.5e4.0 57.1 TO.5 68.62 1.3 58.4e4.3 70.8k3.0 0.11 65.0r1.6 73.6k2.8 37.428.0 44.6k8.0 40.6kO.3 50.0-t0.8 32.6k3.6 42.7r2.9 0.23 43.7k2.6 50.3e1.6 27.922.4 36.71t1.5 24.3kl.O 36.2k0.4 20.421.4 31.Ok1.2 0.33 29.1 e6.4 35.9k5.8 C c 9.7k0.4 18.1 co.3 10.5* 1.0 19.8e1.4 0.43 C C C C -6.821.6 8.920.8 -5.; k1.5 5.5e1.2

a Literature values available are for sucrose 67°C (Kauzmann, 1946), 63.1”C (Weitz and Wunderlich, 1974). 52°C (To and Flink. 1978a). and the estimated value for sucrose/fructose model is 58°C (Soesanto and Williams, 1961).

b Water activity at 25’C. C Moisture content allows crystallization at room temperature.

U Lactose ---ct-- Sucrose ----•--- Sucrose/Fructose - - ) - SucroseIAmioca

.._... A _..... Horseradish -.-.A.-. Strawberries

--._._ .%._, “A

0 10 20 30 40

MOISTURE CONTENT (g/lOOg dry matter)

0.0 0.2 0.4 0.6 0.8 1.0

WATER ACTIVITY

Fig. 2-Effect of moisture content of amorphous food materials on glass transition temperatures. Values for horseradish and

Fig. 3-Glass transition temperatures of food materials plotted

strawberries are from P&3kkijnen and Roos (7990) and Roos against water activity at 25°C used for rehumidification of sam-

(19871, respectively. ples. Values for horseradish and strawberries are from PMk- kdnen and Roos (1990) and Roos (1987), respectively.

(Tgl onset temperature of glass transition, T,, end temperature of glass transition) are shown in Table 2. The effect of moisture content on Ts is shown in Fig. 2 along with glass transition temperatures for some freeze dried foods with high carbohydrate content. The T, values were also plotted against water activity (a, values at 25°C were used) of the samples. This plot showed a linear relationship between water activity and glass transition temperature (Fig. 3).

Effect of moisture content on crystallization and melting

Crystallization temperatures obtained from the dynamic DSC scans are shown in Table 3. T,, was a function of moisture content, and decreased with increasing moisture content showing a similar behavior to glass transition temperature. The increase in moisture content caused about an equal decrease in T, and T,,. The melting temperatures were less sensitive to increasing moisture contents. The T, values are reported in Table 4. The average (T,,-T,) values were: lactose, 50°C; sucrose, 47°C; sucrose/fructose, 74°C; and sucrose/Amoica, 68°C. The AH,, values were close to the AH, values of most samples, and the values were: dry lactose, - 107 J/g solids; sucrose, - 78 J/g solids; sucrose/fructose, - 61 J/g solids; and sucrose/Amioca, -78 J/g sucrose. The heat released during crystallization decreased as moisture content increased. The crystallization of samples containing fructose and Amioca was immediately followed by the melting endotherm. This caused variation in the AH, and AH,,, values obtained by peak integration because of the difficulty of setting the integration baseline.

* Lactose -;-a-- Sucrose ---+-- Sucrose/Fructose - - ) - SucroseIAmioca _........ &.... Horseradish -.-.A.-, Strawberries

Relationship between sticky point and glass transition The Tsl and Ts2 values, and the sticky point of the sucrose/

fructose model for varying moisture contents are shown in Fig. 4. The sticky point values were close to the T,, values, and thus slightly above TB.

Isothermal crystallization of amorphous sugars Crystallization of amorphous sugars occurred above T, and

below T,, as a function of time. Crystallization time was very sensitive to moisture content due to shift in the glass transition temperature. The isothermal DSC crystallization thermograms for the amorphous sucrose and lactose samples are shown in Fig. 5. The latent heat of crystallization (AH,,) was fairly independent of temperature, and the AH, values were: for lactose, -94 J/g; sucrose, -66 J/g; and sucrose (a, 0.23), - 43 J/g. These values were lower than those obtained in dynamic runs because crystallization peaks were broader, and some of the peak area was neglected by peak integration. The crystallization time increased significantly as the (T-T,) decreased. The 6,, values for the amorphous sucrose and lactose samples followed the Arrhenius equation in the temperature range studied (Fig. 6). The experimental 13~~ values were plotted also against (T-T,) together with the 8, values predicted using the WLF equation (Fig. 6). As shown in Fig. 6 experimental values followed both the Arrhenius and WLF equations.

