Ice crystallization is the formation of systematically organized ice from liquid water. Since
ice formation is responsible for most of the detrimental changes (lethality, quality loss) that
occur during freezing, a clear understanding of crystallization is essential to minimize
freezing damage. The crystallization process consists of nucleation and crystal growth. Nucleation is the combining of molecules into an ordered particle of a size sufficient to survive and serve as a site for crystal growth. Crystal growth is simply the enlargement of a nucleus by the orderly addition of molecules.
It was mentioned earlier that the changes taking place during freezing greatly affect the quality of the frozen products since the major component of most food products is water (between 50% and 95%), and the rest consists of animal or vegetable cells containing a low percentage of salts and other solid matter. In order to be able to understand the changes it is first necessary to take a look at the crystallization of water. It is the formation of ice crystals from this water which causes the major problems in frozen foods. Ice crystal formation is a phase change in which water molecules stop moving and form an ordered crystalline structure; in so doing, they give up energy as heat to their surroundings. This slows down the freezing of nearby water molecules, so that during freezing the temperature of the water or the food drops only very slowly, but once freezing is complete there is a sudden fall in temperature (see Figure 7.1).
In a freezing process the high water content is transferred from a liquid phase to a solid state. When a food product is refrigerated below 0°C, ice is formed at the initial freezing point, depending on the concentration of dissolved substances (e.g. salts). The first ice crystals are formed outside the cell, owing to the higher freezing point. The rate of ice crystallization is a function of the speed of heat removal and the diffusion of water from the cell to the intercellular space. If the freezing rate is low, a few crystallization centers (nuclei) are formed in the intercellular space, as crystals grow to a relatively large size. There are two types of nucleations, namely homogeneous nucleation (covering the change orientation and combination of water molecules) and heterogeneous nucleation (covering the formation the formation of a nucleus around suspended particles or at a cell wall). If quick freezing takes place, heterogeneous nucleation dominantly occurs in the food product. In conjunction with this, increased freezing rates result in a large number of ice crystallization nuclei and provide smaller crystals. During the freezing process the cell loses water by diffusion through the membrane, and this water crystallizes to ice on the crystals outside the cell. The size of the ice crystals is directly related to the quality of the frozen food.
Control of crystal size and shape becomes easy if crystals, once formed, are stable. In feet, these are unstable since during frozen storage they undergo metamorphic changes, referring to the recrystallization. This recrystallization encompasses any change in the number, size, shape, orientation, or perfection of crystals following completion of initial solidification. It takes place because systems tend toward a state of equilibrium wherein free energy is minimized and the chemical potential is equalized among all phases. Extensive information on these crystallization and recrystallization may be found elsewhere.
The changes resulting from freezing and storage of frozen products are more evident in organoleptic properties such as texture than in nutritional value. However, the attention must also be paid to deterioration in nutritional value which may be caused by the drip in which soluble materials such as minerals and some vitamins (e.g. vitamin C) are lost during thawing.