Food Freezing Process
As explained earlier, freezing is used for food preservation to reduce or eliminate the physical changes and the chemical and microbiological activity deteriorating the food quality. By lowering the food temperature, the chemical and microbiological activities are reduced or eliminated for extending the shelf life of food.
Food freezing, which is a process used to minimize physical, biochemical and microbiological changes in food, is of great importance in the food industry. This preservative effect is maintained by subsequent storage of the frozen food at a sufficiently cold temperature. The freezing process for foods with a high water content, between 50% and 95%, is divided into three stages:
1. Precooling stage. This involves cooling down from the initial temperature of the product to the temperature at which freezing begins, at which phase change may occur. Heat is transferred to lower the temperature of the product without effecting phase change, and is referred to as sensible heat.
2. Phase change stage. This stage covers the formation of ice in the products and extends from the initial freezing point to a medium temperature about 5°C colder at the center of the product. The major part of the freezable water is converted to ice and this quite small reduction in the temperature is accomplished by a massive enthalpy change. In pure substances this stage is characterized by a plateau at the freezing point, since the heat removed accounts only or mainly for the phase change; this is referred to as latent heat.
3. Tempering stage. This is a cooling-down period to the ultimate temperature for storage and begins when the contribution of the latent heat is negligible compared to that of the sensible heat. It is characterized by an increase in the rate of temperature change. When it leaves the freezer, the frozen product has a nonuniform temperature distribution: warmer in the center and coldest at the surfaces. Its average temperature corresponds to the value reached when the temperature of the product is allowed to equilibrate. In general, it is recommended that the product be cooled in the freezer to an equilibrium temperature of -18°C or colder. Product leaving the freezer with a warmer temperature is stored for some time under relatively unfavorable conditions. Cooling down to the storage temperature may take a few days or weeks. In order to show how these changes occur, a schematic representation of the freezing profile of water and an aqueous solution is shown in Figure 7.1.
Note that reezing involves the change of water contained in the food from a liquid to a solid (ice). When water freezes it expands, and the ice crystals formed cause cell walls of food to rupture. As a result, the texture of the product will be much softer when it thaws. These textural changes are most noticeable in fruits and vegetables that have a high water content. For example, when frozen lettuce thaws, it turns limp and wilted. Thus, vegetables with a high water content, such as celery and salad greens, are not usually frozen. Also, many frozen fruits are best served while they still contain a few ice crystals. The effect of freezing on fruit tissue is less noticeable when fruit is still partially frozen. However, textural changes due to freezing are not as apparent in products that are cooked before eating because cooking also softens cell walls. Textural changes are also less noticeable in high-starch vegetables, such as peas, corn and lima beans.
It is important to note that the freezing rate influences the quality of food. High freezing rates result in small crystals in the product and low concentration gradients across cell membranes, whereas low freezing rates result in large ice crystals. Large ice crystals will grow intracellularly destroying cells and thus deteriorating the quality.