The low thermal tolerance of Salmonella enterica in foods with intermediate moisture levels, such as caramel sauces, ensures that mild heat treatment is sufficient to achieve 5-log reductions of this pathogen. This treatment mitigates the risk posed by salmonellae in raw materials; however, recontamination might occur because of survival of the pathogen in products that are not heated before consumption. This study was conducted to evaluate the effect of water activity (aw) on the thermal tolerance and survival of S. enterica serovars Tennessee and Senftenberg. The D-values at 76, 78, and 80°C, z-values, and survival at 20.0 ± 0.5°C for 32 weeks of these two serovars were determined in goat's milk caramel at three aw values (0.85, 0.90, and 0.93). The highest thermal tolerance was observed at aw = 0.85 for Salmonella Senftenberg (D76°C = 2.9 ± 0.3 min), and the lowest was at aw = 0.93 for Salmonella Tennessee (D80°C = 0.131 ± 0.007 min). After a logarithmic transformation of the z-values, a significant interaction between serovar and aw was found (P < 0.0001), but no consistent trends were observed at the three evaluated aw levels for either serovar. Survival response was modeled using two sigmoidal three-parameter models. A significant interaction was found between nominal variables aw and serovar when comparing inflection points of the resulting curves: P < 0.0016 for the logistic model (R2 = 0.91) and P < 0.0014 for the Gompertz model (R2 = 0.92). Although a >8-log reduction was observed at week 20 of storage, regardless of the product's aw and the serovar, low levels of salmonellae were found in the product up to week 32 of storage. Our findings may assist the food industry with the establishment of critical limits for the safe thermal treatment of milk- and sugar-based foods with intermediate moisture levels. The survival data presented here highlight the relevance of implementing and effectively maintaining good sanitation and hygiene practices during the production of goat's milk caramel and similar food products.
Keywords: Milk caramel; Salmonella enterica; Survival; Thermal tolerance.