Using this simplification and the definition of parameter Z, equation 4 can be rewritten as


This is the familiar equation in food sterilization that provides the equivalent heating time F0 at a reference temperature Tr for a process whose actual temperature T varies with time t.

In any finite mass of liquid (or any material), the temperature history T(t) varies spatially throughout the mass. Thus F0 as given by equation 6 will vary spatially at any given time. Thus in reality, there is always a distribution of F0 values due to spatial variation of temperature history. For bacterial destruction the lowest value of F0 is taken as the measure of the extent of sterilization. The average F0 for nutrients, which is sometimes referred to as the cook value, provides a measure of the nutritional quality of the processed food material.

The complete distribution of quality parameters such as sterilization and nutrient retention is generally quite difficult to obtain due to the complex variations of temperature T with position and time in most processing situations of practical interest. Under these circumstances, design of the heating process is aimed at minimizing the spatial variation of temperature, for example, by introducing turbulence. For many practical calculations, where spatial variation is ignored, mean temperature is used in equation 6.

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