Predictive model for growth of Clostridium perfringens during cooling of cooked uncured meat and poultry

Comparison of Clostridium perfringens spore germination and outgrowth in cooked uncured products during cooling for different meat species is presented. Cooked, uncured product was inoculated with C. perfringens spores and vacuum packaged. For the isothermal experiments, all samples were incubated in a water bath stabilized at selected temperatures between 10 and 51°C and sampled periodically. For dynamic experiments, the samples were cooled from 54.4 to 27°C and subsequently from 27 to 4°C for different time periods, designated as x and y hours, respectively. The growth models used were based on a model developed by Baranyi and Roberts (1994. A dynamic approach to predicting bacterial growth in food. Int. J. Food Micro. 23, 277-294), which incorporates a constant, referred to as the physiological state constant, q(0). The value of this constant captures the cells' history before the cooling begins. To estimate specific growth rates, data from isothermal experiments were used, from which a secondary model was developed, based on a form of Ratkowsky's 4-parameter equation. The estimated growth kinetics associated with pork and chicken were similar, but growth appeared to be slightly greater in beef; for beef, the maximum specific growth rates estimated from the Ratkowsky curve was about 2.7 log(10) cfu/h, while for the other two species, chicken and pork, the estimate was about 2.2 log(10) cfu/h. Physiological state constants were estimated by minimizing the mean square error of predictions of the log(10) of the relative increase versus the corresponding observed quantities for the dynamic experiments: for beef the estimate was 0.007, while those for pork and chicken the estimates were about 0.014 and 0.011, respectively. For a hypothetical 1.5h cooling from 54°C to 27° and 5h to 4°C, corresponding to USDA-FSIS cooling compliance guidelines, the predicted growth (log(10) of the relative increase) for each species was: 1.29 for beef; 1.07 for chicken and 0.95 log(10) for pork. However, it was noticed that for pork in particular, the model using the derived q(0) had a tendency to over-predict relative growth when the observed amount of relative growth was small, and under-predict the relative growth when the observed amount of relative growth was large. To provide more fail-safe estimate, rather than using the derived value of q(0), a value of 0.04 is recommended for pork.