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Microbial contamination of animal carcasses is a result of the necessary procedures required to process live animals into retail meat. The contamination can be minimized by good manufacturing processes, but the total elimination of bacteria of public health significance is difficult, if not impossible. A variety of methods have been developed to improve the microbiological quality of meat, although most of the current methods focus on washing and sanitizing the carcasses, prior to chilling.
The beef slaughter process begins by humanely stunning the animal, bleeding, and then removing the hooves and head. The hide is removed, and the carcass is eviscerated and split into halves. The carcass halves are washed and then cooled to refrigeration temperatures. The initial research on carcass washing was with washing the split carcass which, as the final step before chilling, was intended to remove as much of the total physical and microbiological contamination as possible. Manual washing was refined with equipment that automatically washed the carcasses. The automated systems were more consistent in operation than a manual system, and also reduced the amount of water used in washing. A further refinement of the automated systems was the inclusion of a sanitizing rinse immediately after washing. The sanitizing rinse uses food grade antibacterial compounds to inhibit the growth of any bacteria remaining after the initial wash. The sanitizers typically are organic acids, such as acetic (vinegar) or lactic acid (naturally occurring in cheese).
The automated washing and sanitizing systems were successful in improving the microbiological quality of beef carcasses. However, since much of the contamination of the surface of the carcass occurs during the hide removal, a second washing station was inserted immediately after hide removal and prior to evisceration (termed "pre-evisceration" washing). The process of pre-evisceration has been patented by a major U.S. meat packer.
The traditional method of cooling carcasses was by forced air refrigeration. In the 1970's, a new cooling process was developed which misted cold water on the carcasses in conjunction with refrigeration. This new process increased the cooling rate by evaporative cooling, and reduced the weight loss of the carcass which normally occurred during traditional chilling. The process used chlorinated water to inhibit bacterial growth, and was patented as "chlor-chil." Since that time, other sanitizers have been incorporated into the spray water on an experimental basis.
Although there were some data in the scientific literature on each of these processes individually, we wanted to evaluate the entire system under controlled conditions. Therefore, our objectives were to determine the effectiveness of pre-evisceration and post-evisceration washing and sanitizing, followed by spray chilling, in reducing the population of salmonellae on beef. This research was conducted in the laboratory, as a feasibility study to establish processing guidelines for full-scale equipment currently being installed in the abattoir at the U.S. Meat Animal Research Center.