A Weibullian model for microbial injury and mortality

Abstract

A microbial survival curve is constructed by plotting the number of recoverable cells or their logarithm vs. the exposure time to the hostile agent, be it high or low temperature, a chemical preservative or disinfectant, etc. Since the recovery is usually done in a medium and under conditions that favor growth, the result is insensitive to whether the counted survivors are intact or injured. If or when both the total number of survivors and those remaining intact follow a Weibullian decay pattern (with different parameters), then the momentary number of injured cells will be the momentary difference between the two. Such a scenario can be easily modeled mathematically and the resulting model enables to simulate a variety of survived-injury patterns in thermal and non-thermal food preservation processes. Under certain conditions according to this model, almost all the survivors would be injured to at least some extent and hence may perish during the food's storage and transportation. Isothermal survival-injury curves generated with the Weibullian model based on the above considerations were in general agreement with published experimental data. In principle, the methodology can be extended to simulate mortality-injury patterns under dynamic conditions, i.e., when the temperature or chemical agent's concentration vary with time. Whether a cell is considered injured depends on the recovery method, e.g., on whether it can or cannot grow in a saline medium. Thus recovery in different media may yield somewhat different quantitative results but very unlikely a qualitatively different pattern. Although the model used was based on that microbial mortality and injury both follow the Weibullian model, very similar results would have been obtained had other survival modes been assumed.