The probability of bacterial spores surviving a thermal process: The 12D myth and other issues with its quantitative assessment

Abstract

The concepts of “D-value,” “thermal death time” and “commercial sterility,” innovative and useful at their inception, are based on untenable assumptions, notably that the log-linear isothermal inactivation model has universal applicability, that extrapolation over several orders of magnitude below the detection level is permissible, and that total microbial inactivation is theoretically impossible. Almost all commonly observed inactivation patterns, the log-linear is just a special case, can be described by both deterministic and fully stochastic models, examples of which are given. Unlike the deterministic, the stochastic models predict either complete elimination of the targeted cells or spores in realistic finite time, or residual survival. In most cases, the published survival data do not contain enough information to establish which actually happens. The microbial safety of thermally processed foods can be compromised not only by under-processing but also by a variety of mishaps whose occurrence probabilities are unrelated to the inactivation kinetics. Moreover, the available sampling plans to detect microbial contamination in sterilized containers through incubation alone are insensitive to levels of potential safety concerns. In principle, many of these issues could be resolved by developing new dramatically improved detection methods and/or verifiable methods to predict very low levels of microbial survival.

Keywords:

• Commercial sterility
• kinetics
• microbial inactivation
• microbial risk assessment (QMRA)
• thermal processing
• survival curves

Acknowledgements

The author expresses his deep gratitude to Mark D. Normand who wrote all the computer programs presented or mentioned in the article.

Disclosure statement

The author declares no conflict of interests. The opinions expressed in this review are the author’s own and do not necessarily reflect those of the Department of Food Science at the University of Massachusetts Amherst with which he is affiliated.

Funding

The author(s) reported there is no funding associated with the work featured in this article.