Crystal Growth and Nucleation

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Crystal growth, in contrast to nucleation, occurs readily at temperatures close to the freezing point. It is more difficult to initiate crystallization than to continue it. The rate of ice crystal growth decreases with decreasing temperature. A schematic graphical representation of nucleation and crystal growth rates is given in Figure 1-20. Solutes of many types and in quite small amounts will greatly slow ice crystal growth. The mechanism of this action is not known. Membranes may be impermeable to ice crystal growth and thus limit crystal size. The effect of membranes on ice crystal propagation was studied by Lusena and Cook (1953), who found that membranes freely permeable to liquids may be either permeable, partly permeable, or impermeable to growing ice crystals. In a given material, permeability to ice crystal growth increases with porosity, but is also affected by rate of cooling, membrane composition and properties, and concentration of the solute(s) present in the aqueous phase. When ice crystal growth is retarded by solutes, the ice phase may become dis-

Nucleation And Growth Crystals

FP = temperature at which crystals start to form.

Figure 1-20 Schematic Representation of the Rate of Nucleation and Crystal Growth

FP = temperature at which crystals start to form.

Figure 1-20 Schematic Representation of the Rate of Nucleation and Crystal Growth continuous either by the presence of a membrane or spontaneously.

Ice crystal size at the completion of freezing is related directly to the number of nuclei. The greater the number of nuclei, the smaller the size of the crystals. In liquid systems nuclei can be added. This process is known as seeding. Practical applications of seeding include adding finely ground lactose to evaporated milk in the evaporator, and recirculating some portion of crystallized fat in a heat exchanger during manufacture of margarine. If the system is maintained at a temperature close to the freezing point (FP), where crystallization starts (Figure 1-20), only a few nuclei form and each crystal grows extensively. The slow removal of heat energy produces an analogous situation, since the heat of crystallization released by the few growing crystals causes the temperature to remain near the melting point, where nucleation is unlikely. In tissue or unagitated fluid systems, slow removal of heat results in a continuous ice phase that slowly moves inward, with little if any nucleation. The effect of temperature on the linear crystallization velocity of water is given in Table 1-6.

If the temperature is lowered to below the FP (Figure 1-20), crystal growth is the predominant factor at first but, at increasing rate of supercooling, nucleation takes over. Therefore, at low supercooling large crystals are formed; as supercooling increases, many small crystals are formed. Control of crystal size is much more difficult in tissues than in agitated liquids. Agitation may promote nucleation and, therefore, reduced crystal size. Lusena and Cook (1954) suggested that large ice crystals are formed when freezing takes place above the critical nucleation temperature (close to FP in Figure 1-20). When freezing occurs at the critical nucleation temperature, small ice crystals form. The effect of solutes on nucleation and rate of ice crystal growth is a major factor controlling the pattern of propagation of the ice front. Lusena and Cook (1955) also found that solutes depress the nucleation temperature to the same extent that they depress the freezing point. Solutes retard ice growth at 10°C supercooling, with organic compounds having a greater effect than inorganic ones. At low concentrations,

Table 1-6 Effect of Temperature on Linear Crystallization Velocity of Water

Temperature at Onset of Crystallization (°C)

Linear Crystallization Velocity (mm/min)

230 520 580 680 1,220 1,750 2,800

proteins are as effective as alcohols and sugars in retarding crystal growth.

Once formed, crystals do not remain unchanged during frozen storage; they have a tendency to enlarge. Recrystallization is particularly evident when storage temperatures are allowed to fluctuate widely. There is a tendency for large crystals to grow at the expense of small ones.

Slow freezing results in large ice crystals located exclusively in extracellular areas. Rapid freezing results in tiny ice crystals located both extra- and intracellularly. Not too much is known about the relation between ice crystal location and frozen food quality. During the freezing of food, water is transformed to ice with a high degree of purity, and solute concentration in the unfrozen liquid is gradually increased. This is accompanied by changes in pH, ionic strength, viscosity, osmotic pressure, vapor pressure, and other properties.

When water freezes, it expands nearly 9 percent. The volume change of a food that is frozen will be determined by its water content and by solute concentration. Highly concentrated sucrose solutions do not show expansion (Table 1-7). Air spaces may partially accommodate expanding ice crystals. Volume changes in some fruit products upon freezing are shown in Table 1-8. The effect of air space is obvious. The expansion of water on freezing results in local stresses that undoubtedly produce mechanical damage in cellular materials. Freezing may cause changes in frozen foods that make the product unacceptable. Such changes may include destabilization of emulsions, flocculation of proteins, increase in toughness of fish flesh, loss of textural integrity, and increase in drip loss of meat. Ice formation can be influenced by the presence of carbohydrates. The effect of sucrose on the ice formation process

Table 1-7 Volume Change of Water and Sucrose Solutions on Freezing

Volume Increase During Temperature Change Sucrose (%) from 70°F to 0°F (%)

0

8.6

10

8.7

20

8.2

30

6.2

40

5.1

50

3.9

60

None

70

-1.0 (decrease)

has been described by Roos and Karel (1991a,b,c).

