Abstract
A notable exception to the enhanced antimicrobial activity generally seen with combination treatments is the combined effect of reduced water activity (aw) and heat inactivation where it is generally held that heat resistance or survival of several bacteria including Salmonella increases as the aw is reduced. However, recent reports suggest that reduced aw might increase or decrease heat resistance. Results of the present study showed that heating Salmonella Typhimurium at reduced aw (0.94) was protective at temperatures above 53-55°C but sensitising below this temperature. Using selective enumeration media to determine injury, it was shown that at lower heating temperatures (e.g. 50°C), cells survived at high aw with cytoplasmic membrane injury whereas at low aw these cells were killed. At higher temperatures (e.g. 60°C) ribosome degradation was a more important cause of death and was inhibited by low aw heating media thereby providing greater heat resistance. The time take for repair from injury varied with the different heating conditions and differed in the response to biosynthetic inhibitors such as azide, chloramphenicol and rifampicin. A lower than optimal growth temperature decreased heat resistance, while a high growth temperature increased it at both aw values (0.99 and 0.94). The effect of growth temperature was generally more pronounced at high aw. Habituation of Salmonella cells at low aw had no effect or decreased their heat resistance depending on the length of habituation and the heating condition. D50 reductions were always larger when heating at high aw compared to low aw conditions, whereas the reverse occurred in D60. The observed effect of low aw on Salmonella heat resistance cannot be generalised to all other bacteria. Other members of Enterobacteriaceae tested responded similarly, but Pseudomonas aeruginosa and the Gram positives Staphylococcus aureus and Lactobacillus plantarum all showed greater heat resistance at low aw regardless of temperature. Interestingly, Enterococcus faecalis, an enteric Gram positive, behaved similarly to the Enterobacteriaceae. Listeria monocytogenes, however, showed the opposite behaviour at both temperatures (50°C and 60°C).