Using Thermodynamic Relationships In Food Processing

Prior to discussing specific food processing operations, it is necessary to define the world or system in which the process takes place. That is, boundaries must be defined as a means of isolating the process from the surrounding environment. There is usually some interchange between a system and the surroundings but they do not affect the principal relationships within the system. For example, if a food is being heated in a retort the heat balance must account for the heat interchange between the source and the food plus allow for a heat loss through the insulated or lagged walls of the retort.

The most important thermodynamic applications reflected in food processing involve: (a) transferring mass within the processing environment or between phases in a process, (b) transferring heat to heat or cool a food and, (c) transferring heat to change the phase of a component in a food processing system.

Basic Relationships

In considering units of measurement, it is necessary to define base values from which one can compare changes. For example, when one is standing on a ladder, the height of 3 meters would be compared to the base value of zero, the floor. Otherwise 3 meters would have no meaning. Likewise, the base for measuring temperature, and thus being able to determine heat energy changes in a body, is absolute zero.

The heat content or enthalpy of a food and the change of this parameter is basic to processing. It is defined as:

where E is the internal energy, p is the absolute pressure, and v is the volume. The changes in enthalpy and not the absolute values are important in food processing. Hence,

^-Vaporization temp.

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