Modelling and predicting growth and growth boundary of Bacillus cereus s.l. from phylogroups II, IV, V and VI in starchy foods at or below 12 °C

Veronica Martinez-Rios Resadije Idrizi Paw Dalgaard Lisbeth Truelstrup Hansen Tina Beck Hansen ^*

• Technical University of Denmark, Kongens Lyngby, Denmark

Pathogenic Bacillus cereus s.l. can survive cooking of starchy foods and grow at chilled storage temperatures, highlighting foods with extended chilled shelf life as a risk factor. Some food administrations encourage use of predictive microbiology to establish support decisions of safe shelf lives of such foods. Therefore, the present study embarked on identifying a model from literature and/or expanding an existing model to enable accurate predictions of growth and no-growth responses of relevant B. cereus s.l. in starchy ready-to-eat and ready-to-cook foods when stored at temperatures at or below 12 °C. The study focused on isolates belonging to psychrotolerant or mesophilic-psychrotolerant intermediary thermotypes in panC-groups II, IV, V, or VI and generated data for growth kinetics for various pH (4.8-7.8), aw (0.935-0.999) and storage temperatures (6.0-11.7 °C) in 42 starchy foods (bulgur, couscous, pasta, potatoes, rice) and eight composite foods containing at least one starchy ingredient. Using 21 of the growth kinetics obtained for starchy foods, the five best performing of ten available growth models were selected for improvement by product calibration and/or expansion with terms to consider the effect of interactions between temperature, pH and aw. Of 410 updated models, nine showed promising performance and were evaluated using the remaining 21 growth kinetics obtained in starchy foods. Two models could be considered validated for these products with Bf /Af-values of 0.87/1.21 and 1.01/1.32, respectively. Both models provided ≥75% correct predictions of the growth/no-growth responses and did not provide any fail-dangerous predictions. These models were evaluated for their ability to predict maximum specific growth rates (µmax, h -1 ) and growth/no-growth responses for a broader range of starchy foods, partly by using 33 challenge tests from studies available in the scientific literature, partly by using eight composite starchy foods from the present study. With Bf -values ≤0.64 and Af-values ≥1.96, performance of both models was poor for composite protein-rich starchy foods and should not be used as µmax might be under-predicted creating unsafe situations. For other starchy foods, one of the validated models was found to be acceptable on the safe side with Bf-and Af-values of 1.34 and 1.57, respectively.