Microbiological Process Discussion Analytical Method for Calculating Heat Sterilization Times

The determination of conditions necessary to adequately heat sterilize various media is a problem of considerable importance in the fermentation and food industries, or other such areas where a lack of sterile techniques preclude satisfactory performance. Until recently, however, practical ways to handle this problem were either wholly empirical ones, or theoretical ones which depended on some simplifying assumptions regarding the shape of the heating curve. Along the latter line is the development of design procedures for the sterilization of canned foods by Ball (1923) and Levine (1956), among others. A theoretical, yet practical and completely general, method for calculating heat sterilization times for fermentation media (as well as other liquid media), based on the thermal-death kinetics of bacterial spores and the actual heating curve, was proposed by Deindoerfer (1957). For batch sterilizations, the method involved a graphical integration of the specific reaction rate for thermal spore destruction along the heating curve. All that was required for this integration was a knowledge of the temperature dependence of the specific reaction rate. For isothermal portions of the heating curve a simple analytical integration was possible, as the specific reaction rate remained constant. In certain types of continuous sterilizers this simple integration was sufficient to determine the time-temperature relationship for any desired degree of sterilization. Humphrey (1957) was successful in analytically integrating the specific reaction rate over certain nonisothermal periods of a batch sterilization. Also, the integrated solutions for several temperature-time profiles of an element of medium flowing through continuous sterilizers were presented by Deindoerfer and Humphrey (1958) in a discussion of the principles entering into the design of these units. This paper reviews briefly the underlying theory of heat sterilization and develops, using this theory and common expressions for heat transfer in various equipment, the analytical integrations mentioned above. The practical solution of many sterilization problems can be carried out directly by employing the equations developed. Values of difficult to compute exponential