The Glass Transition

In aqueous systems containing polymeric substances or some low molecular weight materials including sugars and other carbohydrates, lowering of the temperature may result in formation of a glass. A glass is an amorphous solid material rather than a crystalline solid. A glass is an undercooled liquid

Table 1-8 Expansion of Fruit Products During Freezing

Volume Increase During Temperature Change from Product 70°Fto 0°F (%)

Apple juice 8.3

Orange juice 8.0

Whole raspberries 4.0

Crushed raspberries 6.3

Whole strawberries 3.0

Crushed strawberries 8.2

of high viscosity that exists in a metastable solid state (Levine and Slade 1992). A glass is formed when a liquid or an aqueous solution is cooled to a temperature that is considerably lower than its melting temperature. This is usually achieved at high cooling rates. The normal process of crystallization involves the conversion of a disordered liquid molecular structure to a highly ordered crystal formation. In a crystal, atoms or ions are arranged in a regular, three-dimensional array. In the formation of a glass, the disordered liquid state is immobilized into a disordered glassy solid, which has the rheological properties of a solid but no ordered crystalline structure.

The relationships among melting point (Tm), glass transition temperature (Tg), and crystallization are schematically represented in Figure 1-21. At low degree of supercooling (just below Tm), nucleation is at a minimum and crystal growth predominates. As the degree of supercooling increases, nucleation becomes the dominating effect. The maximum overall crystallization rate is at a

Supercooling Nucleation Growth
Figure 1-21 Relationships Among Crystal Growth, Nucleation, and Crystallization Rate between Melting Temperature (Tm) and Glass Temperature (Tg)

point about halfway between Tm and Tg. At high cooling rates and a degree of supercooling that moves the temperature to below Tg, no crystals are formed and a glassy solid results. During the transition from the molten state to the glassy state, the moisture content plays an important role. This is illustrated by the phase diagram of Figure 1-22. When the temperature is lowered at sufficiently high moisture content, the system goes through a rubbery state before becoming glassy (Chir-ife and Buera 1996). The glass transition temperature is characterized by very high apparent viscosities of more than 105 Ns/m2 (Aguilera and Stanley 1990). The rate of diffusion limited processes is more rapid in the rubbery state than in the glassy state, and this may be important in the storage stability of certain foods. The effect of water activity on the glass transition temperature of a number of plant products (carrots, strawberries, and potatoes) as well as some biopolymers (gelatin, wheat gluten, and wheat starch) is shown in Figure 1-23 (Chirife and Buera 1996). In the rubbery state the rates of chemical reac

liquid

7

ice + liquid

glass

Figure 1-22 Phase Diagram Showing the Effect of Moisture Content on Melting Temperature (Tm) and Glass Transition Temperature (Tg)

concentration %

Figure 1-22 Phase Diagram Showing the Effect of Moisture Content on Melting Temperature (Tm) and Glass Transition Temperature (Tg)

tion appear to be higher than in the glassy state (Roos and Karel 1991e).

When water-containing foods are cooled below the freezing point of water, ice may be formed and the remaining water is increasingly high in dissolved solids. When the glass transition temperature is reached, the remaining water is transformed into a glass. Ice formation during freezing may destabilize sensitive products by rupturing cell walls and breaking emulsions. The presence of glass-forming substances may help prevent this from occurring. Such stabilization of frozen products is known as cryoprotection, and the agents are known as cryoprotectants.

When water is rapidly removed from foods during processes such as extrusion, drying, or freezing, a glassy state may be produced (Roos 1995). The Tg values of high molecu lar weight food polymers, proteins, and polysaccharides are high and cannot be determined experimentally, because of thermal decomposition. An example of measured Tg values for low molecular weight carbohydrates is given in Figure 1-24. The value of Tg for starch is obtained by extrapolation.

The water present in foods may act as a plasticizer. Plasticizers increase plasticity and flexibility of food polymers as a result of weakening of the intermolecular forces existing between molecules. Increasing water content decreases Tg. Roos and Karel (1991a) studied the plasticizing effect of water on thermal behavior and crystallization of amorphous food models. They found that dried foods containing sugars behave like amorphous materials, and that small amounts of water decrease Tg to room temperature with

Wheat Starch

Gelatin

^Wheat^G^

Strawberry

\

Potatoes

- Carrots

Water activity

Water activity

Figure 1-23 Relationship Between Water Activity (aw) and Glass Transition Temperature (Tg) of Some Plant Materials and Biopolymers. Source: Reprinted with permission from J. Cherife and M. del Pinar Buera, Water Activity, Water Glass Dynamics and the Control of Microbiological Growth in Foods, Critical Review Food Sci. Nutr., Vol. 36, No. 5, p. 490, © 1996. Copyright CRC Press, Boca Raton, Florida.

250

— Starch

230

o

210

lu

190

' MaltohexaossO

<M040

ce

170

< E

150

MaltotrioseN

3M200

U1 O.

