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of fat crystals is too high, the product is firm and difficult to spread, and if it is too low, the product is soft and tends to collapse under its own weight. The creation of food emulsions with desirable properties therefore depends on an understanding of the major factors which influence the crystallization and melting of lipids in foods (Birker and Padley 1987).

The arrangement of triacylglycerol molecules in the solid and liquid state is shown schematically in Figure 4.2. The physical state of a triacylglycerol at a particular temperature depends on its free energy, which is made up of contributions from enthalpic and entropic terms: AGS^L = AHS^L - TASs^l (Atkins 1994). The enthalpy term (AHS^L) represents the change in the overall strength of the molecular interactions between the triacylglycerols when they are converted from a solid to a liquid, whereas the entropy term (ASs^l) represents the change in the organization of the molecules that is brought about by the melting process. The strength of the bonds between the molecules is greater in the solid than in the liquid state because the molecules are able to pack more efficiently, and so AHS^L is positive, which favors the solid state. On the other hand, the entropy of the molecules in the liquid state is greater than that in the solid state, and therefore ASs^l is positive, which favors the liquid state. At low temperatures, the enthalpy term dominates the entropy term (AHS^L > TASs^l), and therefore the solid state has the lowest free energy (Atkins 1994). As the temperature increases, the entropic contribution becomes increasingly important. Above a certain temperature, known as the melting point, the entropy term dominates the enthalpy term (TASs^l > AHs^l), and so the liquid state has the lowest free energy. A material therefore changes from a solid to a liquid when its temperature is raised above the melting point. This process

FIGURE 4.2 The arrangement of triacylglycerols in the solid and liquid states depends on a balance between the organizing influence of the attractive interactions between the molecules and the disorganizing influence of the thermal energy.

Solid fat

Liquid oil

Solid fat

Liquid oil

Free Energy

MeltingiPoint

FIGURE 4.3 Temperature dependence of the free energies of the solid and liquid states. At low temperatures, the solid state is thermodynamically favorable, but above the melting point, the liquid state is more favorable.

MeltingiPoint

Temperature

FIGURE 4.3 Temperature dependence of the free energies of the solid and liquid states. At low temperatures, the solid state is thermodynamically favorable, but above the melting point, the liquid state is more favorable.

leads to the release of heat because of the reduction in the amount of energy stored in the intermolecular interactions when the material changes from a solid to a liquid (i.e., it is an exothermic process).

The temperature dependence of the free energies of the solid and liquid states clearly shows that below the melting point, the solid state has the lowest free energy, but above it, the liquid state has the lowest (Figure 4.3). Thermodynamics informs us whether or not a phase transition can occur, but it tells us nothing about the rate at which this process occurs or about the physical mechanism by which it is accomplished (Atkins 1994). As will be seen below, an understanding of lipid phase transitions requires a knowledge of both the thermodynamics and kinetics of the process. The crystallization of fats can be conveniently divided into three stages: supercooling, nucleation, and crystal formation (Boistelle 1988, Mullin 1993).

4.2.3.1. Supercooling

Crystallization can only take place once a liquid phase is cooled below its melting point (Garside 1987, Walstra 1987). Even so, a material may persist as a liquid below its melting point for a considerable time before any crystallization is observed (Skoda and van den Tempel 1963, Phipps 1964, Mulder and Walstra 1974). This is because of an activation energy which must be overcome before the liquid-solid phase transition can occur (Figure 4.4). If the magnitude of this activation energy is sufficiently high compared to the thermal energy of the system, crystallization will not occur, even though the transition is thermody-

Solid

FIGURE 4.4 When there is a sufficiently high activation energy between the solid and liquid states, a liquid oil can persist in a metastable state below the melting point of a fat.

Liquid

Liquid

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