Changing The State Of A Food Product

There are three states in which a food can exist, namely solid, liquid, or vapor. Because most natural foods are in some state of equilibrium with water, most of the significant changes of state in food products involve water. Food processes involving a change in state include freezing, heating to the point of evaporating a component (eg, vaporization or dehydration), condensing vapors to liquids (eg, solvent extraction), and subliming water directly from the frozen state to vapor.

Freezing Food

Freezing foods involves removing heat, resulting in the changing of liquid water in the food to ice. This has been demonstrated in the frozen fish industry where sensible heat is removed until the temperature reaches approximately - 1°C (30°F). It then takes about 45 min during the so-called critical period to remove the latent heat of fusion from the water. After this period, the temperature drops rapidly as sensible heat is again removed (see Fish and shellfish products). It should be noted that foods are not pure substances so do not have precise temperatures at which there is a change in phase. This is because water has dissolved materials that increase or decrease in amount as a food is being processed and some of the water is bound to components of the food rather than being free to flow and act like pure water.

Vaporization Processes

The three vaporization processes involve the change in basic characteristics of a food or material by applying heat or other forms of energy that is converted to heat within the product.

Evaporation. Evaporization is the concentration of a liquid by supplying sufficient energy to vaporize the more volatile component or components. The process is used to concentrate solutions for stability or prior to further processing and to recover solvent from an extraction system.

Drying or Dehydrating. Drying or dehydration involves adding energy, usually heat, to vaporize a liquid, usually water, from a solid food. Foods are dried by supplying sufficient heat energy to vaporize water. This (1) preserves the product by reducing water activity; (2) reduces the cost or difficulty of packaging, handling, storing and shipment; and (3) produces convenience items (eg, instant coffee). In certain cases energy is supplied through other energy forms and then converted to heat in the product. For example, in processing by microwave heating, the energy is supplied to the system in the form of hertzian waves that are absorbed by the food, the resulting friction between vibrating molecules converts the wave energy to heat energy.

There are two distinct periods of drying in which different mechanisms control the rate of drying. During the first period, the constant rate period, the heat transfer predominates; all of the heat added is directly used in evaporating water and the rate of drying is independent of the nature of the food. In this case the moisture movement near the surface is rapid enough to maintain a saturated condition at the surface and the temperature of the food remains constant.

Each food has a critical moisture content at which the moisture can not migrate to the surface by diffusion rapidly enough to utilize all of the heat for evaporation. At this point the second phase, or falling rate period, begins and the food begins to absorb heat and rise in temperature.

A special form of drying involves sublimation of water directly from a frozen product to vapor, that is without passing through the liquid state. This is accomplished by placing a frozen food in a chamber under high vacuum, if the partial pressure of water vapor in the chamber is maintained below that of the ice at 0°C or 4.58 mm Hg, the water does not thaw prior to becoming a vapor.

Distillation. Distillation is the separation of two or more liquids through vaporizing the more volatile component or components. A principal use of this process in the food industry is in the recovery of a product after solvent extrac tion. In the case of organic solvent extraction of oilseeds, the solvent is recovered for reuse and the oil is retained as a pure food product.

Heat energy is certainly the most important factor involved in the processing of food products. In dealing with the science and engineering aspects of the food industry it is necessary to be constantly aware of the units involved in measuring the amount of energy being added to or removed from a food product. Unit equations (those showing the units as well as the numerical values) ensure the consistency of units and dimensions.

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