Effect of Water Activity on the Thermal Tolerance and Survival of Salmonella enterica Serovars Tennessee and Senftenberg in Milk Caramel

Introduction: Ensuring the safety of water activity (a_w) controlled products is challenging due to the enhanced heat tolerance and survival mechanisms that S. enterica has shown in low a_w environments. Few studies regarding thermal tolerance and survival of S. enterica in foods with a_w ranging between 0.85 and 0.93 have been reported. 

Purpose: We evaluated the effect of a_w on the D- and z-values, as well as survival of S. enterica serovars Tennessee and Senftenberg in milk caramel. 

Methods: The decimal reduction time (D-value) at 76, 78 and 80ºC, change in temperature necessary to produce a 10-fold change in D-value (z-value), and survival at 20.0 ± 0.5ºC for 32 weeks of two S. enterica serovars were determined in goat milk caramel samples adjusted to three a_w values (0.85, 0.90 and 0.93). Experiments were performed in triplicate. 

Results: A three-way ANOVA with log-transformed D-value as response (r² = 0.9969) showed a significant triple interaction between nominal factors serovar, temperature and a_w (P < 0.0001). Highest tolerance was observed at a_w 0.85 for S. enterica Senftenberg (D_76ºC 2.9 ± 0.3 min), and lowest at a_w 0.93 for the Tennessee serovar (D_80ºC 0.131 ± 0.007 min). A two-way ANOVA with log-transformed z-value as response (r² = 0.9834) showed a significant interaction between nominal factors serovar and a_w (P < 0.0001). Regardless of the tested serovar, no consistent trends were observed for the z-values at the evaluated a_w levels. A logistic decay model was fitted to all survival curves (r² ranged between 0.9857 and 0.9963). Although greater than 8-log reductions were observed at week 30 of storage, low levels of Salmonella were still found in the product at week 32.

Significance: Our findings contribute to the establishment of critical limits for the safe thermal processing of a_w controlled products. We also stress the relevance of maintaining good manufacturing and sanitation practices as a preventative action during processing of a_w controlled products.