130

Ul

110

1-

90

70

Maltose

Figure 1-24 Glass Transition Temperature (Tg) for Maltose, Maltose Polymers, and Extrapolated Value for Starch. M indicates molecular weight. Source: Reprinted with permission from Y.H. Roos, Glass Transition-Related Physico-Chemical Changes in Foods, Food Technology, Vol. 49, No. 10, p. 98, © 1995, Institute of Food Technologists.

the result of structural collapse and formation of stickiness. Roos and Karel (1991e) report a linearity between water activity (aw) and Tg in the aw range of 0.1 to 0.8. This allows prediction of Tg at the aw range typical of dehydrated and intermediate moisture foods.

Roos (1995) has used a combined sorption isotherm and state diagram to obtain critical water activity and water content values that result in depressing Tg to below ambient temperature (Figure 1-25). This type of plot can be used to evaluate the stability of low-moisture foods under different storage conditions. When the Tg is decreased to below ambient temperature, molecules are mobilized because of plasticization and reaction rates increase because of increased diffusion, which in turn may lead to deterioration. Roos and Himberg (1994) and Roos et al. (1996) have described how glass transition temperatures influence nonenzymatic browning in model systems. This deteriorative reaction

Water Activity Content Isotherm

water activity

Figure 1-25 Modified State Diagram Showing Relationship Between Glass Transition Temperature (Tg), Water Activity (GAB isotherm), and Water Content for an Extruded Snack Food Model. Crispness is lost as water plasticization depresses Tg to below 24 °C. Plasticization is indicated with critical values for water activity and water content. Source: Reprinted with permission from Y.H. Roos, Glass Transition-Related Physico-Chemical Changes in Foods, Food Technology, Vol. 49, No. 10, p. 99, © 1995, Institute of Food Technologists.

water activity

Figure 1-25 Modified State Diagram Showing Relationship Between Glass Transition Temperature (Tg), Water Activity (GAB isotherm), and Water Content for an Extruded Snack Food Model. Crispness is lost as water plasticization depresses Tg to below 24 °C. Plasticization is indicated with critical values for water activity and water content. Source: Reprinted with permission from Y.H. Roos, Glass Transition-Related Physico-Chemical Changes in Foods, Food Technology, Vol. 49, No. 10, p. 99, © 1995, Institute of Food Technologists.

showed an increased reaction rate as water content increased.

Water Activity and Reaction Rate

Water activity has a profound effect on the rate of many chemical reactions in foods and on the rate of microbial growth (Labuza 1980). This information is summarized in Table 1-9. Enzyme activity is virtually nonexistent in the monolayer water (aw between 0 and 0.2). Not surprisingly, growth of microorganisms at this level of aw is also virtually zero. Molds and yeasts start to grow at aw between 0.7 and 0.8, the upper limit of capillary water. Bacterial growth takes place when aw reaches 0.8, the limit of loosely

Table 1-9 Reaction Rates in Foods as Determined by Water Activity

Monolayer

Loosely Bound

Reaction

Water

Capillary Water

Water

Enzyme activity

Zero

Low

High

Mold growth

Zero

Low*

High

Yeast growth

Zero

Low*

High

Bacterial growth

Zero

Zero

High

Hydrolysis

Zero

Rapid increase

High

Nonenzymic browning

Zero

Rapid increase

High

Lipid oxidation

High

Rapid increase

High

"Growth starts at aw of 0.7 to 0.8.

"Growth starts at aw of 0.7 to 0.8.

bound water. Enzyme activity increases gradually between aw of 0.3 and 0.8, then increases rapidly in the loosely bound water area (aw 0.8 to 1.0). Hydrolytic reactions and nonenzymic browning do not proceed in the monolayer water range of aw (0.0 to 0.25). However, lipid oxidation rates are high in this area, passing from a minimum at aw 0.3 to 0.4, to a maximum at aw 0.8. The influ ence of aw on chemical reactivity has been reviewed by Leung (1987). The relationship between water activity and rates of several reactions and enzyme activity is presented graphically in Figure 1-26 (Bone 1987).

Water activity has a major effect on the texture of some foods, as Bourne (1986) has shown in the case of apples.

I Free (Solute « Caplllaryl

Free Fatty Acids

Free Fatty Acids

The Effect Moisture Crystal Growth

Figure 1-26 Relationship Between Water Activity and a Number of Reaction Rates. Source: Reprinted with permission from D.P. Bone, Practical Applications of Water Activity and Moisture Relations in Foods, in Water Activity: Theory and Application to Food, L.B. Rockland and L.R. Beuchat, eds., p. 387, 1987, by courtesy of Marcel Dekker, Inc.

Figure 1-26 Relationship Between Water Activity and a Number of Reaction Rates. Source: Reprinted with permission from D.P. Bone, Practical Applications of Water Activity and Moisture Relations in Foods, in Water Activity: Theory and Application to Food, L.B. Rockland and L.R. Beuchat, eds., p. 387, 1987, by courtesy of Marcel Dekker, Inc.

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