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Bacterial contamination of fresh meats can occur during normal slaughter and handling procedures, although this contamination can be minimized by adhering to good hygienic practices during slaughter. Since the bacteria are confined almost exclusively to the carcass surface as compared to the deep muscle tissue, procedures which can control the survival and growth of bacteria on tissue surfaces are of interest to both the meat industry and regulatory agencies. Chilling, either by forced air or water spray systems, is used universally to reduce the growth of bacteria on animal carcasses. However, because of the initial heat in an animal carcass, it takes several hours for the temperature to fall low enough to prevent bacterial growth. During the carcass cooling process, the contaminating bacteria can grow, resulting in a bacteria population many times greater than that of the initial contamination.
Bacterial growth progresses through several distinct phases. The first phase, called the lag phase, occurs as the bacteria adjust to a new environment. Although the bacteria are metabolically active during this phase, there is no net increase in numbers of bacteria. The second phase of growth is the logarithmic growth phase, where there is a rapid increase in the bacterial population. Eventually, the bacterial population exhausts most of the available nutrients and reaches a stable population, called the stationary phase. When graphed, the growth of bacteria resembles an "S" curve. The time required for a bacterial population to move from a static population (lag phase) to an actively growing state (logarithmic growth) is defined as the lag time. The time required for the bacterial population to double during the logarithmic growth phase is referred to as the generation time.
Bacteria generally have shorter lag and generation times as temperature increases, with the optimum temperature for bacteria of public health significance being very close to that of the body temperature of a cow (approximately 100-104°F). Since temperature has a significant effect on bacterial growth rate, the temperature history of food products has been used to estimate the potential bacterial population on a given product or, in practice, to predict relative rates of microbial growth for different cooling processes, with this process currently being referred to as temperature function integration. Although much of the previous research has focused on spoilage, this area also has applications for foodborne bacteria of public health significance.
The temperature function integration technique has been used for assessing beef carcass cooling processes. Researchers have used the temperature history of beef carcasses to predict the growth of E. coli during cooling, based on the growth of the bacterium in liquid cultures. Our intent was to construct a predictive model for the growth of salmonellae using intact beef tissue, with the specific purpose of evaluating beef carcass cooling procedures.