DISCUSSION THE THERMAL TRANSITIONS of dried food models could be detected by using DSC, and the scans showed the expected

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Tab/e 3-Crystallization temperatures Yr,, onset temperature of crystallization, TJJ peak temperature of the crystallization exotherm) for freeze-dried amorphous food models rehumidified over saturated salt solutions

Crystallization temperature (“C)

a-Lactose Sucrose Sucrose/Fructose Sucrose/Amoica

a,’ TC, TC? TC, TC? Tc, TcrP TC, TC,P 0.0 1G?.5+1.1 166.7-cl.5 104.4 + 2.3 108.9+2.5 137.0r5.3 149.8k2.2 129.5T8.1 144.7k8.3 0.11 113.3r6.1 115.7k6.0 83.7~~ 7.6 86.527.1 116.0~0.8 125.9k1.3 104.7k2.0 116.323.7 0.23 93.3r1.1 97.2 IT 0.8 75.1 +4.1 78.9k4.1 103.122.7 114.8e2.6 100.3 f 2.8 112.9r3.5 0.33 74.7 + 5.8 78.6 t 6.0 81.4e2.4 88.4k 2.3 73.4k3.6 81.7~3.5 0.43 b b 55.851.6 78.1 k1.8 49.6r3.6 58.9 f 1.8

a Water activity at 25°C. b Moisture content allows crystallization at room temperature.

Table 4-Melting temperatures Yr, peak temperature of the melting endotherm) for freeze-dried amorphous food models rehumidified over saturated salt solutions’

awb a-Lactose

Melting temperature PC)

Sucrose Sucrose/Fructose Sucrose/Amioca

8% 214.1 eO.4 0:23

183.5kl.2 172.1 k4.2 171.621.0 168.1 eo.2 178.1 167.6 e2.2 + 2.0 165.0r2.1 158.8*0.5 156.7k1.8

0.33 145.8r0.8 144.2~2.8 0.43 138.4 + 2.6

‘Literature values are for (r-lactose 210°C (Raemy and Schweizer, 1983). and for wcroe 186°C (Weitz and Wunderlich, 1974).

b Water activity at 25%

--LF To1 -- U- Tg2 --•- Downton et al. 1982

0 2 4 6 8 10

MOISTURE CONTENT (g/l OOg dry matter)

Fig. 4-Moisture content dependence of glass transition temperature /T,, onset temperature of glass transition, T,, end temperature of glass transition) and sticky point of amorphous sucrose/fructose (7:l) model.

thermal transitions reported by Simatos and Blond (1975), To and Flink (1978a), Roos (1987) and Slade et al. (1989). The thermograms were typical of crystallizable amorphous materials showing glass transition, and subsequent exothermal crystallization and endothermal melting. Literature on heats of fusion of dry sugars is limited, and mostly based on older studies (Raemy and Schweizer, 1983). The crystallization and melting heats had about the same but opposite values which indicated the samples obtained from freeze drying were completely amorphous. However, both AH,, and AH,,, decreased with increasing moisture content. The AH,, values of the isothermal exotherms were almost independent of temperature. This showed an increased mobility of molecules caused by water plasticization as predicted by free volume theory (Slade et al., 1989).

Plasticizers in amorphous polymers decrease their glass transition temperatures. At the glass transition temperature free volume and mobility of molecules is increased which leads to decreased viscosity, and therefore a changed physical structure of amorphous substances. In amorphous food materials when plasticized by water the changes of the physical structure as

kxol

80°C

12 3 4 5 6 7 8 TIME (lo3 s)

Fig. 5-Isothermal DSC crystallization scans for amorphous lactose (upper) and sucrose (lower) samples dried over P,O, at various temperatures above their glass transition temperature.

characterized by collapse temperature or sticky point have a similar decrease as the glass transition of amorphous polymers in presence of plasticizers (Tsourouflis et al., 1976; To and Flink, 1978b; Flink, 1983). In our study the glass transition temperature of the models decreased with increasing moisture content, which showed typical behavior of water plasticization as pointed out by Slade et al. (1989). The decrease of the Tg was most significant as moisture content increased from 0 to 5 gH,O/lOOg dry matter. At that moisture content the glass transition of all models was close to or below room temperature. In amorphous dry foods this leads to rapid collapse of structure, stickiness and probably to increased rates of deteriorative reactions in the plasticized rubbery state (Simatos and Karel, 1988).

