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Models may not always offer the right predictions.

Cooking pork sausage makes us question how much heat is necessary to ensure product safety and maintain sensory quality. Food processors must consider such factors as the cost and the effectiveness of heat treatments in killing pathogenic bacteria as well as their effects on taste. Processors need to run their own tests to find the answers to these questions because models won't necessarily tell the true story about microbial destruction in meat.

Scientists at Iowa State University's Food Safety Consortium unit (2312 Food Sciences Building, Ames, IA 50011) examined the effects of phosphates on pork processing and found that one uniform treatment does not fit all scenarios. A number of research groups have developed models that are designed to predict the extent of heat destruction of certain foodborne pathogens as affected by environmental factors, such as phosphates. Phosphates increase pork's water-holding capacity and improve product safety by making L. monocytogenes less resistant to heat and more susceptible to destruction.

Although USDA scientists and others have developed predictive models, processors should validate predictions from models by running tests with their own meat products. The model system can never provide the exact conditions of a meat product, no matter how hard we try, investigators indicate. There are some factors in meat that will not be present in a model system. Validation involves comparing results from tests in actual meat products with predictions from models.

Predictive models, when validated, can be useful in indicating when a food product might spoil or under what conditions foodborne pathogens might pose a health risk to consumers. Processors should know what cooking temperature is best for killing pathogens without negatively affecting palatability, flavor and texture of the meat product. By using salt, phosphates, lactate and certain other ingredients, a processor can put enough stress on the organisms so that less heat is required to kill them. Lower heat means lower production costs.

You can estimate the behavior of foodborne microorganisms by evaluating their growth or destruction in broth cultures or other model food systems. However, exceptions occur when an additional significant factor is present in the food, but not in the model system. For example, a relatively high fat content or phosphate level in ground pork could make a model unreliable for predicting the microbial safety or shelf life of pork sausage if the model lacks the same levels of fat or phosphate.

Data on pathogen destruction from the use of model systems should just serve as a general guideline for processors to follow, according to Iowa researchers. They should always validate their results. Investigators examined the effect of a phosphate, sodium acid pyrophosphate (SPP), on the heat destruction of L. monocytogenes in a model system involving pork slurry, which is a mixture of pork with peptone water. They also ran the same test using freshly ground pork.

The Iowa research team inoculated the pork samples with L. monocytogenes and heated them at temperatures ranging from 55 C to 62.5 C. In the presence of SPP, the heat resistance of L. monocytogenes decreased greatly in the model system that used pork slurry. But researchers could not repeat these results in the fresh pork. They realized that fresh meat has some enzymes that degrade into orthophosphates, which have little or no antimicrobial activity. They demonstrated that the degradation of SPP by natural phosphates in freshly ground pork contributes to its ineffectiveness against L. monocytogenes.

Further information. Aubrey Mendonca; phone: 515-294-2950; fax: 515-294-8181; email:
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Publication:Microbial Update International
Date:Oct 1, 2001
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