The influence of temperature and water activity on thermal resistance of Salmonella in milk chocolate

Salmonella contamination of chocolate-derived products has caused several outbreaks and recalls in recent years. Earlier research found that reducing moisture content or water activity of low-moisture foods sharply enhances the resistance of Salmonella during thermal treatments. However, there is a lack of data that correlates the relationship between temperature, water activity (a(w)), and thermal resistance of Salmonella in milk chocolate. In this study, milk chocolate was inoculated with Salmonella Enteritidis PT 30 and conditioned to a(w) of 0.23, 0.33, and 0.43 at room temperature (21 +/- 2 degrees C). The chocolate samples were heated at 70, 75, and 80 degrees C to obtain the D-values (time to inactivate 90% of the test microorganisms at a given temperature) of S. Enteritidis PT 30. The change of a(w) of milk chocolates at elevated temperatures (up to 80 degrees C) was also investigated. The results showed that the D-value of S. Enteritidis PT 30 decreased exponentially with the increase of a(w) or temperature. The maximum D-value was 47.4 +/- 3.7 min obtained at 70 degrees C and a(w) of 0.23. The minimum D-value was 5.2 +/- 1.0 min at 80 degrees C and a(w) of 0.43. The z-values were found as the followings: z(aw), 70 degrees C = 0.42, z(aw), 75 degrees C = 0.36, and z(aw), 80 degrees C = 0.46 degrees C; z(T,) (0.23) = 18.8 +/- 2.5, z(T, 0.3)3 = 20.6 +/- 4.1, z(T, 0.43) = 18.1 +/- 0.5 degrees C. Overall, the results from this research may provide useful information to help the industry control the risk of Salmonella contamination and improve microbial safety in chocolate production.