Soesanto and Williams (1981) showed the change of viscosity as a function of temperature of a sucrose/fructose model followed the WLF equation in the rubbery state. The same model was used by Downton et al. (1982) to determine the effect of moisture content on mechanism of stickiness and sticky


WATER PLASTICRATION IN FOODS. . ,

0 10 20 30 40 50 60

T-Tg (“C) Fig. 6-Arrhenius and WLF type temperature dependence of crystallization time of amorphous lactose and sucrose.

point in spray drying. The sticky point decreased with increasing moisture content which was related to the change in surface viscosity of the samples. In our study glass transition temperatures for that model were determined. The sticky point and glass transition temperature were similarly affected by increasing moisture content. This was in good agreement with the close relationship between amorphous food and polymeric materials reported by Tsourouflis et al. (1976) and To and Flink (1978b), and the effects of glass transition in food processing and storage reported by Slade et al. (1989).

The glass transition temperature of dried foods is extremely important in prediction of conditions for proper drying, agglomeration, and storage. A model which could be used to predict collapse temperature of dried materials was reported by To and Flink (1978b). They showed the effect of moisture content on the collapse temperature followed two linear equations: one below, and one above the monolayer moisture content. However, a simpler equation may be better for practical applications and industrial food processing. Roos (1987) reported the glass transition temperature of freeze dried strawberries was a linear function of water activity used for rehumidification of the samples at 25°C. In our study all models used were found to follow a similar linear relationship when plotted against water activity at 25°C (Fig. 3). Theoretically the T, decreases from the Ta of the pure amorphous material to a theoretical T, of pure water at - 135°C (Levine and Slade, 1986). Therefore the regression equations obtained for different materials between their a, and T were used to extrapolate Ts values at a,,, 1. The average va&te for T, of water was - 92°C. This value was higher than the often cited - 135”C, but may be considered a constant, useful for prediction of T, of materials containing low molecular weight carbohydrates. It may also be noted that T, for pure water has not been experimentally verified, and other higher values than -135°C have been reported (Angell, 1983). Using the linear relationship between a,., and Te requires only determination of T, of a sample having “zero” moisture content, and thus the two points can be used to obtain the glass transition temperature of samples at any water activity according to Eq. 4:

T, = (-92°C - To,,) a, + To,, (4)

where a, is water activity and Tar” is glass transition temperature of a sample of “zero” moisture content. This equation predicts the glass transition temperatures for the materials reported in our study reasonably well, and may be applicable to other amorphous food materials containing sugars. However, at high water contents freeze-concentration of solids occurs at

low temperatures which increases the glass transition temperature (Levine and Slade, 1988).

The crystallization and melting temperatures of the samples decreased with increasing moisture content. This is typical of plasticization of amorphous polymers (Slade et al., 1989), and shows further similarities between amorphous food materials and polymers. The effect of moisture content was about the same for T,, and T as indicated by a fairly constant value for (T.=, - T ). This co&rmed that crystallization was possible only above tl?e glass transition temperature. The rubbery state above the glass transition temperature was metastable, a supercooled liquid-like state, and the rate of crystallization depended on viscosity. At temperatures above T, viscosity is rapidly decreased as expressed by the WLF equation (Williams et al., 1955). In our study the time to allow complete crystallization of amorphous lactose and sucrose was studied. Time to crystallization increased with decreasing temperature. The temperature dependence of crystallization time within the temperature range which we were able to follow experimentally followed both the Arrhenius and WLF type equations. However, the (T - Tg) values tested were high, and therefore tended to bias the correlation toward the Arrhenius type temperature-dependence (Williams et al., 1955). If the Arrhenius equation is used to predict crystallization times for amorphous sucrose, the time is much shorter than that observed experimentally by Makower and Dye (1956) (Fig. 6). The WLF equation predicts crystallization of amorphous sucrose and lactose at Tg to occur after about 200 and 4600 years, respectively. Makower and Dye (1956) found the crystallization time of amorphous sucrose stored at 24% rH and 25°C was over 277 days. This agreed well with the WLF type temperature dependence of the crystallization time obtained in our study (Fig. 6). The crystallization time over long periods can be further studied by determining weight change and by microscopy.

