Effect of challenge temperature and solute type on heat tolerance of Salmonella serovars at low water activity

Salmonella spp. are reported to have an increased heat tolerance at low wat er activity (a(w)) measured by relative vapor pressure [rvp]), achieved eit her by drying or by incorporating solutes. Much of the published data, howe ver, cover only a narrow treatment range and have been analyzed by assuming first-order death kinetics. In this study, the death of Salmonella enteric a serovar Typhimurium DT104 when exposed to 54 combinations of temperature (55 to 80 degreesC and a(w) (rvp 0.65 to 0.90, reduced using glucose-fructo se) was investigated. The Weibull model (LogS = -bt(n)) was used to describ e microbial inactivation, and surface response models were developed to pre dict death rates for serovar Typhimurium at all points within the design su rface. The models were evaluated with data generated by using six different Salmonella strains in place of serovar Typhimurium DT104 strain 30, two di fferent solutes in place of glucose-fructose to reduce a(w), or six low-a(w ) foods artificially contaminated with Salmonella in place of the sugar bro ths. The data demonstrate that, at temperatures of greater than or equal to 70 degreesC, Salmonella cells at low a(w) were more heat tolerant than tho se at a higher aw but below 65 degreesC the reverse was true. The same patt erns were generated when sucrose (rvp 0.80 compared with 0.90) or NaCl (0.7 5 compared with 0.90) was used to reduce a(w), but the extent of the protec tion afforded varied with solute type. The predictions of thermal death rat es in the low-a(w) foods were usually fail-safe, but the few exceptions hig hlight the importance of validating models with specific foods that may hav e additional factors affecting survival.