HEAT-SHOCK, ANAEROBIC JAR INCUBATION AND FLUID THIOGLYCOLATE MEDIUM HAVE CONTRASTING EFFECTS ON D-VALUES OF ESCHERICHIA-COLI

This study investigated the effects of heat shock and post-heat environmental conditions on the survival of Escherichia coli ATCC 25922 at 52, 54 and 56 degrees C. Significantly (P <0.05) higher decimal reduction time-values (D-values) in nutrient broth at all three temperatures were obtained when the cells had been previously heat shocked at 46 degrees C for 1 h. This heat shock effect was greatest at 56 degrees C, followed by 54 and 52 degrees C. Anaerobic jar incubation of heat-stressed cells led to D-values higher than those obtained using aerobic incubation, but the difference was only statistically significant at 56 degrees C. Previous studies showed that fluid thioglycollate medium and thioglycollic acid enhanced the recovery of heat-stressed Salmonella enteritidis and E. coli, respectively. However, a fluid thioglycollate medium overlay of spread plates significantly decreased the D-values of E. coli ATCC 25922 at 52, 54 and 56 degrees C. When the fluid thioglycollate medium was used to overlay pour plates, no significant changes in D-values were seen.