Knowledge of temperature and moisture content dependence of crystallization time is important to evaluation of storage conditions of amorphous or partially amorphous foods. Storage temperatures below glass transition temperature are optimal for their stability. However, short periods of temperature expo- sures above T, are permissible if (T-T ) is not large. Crys- tallization of amorphous sucrose (at a, 8.33) and lactose (at a,,. 0.43), occurred rapidly at room temperature because the glass transition temperatures were shifted well below room temperature. This crystallization is shown as loss of moisture in sorption isotherms, and values obtained in our study were close to those reported for amorphous sugars (Sharp and Doob, 1941; Berlin et al., 1971; Flink and Karel, 1972; Saltmarch and Labuza, 1980). As shown in Fig. 2 very small amounts of water were adequate to cause the shift from glassy state to rubbery state, and lead to crystallization at room temperature. In a vapor-impermeable package crystallization increases the relative humidity which further facilitates crystallization. If the glass transition temperature of an amorphous food is known, the moisture content or storage conditions can be adjusted to ensure storage below T as recommended by White and Cak- ebread (1966) and Flint and Karel (1970). Storage below Ta is also required to avoid oxidation of entrapped oil in the amorphous matrix (Gejl-Hansen and Flink, 1977; To and Flink, 1978c).

According to Iglesias and Chirife (1978) environmental conditions govern caking, stickiness and crystallization of amorphous sugars. For some food products the crystalline state may be preferred, to assure low hygroscopicity. If the glass transition temperature and crystallization behavior are known, the crystalline state can be achieved by controlled increase of moisture content and temperature with subsequent drying. The same method can be also used for agglomeration using lower temperatures. However, some materials containing polymeric substances are difficult to crystallize because of delayed crystallization. Most sugars interfere with sucrose crystallization by viscosity or surface effects (Van Hook, 1961). In our study

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both fructose and Amioca delayed crystallization of amorphous sucrose probably because of increased viscosity above glass transition temperature. Fructose increased crystallization temperature of sucrose about 30°C and Amioca about 25°C. The crystallization exotherms were also broad showing delayed crystallization. The addition of fructose to inhibit sucrose crystallization is used in manufacturing glassy hard sugar candies, and has been studied by Herrington and Brandfield (1984). In some amorphous foods crystallization of sugars seems to occur only at high temperatures or probably very slowly in a fully plasticized state (Roos, 1987).

The aroma retention during drying and storage of dried foods is closely related to collapse during drying and crystallization (Flink and Karel, 1972; Chirife and Karel, 1974; Tsourouflis et al., 1976; To and Flmk, 1978c; Gerschenson et al., 1981). Volatile losses are high above a critical moisture content (Flink and Karel, 1972). The control of temperature and water plasticization can be used to control release of volatiles. In the glassy state volatiles are encapsulated in the amorphous glass. Above Tg, collapse and sometimes crystallization takes place releasing encapsulated volatiles. Some volatiles may also decrease crystallization temperature of the amorphous matrix (To and Flink, 1978c). Exceeding the T, by increasing temperature or moisture content increases diffusion coefficients (Omatete and King, 1978; Simatos and Karel, 1988) which enhances loss of volatiles and possibly increases reaction rates such as browning (Flink et al., 1974; Saltmarch et al., 1981). The physical state of amorphous foods governs rates of various chemical and physical changes during food processing and storage. The rates of such changes should be studied and com- pared with physical state of the materials.

CONCLUSIONS DRIED FOOD MATERIALS containing sugars show typical behavior of amorphous materials. In such materials water is an extremely good plasticizer. Small amounts of water decrease glass transition temperatures to room temperature, which decreases viscosity, and causes time dependent structural fail- ures like collapse and stickiness. The water plasticization has an identical effect on glass transition temperature, collapse temperature, and sticky point. At high enough temperatures above these transitions crystallization takes place, being time dependant according to the temperature difference between T and T,. The temperature dependence of crystallization time follows WLF equation probably because of decreased viscosity and increased molecular mobility. Other compounds like fructose and starch in the amorphous matrix increase crystallization temperature of sucrose and cause delayed crystallization. In the rubbery state above the glass transition temperature rates of chemical reactions seem to increase.

