Taking injuries of surviving bacteria into account for optimising heat treatments

The main assets of early conventional models applied in the field of canned food industries are their simplicity and their robustness. Moreover, a certain standardisation of these models allows the intrinsic quantification of a food process like sterilisation, regardless of the nature of concerned microbial populations. However, a first drawback of conventional models is their monofactorial nature: only temperature is considered for the evaluation of microbial heat resistance. A second limit of early survival models is that conventional estimates of heat resistance are made by recovering heated surviving cells at optimal incubation conditions. However, many investigators observed that culture medium and incubation temperature influence both the ratio of injured cell recovery and estimated heat resistance values. Mafart and Leguérinel [Mafart, P., Leguérinel, I., 1998. Modeling combined effects of temperature and pH on heat resistance of spores by a linear-Bigelow equation. J. Food Sci. 63, 6-8] developed a model describing the heat resistance of spores as a function of temperature and pH which is an extension of the Bigelow equation. A short while later, they added a further term to their model in order to consider the water activity of the heating medium (Gaillard, S., Leguérinel, I., Mafart, P., 1998. Model for combined effects of temperature, pH and water activity on thermal inactivation of Bacillus cereus spores. J. Food Sci. 63, 887-889). From their model, the authors proposed an extension of the concept of biological destruction value (BDV) noted L(T, pH, a(w)), that they called primary BDV. More recently, we developed a second multifactorial model describing the effect of the temperature, the pH and the water activity of the recovery medium on the estimated D-value of heated spores (unpublished). From this new model we propose the concept of secondary BDV noted L'(T, pH, a(w)). We show that, for calculations of heat processes, the effective BDV, M, to be considered is the overall function M = LL'. From another model [Mafart, P., Leguerinel, I., 1997. Modelling the heat stress and the recovery of bacterial spores. Int. J. Food Microbiol. 37, 131-135], it can be shown that the secondary BDV is also the corrective factor to be considered to assess the overall decimal reduction ratio q: q = L'n where n is the conventional decimal reduction ratio without taking into account the effect of environmental factors of the recovery medium on the effective heat resistance of injured spores.