Mechanisms of heat inactivation in Salmonella serotype Typhimurium as affected by low water activity at different temperatures

Aims:  To determine the effect of reduced water activity ( a w ) on thermal inactivation of Salmonella serotype Typhimurium at different temperatures and its mechanism. Methods and Results:  D ‐value determinations at a range of different temperatures showed that heating at reduced a w (0·94, produced by addition of glucose or sodium chloride to nutrient broth) was protective at temperatures above 53–55°C but sensitizing below this temperature. Using selective enumeration media to determine injury, it was shown that at lower heating temperatures cells survived at high a w with cytoplasmic injury whereas at low a w these cells were killed. At higher temperatures ribosome degradation was a more important cause of death and was inhibited by low a w heating media thereby providing greater heat resistance. Conclusions:  The observed change in behaviour reflects the different reactions responsible for thermal death at different temperatures and their different response to reduced a w . Significance and Impact of the Study:  This work qualifies the previous assumption that reduced a w is protective and suggests that the efficacy of low temperature pasteurization regimes may be increased by reduced a w .