Bacterial thermal death kinetics based on probability distributions: the heat destruction of Clostridium botulinum and Salmonella Bedford.

Despite the long history and excellent record of inactivation models used in thermal processing, there are relatively few approaches that attempt to describe the kinetics commonly observed. There are even fewer examples of models that allow the user to deal with the environmental conditions that influence these kinetics. We describe an approach that assumes a distribution of inactivation times within a population of bacterial cells. The concept allows for alternative interpretations of death kinetics and provides excellent descriptions of data generated with two important foodborne pathogens, Clostridium botulinum and Salmonella Bedford. The Salmonella Bedford data set used is unusual and perhaps unique in that it provides information where more than 50% of the population survival has been measured. These measurements are often overlooked or missed in experimental work but are essential when using a vitalistic approach, enabling calculation of a 50% lethal dose for destruction of bacteria. Use of the normal or Prentice distribution provided better fits to the data than other models commonly used to describe thermal death. There was no obvious bias in the fits even though significant tailing was evident. In addition, the procedure described allows data from all the conditions to be fitted rather than individual independent series. This enables a single equation to be derived that can be judged against the whole domain of the data. Approaches that provide accurate and unbiased descriptions of thermal death are likely to become increasingly important to ensure the safety of more marginal heat processes.

Food microbiology: the challenges for the future.

Food microbiology has become a mature science in the twentieth century and has made great advances. While recognising these achievements, it is also necessary to consider how the science may need to change. This paper addresses this by reference to three areas. These are possible changes in foodborne diseases of concern and the impact of molecular and genetic techniques on our current methodology. The recognition of the role of food and associated microbial contaminants in chronic diseases could become a major concern. New developments in our understanding of microbial genetics could affect our concepts of bacterial taxonomy. The current methodologies we use, based upon genotypically identical populations, may need to be addressed. If the trends indicated here are realised, they indicate a major challenge and opportunity for the food microbiologist.

Hazard analysis applied to microbial growth in foods: development of mathematical models describing the effect of water activity.

Mathematical models have been developed which describe the effect of lowering the water activity on the growth kinetics of Staphylococcus aureus and Salmonella typhimurium. By treating the lag phase and exponential phase kinetics separately predictions can be made on the extent of microbial growth over successive time/temperature cycles. Staph. aureus was far more tolerant than Salm. typhimurium to lowered water activity and under near growth limiting conditions of water activity and temperature was showing lag periods as long as ca 40 d. The maximum lag period observed for Salm. typhimurium was ca 5 d. Under these conditions the predicted generation times for Staph. aureus were 2-3 d and for Salm. typhimurium.

Technique and apparatus for rapid and inexpensive enumeration of bacteria.

A foot-operated diluter/dispenser and a projection viewer were developed for use with a rapid bacterial counting technique employing agar droplets. The equipment allows the advantages of the technique to be properly realized and assists many conventional bacteriological tests which may be made at the same time. Significant cost and labor savings are made, with reductions in incubation time, incubator space, and preparative work.