ABSTRACT
Th e lethal effect of high temperature on microorganisms and spores has been a topic of intensive study during past decades. As a result, there are now generally accepted criteria for judging the safety of existing thermal
preservation methods and procedures for calculating the safety of new ones. The impressive safety record of the food industry, especially in its canning and bottling operations, has served as proof of the efficacy of these criteria and procedures. Therefore, although the theory on which these criteria and procedures is based has been continuously challenged and criticized (e.g., Casolari et al., 1988; Casolari, 1994; Cole, 1997), there has been little incentive to revise it. However, the situation has recently changed with the emergence of novel nonthermal preservation methods, such as ultra-high pressure pulsed high-voltage electric fields (Barbosa-Canovas et al., 1998), and the tendency to reduce the amount of heat in conventional thermal processes in order to preserve freshness and nutrients. There is also a growing interest in new combinations of two or more preservation methods as a compromise between the conflicting demands of ensuring safety and maintaining freshness. Such combinations allow reduction of the overall level of delivered energy through the exploitation of synergistic effects. (In the case of ionizing radiation, as in pork products, this is necessary so that they remain edible). The safety of any new or modified preservation method in a particular product will always require confirmation by direct microbiological analysis. Nevertheless, commercial application of these new technologies will also raise theoretical questions whose answers may be of more than academic interest. The main issues will
be how the microbial survival data of novel processes can be used to determine safety and what constitutes an equivalent treatment in processes where the microbial mortality pattern is different. Addressing these areas will obviously require knowledge of the biology, physiology, biochemistry, and biophysics of microorganisms in order to elucidate how the lethal agent works at the molecular and cellular levels. It will also require familiarity with the properties of statistical distributions in order to understand the microbial response to a preservation treatment at the population level. The objectives of this review are to present a statistical approach to the interpretation of various common types of microbial survival curves and to explore the possibility of expressing their characteristics, not in terms of reactions kinetics, but as cumulative forms of distributions of lethal events having different mode, mean, variance, and skewness coefficients.