REFERENCES Alexander, K. and King, C.J. 1985. Factors governin surface morphology of

spra -dmd amo Angel, CA 1983.

hous substances. Drying Techno 3: 321. Y %

f upercooled water. Ann. Rev. Phys. Chem. 34: 593.

Berg van den, C. 1985. Water activity. In Concentration and Drying of Foods, p.11. D. MacCarthy (Ed.). Elsevier Applied Science Publishers, London.

Berlin, E., Anderson, A., and Pallansch, M.J. 1970. Effect of temperature on water vapor sorption by dried milk powders. J. Dairy Sci. 53: 146.

Bushill, J.H., Wright, W.B., Fuller, C.H.F., and Bell,,A.V. 1965. The crystallization of lactose with particular reference to its occurence in milk powder. J. Sci. Food Agric. 16: 622.

Chirife, J. and Karel, M. 1974. Effect of structure disrupting treatments on volatile release from freeze dried maltose. J. Food Technol. 9: 13.

Downton, G.E., Flares-Luna, J.L., and King, C.J. 1982. Mechanism of stickiness in hygroscopic, amorphous powders. Ind. Eng. Chem. Fundam. 21: 447.

Flink, J.M. 1983. Structure and structure transitions in dried carbohydrate materials, In Physical Properties of Foods, p.473. M. Peleg and E.B. Bagley (Ed.). AVI Publishing Company, Westport, CT.

Flink, J. and Karel, M. 1970. Effects of Process Variables on Retention of Volatiles in Freeze Drying. J. Food Sci. 35: 444.

Flink, J.M. and Karel, M. 1972. Mechanisms of retention of organic volatiles in freeeze-dried systems. J. Food Technol. 7: 199.

Flink, J.M., Hawkes, J., Chen, H., and Won E. 1974. Properties of the freeze drying “scorch” temperature. J. Foo dg’. Sci. 39: 1244.

Gejl-Hansen, F. and Flink, M. 1977. Freeze-dried carbohydrate containing oil-in-water emulsions: microstructure and fat distribution. J. Food Sci. 42: 1049.

Gerschenson, L.N., Bartholomai, G.B., and Chirife, J. 1981. Structural collapse and volatile retention during heating and rehumidification of freeze-dried tomato juice. J. Food Sci. 46: 1552.

Herrington, T.M. and Brandfield, AC. 1984. Physico-chemical studies on sugar glasses. I. Rates of crystallization. 3. Food Technol. 19: 409.

Iglesias, HA. and Chirife, J. 1978. Delayed crystallization of amorphous sucrose in humidified freeze d&d model systems. J. Food Technol. 13: 137.

Karel, M. 1985. Effects of water activity and water content on mobility of food components, and their effects on phase transitions in food systems. In Properties of Water in Foods, p.153. D. Simatos and J.L. Multon (Ed.). Martinus Nijhoff Publishers, Dordrecht.

Karel, M. and Nickerson, J.T.R. 1964. Effects of relative humidity, air and vacuum on browning of dehydrated oran

Kauzmann, W. 1948. The nature of the g assy 7 e juice. Food Technol. 18: 1214.

state and the behavior of liquids at low tern ratures. Chem. Rev. 43: 219.

Levine, H. and Sla 8” e, L. 1986. A polymer physico-chemical approach to the study of commercial starch hydrolysis products @HP& Carboh. Pol- ymers 6: 213.

Levine, H. and Slade, L. 1988. Principles of “cryostabilization” technology from structure/property relationships of carbohydrate/water systems-A review. f&o-Letters 9: 21.

Makower, B. and Dye, W.B. 1956. E tallization of amorphous sucrose an

uilibrium moisture content and crys- 1 glucose. J. Agric. Food Chem. 4: 72.

Masters, K. and Stolte, A. 1973. Agglomeration advances. Food Eng. (21: 64. Moreyra, R. and Peleg, M. 1981. Effect of equilibrium water activity on

the bulk properties of selected food powders. J. Food Sci. 46: 1918. Omatete, 0.0. and Kin

ii&ion of freeze drie f C.J. 1978. Volatiles retention during rehumidi-

food models. J. Food Technol. 13: 265. P5&kkiinen, K. and Roes, Y. 1990. Effects of drying conditions on water

sorption and phase transitions of freeze dried horseradish roots. J. Food Sci. 55: 206.

