Chapter 17 - Thermal Processing

Abstract

Depending on their intensity, thermal preservation processes are classified into two categories: pasteurization and sterilization. Pasteurization destroys vegetative cells but has almost no effect on spores. Sterilization is aimed at the inactivation of all forms of microorganisms, including spores. For the kinetics of thermal inactivation of microorganisms, the decimal reduction time or log-linear model is widely accepted. This model assumes that thermal destruction of microorganisms follows the kinetics of a first-order reaction. The relationship between the decimal reduction time and the temperature is also assumed to be log-linear, leading to the definition of the z value as the temperature increment needed to accelerate the destruction of microorganisms by a factor of 10. Both the first-order kinetics and the Arrhenius-type effect of temperature are contested. The lethality of a thermal process is expressed as its F0 value. The F0 value of any thermal sterilization process is the number of minutes of heating at 121°C required to achieve the same thermal destruction ratio of a specified target microorganism, having a z value of 10°C. Thermal process optimization is the selection of processing conditions resulting in minimal damage to quality. One of the conclusions of optimization is the superiority of high temperature–short time (HTST) processes. Heat penetration considerations are of central importance in the design and conduct of thermal processes.