Modeling Survival of Salmonella Enteritidis during Storage of Yoghurt at Different Temperatures

Introduction: Yoghurt has an important role in the human diet due to its nutritional value and positive effect on health. This product has been recently included in a vulnerability assessments of food systems developed by Food and Drug Administration (FDA, 2012), suggesting that yoghurt could be a potential target for bioterrorist attack.

Purpose: To investigate the behavior of Salmonella Enteritidis in yoghurt at 4, 12, 20, and 25°C and develop predictive microbiology models for vulnerability assessment purposes.

Methods: Survival data were obtained at different temperatures by plate count method and used to fit survival models (Geeraerd model, Weibull model, the modified Weibull model, the trilinear model, the bilinear model) by using the package of nlsMicrobio in R software. To evaluate the effect of storage temperature on kinetic parameters such as inactivation rate (k_max) and shoulder (S_l), secondary models were developed by using two empirical models.

Results: According to the survival curves and smaller goodness of fit indices (RMSE, ACC_C), Geeraerd model with shoulder and tailing was selected as the most appropriate model to describe the survival of Salmonella in yoghurt during storage at different temperatures. At 4°C, Salmonella displayed the lowest inactivation rate (0.05 h^-1), whereas at 25°C, the maximum temperature assayed, it showed the highest inactivation rate (0.32 h^-1). S_l was the longest in samples stored at 4°C (55.93 h), whereas in samples stored at 25°C it was the shortest (4.28 h). In addition, the tested empirical models were able to accurately predict Salmonella survival as a function of temperature.

Significance: Results suggest that contamination by Salmonella in yoghurt could pose a significant risk to consumers. The predictive models herein developed could be applied to better support quantitative vulnerability and risk assessment studies, providing more accurate estimates.