The effect of water activity on thermal resistance of Salmonella in chocolate products with different fat contents

Several instances of Salmonella outbreaks have been documented in association with chocolates. Conching, an important chocolate manufacturing step, may be designed for control of Salmonella if proper product temperature is maintained for sufficient time. A better understanding of inactivation kinetics of Salmonella in different chocolate products is, however, highly desirable in evaluating the effectiveness of an existing conching process for Salmonella reduction or assessing the need for modification of manufacturing operations. Our previous study on milk chocolate (41.8% fat content) noted that aw and temperature are two key factors to Salmonella inactivation in a thermal process. In this research we studied the effect of different fat levels on Salmonella inactivation in chocolates at 80 °C. White and dark chocolates (fat content: 53.0% and 74.3%) were used in this study. The samples were conditioned to three water activity (aw) levels, 0.23, 0.33, and 0.43, at room temperature. The changes of aw in these samples were quantified when heated to 70 °C and 80 °C to reflect the real humidity conditions during the process. The data indicated an increase in the aw in the chocolate products, correlating with elevated temperatures. The thermal inactivation tests showed that D-values (time to achieve one log reduction) of Salmonella were between 33.9 and 46.5 min when treated at 80 °C, the corresponding aw, 80 °C were 0.29 and 0.28 in white and dark chocolate samples, respectively. Our data suggests that the higher fat content contributed to a less increase in aw, leading to a higher heat tolerance of bacteria in the chocolates. The research results may offer useful insights for the chocolate manufactures to utilize conching process for the pathogen control of chocolate products with different recipes.