This chapter deals with some of the fundamental concepts concerning thermal processing of foods to eliminate foodborne pathogens. For most foodborne pathogens in a food matrix under isothermal conditions, the cell population generally decreases exponentially with heating time and, therefore, can be described by 1 "-order kinetics. The resistance of microorganisms to heat is conventionally characterized by D and z values. For mixed cultures, if both D and z values follow lst-order kinetics, a mixed-culture model can be used. With the latter model, the more heat-sensitive microorganisms will be preferentially inactivated, followed by the heat-resistant ones, explaining the "tail" effect observed in some survival curves. For more complex survival curves, a general "Weibull"-type model can be used. This model offers more flexibility in describing either convex, concave, or linear survival curves. This model is generally more accurate than the transitional linear model when used to describe convex and concave curves.
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