Understanding and modeling Clostridium perfringens growth during cooling

Proper temperature control is essential in halting Clostridium perfringens germination, and growth. To prevent foodborne outbreaks of C. perfringens, the United States Department of Agriculture (USDA) Food Safety and Inspection Service (FSIS) offers two options for the cooling of meat products: follow a standard time-temperature schedule or validate that alternative cooling regimes result in no more than a 1-log colony forming unit (CFU) increase of C. perfringens. FSIS also allows the use of mathematical models to determine if temperature deviations during cooling have increased the risk of foodborne illness. A model has been developed by Juneja et al. (1999) but has not yet been extensively validated. Preliminary experiments in our lab also showed apparent growth at 4.4°C after cooling under select conditions which has important regulatory implications.; The objectives of this project are: (1) to validate the current mathematical model under a variety of changing temperature and temperature abuse situations; (2) to develop a new predictive model if necessary and (3) understand why C. perfringens appears to grow at 4.4°C after cooling under select conditions.; The Juneja model consistently under-predicted C. perfringens growth during cooling in ground beef and nutrient broth. These under-predictions would lead processors to conclude that a cooling deviation would allow less than a 1-LOG10 CFU/g increase, where our experiments showed growth in excess of the performance standard. The model error is due to faster exponential growth rates (EGR) in ground beef under non-isothermal conditions when compared to growth rates in broth during isothermal growth experiments. The shortest GOL time at each temperature and the maximum EGR at each effective temperature were used to create new mathematical models. The improved models accurately predicted the growth of C. perfringens during cooling of ground beef.; The "apparent growth" observed at 4.4°C following a normal cooling process appears to be due to cell elongation during low temperature stress, followed by subsequent division of those elongated cells. This finding is of great practical significance since apparent growth might cause otherwise acceptable cooling regimes to violate the USDA-FSIS performance standard for cooling.