Model takes into account bacterial heat resistance.

Meat, poultry and pork processors want to avoid contamination from L. monocytogenes. But the more the pathogenic bacterium is sanitized, the stronger and more resistant to sanitation efforts it becomes. To counter this, a team of researchers at Iowa State University developed a model that helps processors take into account the heat resistance of L. monocytogenes.

Processors of cooked and ready-to-eat meats generally heat-treat their products to kill bacteria. But some of the bacteria can tolerate high levels of heat, which is contrary to research and models that indicate they shouldn't be able to survive under those conditions. Many predictive models focus on bacteria that grow in the lab, but the bacteria that survive in the processing plant environment are more hardy.

When bacteria are starving for nutrients, they produce stress proteins that make them stronger. They are not strong enough to multiply, but they are not going to be eliminated. Investigators found that when conditions in the lab are created to starve organisms, they become more resistant to high-temperature conditions.

There are practical implications for processors. Organisms that survive cleaning and sanitation efforts may remain without nutrients on food contact surfaces and develop heat resistance. The model designed by the Iowa State scientists considers the heat-resistant characteristics of L. monocytogenes when the bacteria are heat-treated. In some applications, the level of heat should be increased to lessen pathogen resistance.

Also, the use of salt and sodium pyrophosphate in meat can help L. monocytogenes to survive. Products with high salt levels may need more heat to kill the bacteria because there is less water available in a product. Heat treatments work best when more moisture is available. An increase in salt content causes a decrease in the amount of free water in the product, so more heat is needed to make up for that deficiency to kill the pathogens.

Processors may still be able to use predictive models developed under less stringent laboratory conditions and avoid any resurgence of bacteria when a product is heated. But this would occur only if the number of pathogens is consistently low. If the bacterial count remains high, processors are going to run into a problem.

Further information. Aubrey Mendonca, Iowa State University, Department of Food Science, 2312 Food Sciences Building, Ames, IA 50011; phone: [phone]; fax [phone]; email: [e-mail] <mailto:[e-mail]>.