Raem A. and Schweizer, T.F. 1983. Thermal behaviour of carbohydrates stu d! led by heat flow calorimetry. J. Thermal Anal. 28: 95.

Roos, Y.H. 1987. Effect of moisture on the thermal behavior of strawberries studied using differential scanning calorimetry. J. Food Sci. 52: 146.

Saltmarch, M. and Labuza, T.P. 1980. Influence of relative humidity on the physicochemical state of lactose in spray-dried sweet whey powders. J. Food Sci. 45: 1231.

Saltmarch, M., Vagnini-Ferrari, M., and Labuza, T.P. 1981. Theoretical basis and application of kinetics to browning in spray-dried whey food systems. Pro

d . Food Nutr. Sci. 5: 331.

Sharp, P.F. an Doob, H. 1941. Effect of humidity on moisture content and forms of lactose in dried whey. J. Dairy Sci. 24: 679.

Simatos, D. and Blond, G. 1975. The B

orous texture of freeze dried rod- ucts. In Freeze D ing and Advance Food Technology, p.401. S.A. &old- blith, L. Rey, an 7 W.W. Rothmayr (Ed.). Academic Press, London.

Simatos, D. and Karel, M. 1988. Characterization of the condition of water in foods-ph sico-chemical aspects. In Food Preservation by Water Ac- tivity Contra f , p.1. CC. Seow (Ed.). Elsevier, Amsterdam.

Simatos, D., Faure, M., Bonjour, E., and Couach, M. 1975. The physical state of water at low temperatures in plasma with different water contents as studied by differential thermal analysis and differential scanning calorimetry. Cryobiology 12: 202.

Slade, L., Levine, H. and Finlay, J.W. 1989. Protein-water interactions: Water as a plasticizer of gluten and other protein olymers. Quality and the Effects of Processing, p.9. R.D. Phil ips and J.W. Finlay F

In Protein

(Ed.). Marcel Delcker, Inc., New York. Soesanto, T. and Williams, MC. 1981. Volumetric inte

for concentrated and dilute sugar solutions. J. Phys. z retation of viscosity hem. 85: 3338.

To, E.C. and Flink, J.M. 1978a. Collapse, a structural transition in freeze dried carbohydrates. I. Evaluation of analytical methods, J. Food Tech- nol. 13: 551.

To, E.C. and Flink, J.M. 1978b. Collapse, a structural transition in freeze dried carbohydrates. II. Effect of solute composition. J. Food Technol. 13: 567.

To, E.C.and Flink, J.M. 1978c. Collapse, a structural transition in freeze dried carbohydrates III. Prerequisite of recrystallization. J. Food Tech- nol. 13: 583.

Troy, H.C. and Sharp, P.F. 1930. a and 8 lactose in some milk products. J. Dai

iif S&13: 140.

Tsourou IS, S., Flink, J.M. and Karel, M. 1976. Loss of structure in freeze dried carbohydrates solutions: Effect of temperature, moisture content and composition. J. Sci. Food Agric. 27: 509.

Van Hook, A. (Ed.). 1961. Crystallization Theory and Practice, 325 p. Rein- hold Publ. Corp., New York.

Weitz, A. and Wunderlich, B. 1974. Thermal analysis and dilatometry of $as;;;ormed under elevated pressure. J. Polym. Sci. Polym. Phys. Ed.

White, G W. and Cakebread, S.H. 1966. The glassy state in certain sugar- containing food products. J. Food Technol. 1: 73.

Williams, M.L., Landel, R.F., and Ferry, J.D. 1955. The temperature dependence of relaxation mechanisms in amorphous polymers and other glass-forming liquids. J. Am. Chem. Sot. 77: 3701.

MS received 11/l/89; revised 4/24/90; accepted 6/4/90. We thank Academy of Finland, Center for Advanced Food Technology, at Rutgers

University and New Jersey Agricultural Experiment Station for financial support, and M&M Mars, Inc. for donation of wne of the equipment. This is contribution no. D-


1991 - Roos, Karel - Plasticizing Effect of Water on Thermal Behavior and Crystallization of...

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