Combined effect of chilling and desiccation on survival of Escherichia coli suggests a transient loss of culturability
Mellefont, LA and Kocharunchitt, C and Ross, T, Combined effect of chilling and desiccation on survival of Escherichia coli suggests a transient loss of culturability, International Journal of Food Microbiology, 208 pp. 1-10. ISSN 0168-1605 (2015) [Refereed Article]
Dry air carcass chilling regimes used in some Australian meat works, which not only rapidly reduce the temperature of the carcasses but also dry the meat surface initially, are reported to cause reductions in the number of Escherichia coli present on carcasses after processing. This study used a laboratory broth model system to systematically investigate the basis of such reductions by simulating chilling and desiccation profiles observed on carcasses separately and, finally, in combination. Observed growth was compared to the predictions generated by a strain-specific modification of a validated E. coli growth model (Mellefont et al., 2003; Performance evaluation of a model describing the effects of temperature, water activity, pH and lactic acid concentration on the growth of E. coli). Good agreement between observed and predicted growth was evident when chilling or desiccation profiles were simulated individually. However, when chilling and desiccation profiles were applied simultaneously the observed population kinetics deviated from those predicted by the model. An initial reduction in cell numbers, not predicted by the model, was observed followed by an anomalously rapid increase in population density before growth resumed at a rate expected for the conditions imposed. From our analysis of the kinetics of the population changes, we suggest that the initial decrease in cell numbers was unlikely due to cell death, because conditions were growth permissive. Considering all possible explanations from the observed population kinetics, we propose that a temporary loss of the ability to produce colonies on agar plates may occur. These results may explain reports of increases in E. coli numbers two to three days after commencement of chilling, compared to those observed after 16–24 h, despite the imposition of growth-preventing temperatures.