Effect of Water Activity on Heat Survival of Staphylococcus aureus, Salmonella typhimurium and Salm. senftenberg

The viability of Staphylococcus aureus heated at 55° in phosphate buffer was determined on recovery media of different water activity ( a_w ) levels. The basal recovery medium, tryptone–soya agar ( a_w 0.997) was adjusted to lower a_w levels by the addition of NaCl, glycerol or sucrose. Maximum survival occurred at a_w 0.997. Viability was reduced to 1/10 of the maximum at a_w 0.98 when a_w , was controlled by sucrose or NaCl but not until a_w 0.93 with glycerol. To eliminate effects such as incomplete mixing or post-heating dilution and in order to use conditions comparable to those occurring in foods, a solid medium heating/recovery method was also used. This involved heating, by immersion, of surface-inoculated agar plates and recovery of survivors in situ . Heat resistance studies could thus only be carried out at a_w levels permitting growth on the heating/recovery medium. Staphylococcus aureus, Salmonella typhimurium and Salm. senftenberg 775W were heated at 55° and recovered in situ on tryptone-soya agar adjusted to lower a_w levels as above. Maximum survival of Staph. aureus occurred at a higher a_w with glycerol ( a_w 0.965) than with NaCl (0.92) or sucrose (0.90). The maximum survival of both Salm. typhimurium and Staph. aureus heated at 55° occurred at the same a_w (0.965) with glycerol. This maximum was not affected by the duration of heating. As a contrast, heat resistance of Salm. senftenberg 775W was virtually unaffected by reduction in the a_w of the heating/recovery medium.