Validation and analysis of modeled predictions of growth of Bacillus cereus spores in boiled rice

The growth of psychrotrophic Bacillus cereus 404 from spores in boiled rice was examined experimentally at 15, 20, and 30 degrees C. Using the Gompert z function, observed growth was modeled, and these kinetic values were comp ared with kinetic values for the growth of mesophilic vegetative cells as p redicted by the U.S. Department of Agriculture's Pathogen Modeling program, version 5.1. An analysis of variance indicated no statistically significan t difference between observed and predicted values. A graphical comparison of kinetic values demonstrated that modeled predictions were "fail safe" fo r generation time and exponential growth rate at all temperatures. The mode l also was fail safe for lag-phase duration at 20 and 30 degrees C but not at 15 degrees C. Bias factors of 0.55, 0.82, and 1.82 for generation time, lag-phase duration, and exponential growth rate, respectively, indicated th at the model generally was fail safe and hence provided a margin of safety in its growth predictions. Accuracy factors of 1.82, 1.60, and 1.82 for gen eration time, lag-phase duration, and exponential growth rate, respectively , quantitatively demonstrated the degree of difference between predicted an d observed values. Although the Pathogen Modeling program produced reasonab ly accurate predictions of the growth of psychrotrophic B. cereus from spor es in boiled rice, the margin of safety provided by the model may be mure c onservative than desired for some applications. It is recommended that if m icrobial growth modeling is to be applied to any food safety or processing situation, it is best to validate the model before use. Once experimental d ata are gathered, graphical and quantitative methods of analysis can be use ful tools for evaluating specific trends in model prediction and identifyin g important deviations between predicted and observed data.