Foodprocessing Operations Requiring Heat Exchangers

It is common to think of all heat exchangers as being composed of two adjoining compartments through which fluids are flowing, each cooling or heating the other. This concept is natural because the basic engineering approach to studying heat transfer is to discuss each type of heat exchange operation as a specific process (eg, product freezing, refrigeration systems, batch heating, nonsteady-state operations, etc) with steady-state, liquid-liquid heat exchangers being studied under the subject of heat exchangers. However, when processing food, it is important to think of heat exchangers as being the total range of equipment that is used for cooking, blanching, pasteurizing, sterilizing, cooling, freezing, and cold-storage holding. In addition, there are many processes, such as drying and extraction, in which the transfer of heat is important to accomplish the basic goal of the process. Food products that are heated or cooled during processing, including the heat transfer medium, cover a wide range of vapors, liquids, and solids. Table 1 indicates how machinery and equipment must be used for the transfer of heat in the many different food-processing operations.

In addition to the type of heat transfer taking place during processing, the physical and chemical condition of the food before, during, and after heating or cooling must be considered. Products requiring different approaches to heat transfer during the food-processing operation include

1. Dense, hard, solid foods (eg, potatoes) that can stand considerable mechanical abuse during processing but become somewhat fragile during the final phase of heating.

2. Soft, solid foods (eg, tomatoes) that cannot stand any mechanical abuse during or after cooking.

3. Purées or soft items that flow like a liquid but congeal or gel to a solid when heated (eg, surimi being processed into seafood analogues and extruded cereals).

4. Liquids that range from heat stable to extremely heat labile.

5. Foods, particularly those high in protein, that congeal with heat and cause baked-on deposits on the heat-transfer surfaces.

Table 1. The State of a Food Product as Related to Classification of Heat Exchanger

Food form

Heat transfer media

Classification

Example

Vapor

Liquid

Noncontact

Condensing vapors

Liquid

Vapor

Contact

Steam infusion, steam injection

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