Models predict the heat inactivation of L. monocytogenes in biofilms.

Under most circumstances in food processing facilities, it is possible to control biofilms by properly cleaning equipment surfaces and chemically sanitizing them. However, in some cases, usually due to poor equipment design, a biofilm may not be accessible to cleaning agents. In this case, the best control strategy may be to inactivate the microbes by using heat.

Pathogens attached to surfaces or imbedded in biofilms may have greater heat resistance then their planktonic counterparts, so heat-inactivating them cannot be modeled using data from cell suspensions. Kinetic models that require CFU data are not readily applicable to biofilm cells because it is difficult to quantitatively remove the cells from a surface. In addition, detached cells are clumped. This may lead to an underestimation of the number of cells and a tailing of the heat inactivation curve.

To avoid these difficulties, scientists at the University of Georgia developed models for the heat inactivation of L. monocytogenes in biofilms using fraction negative data. These data were obtained by completely immersing biofilm-containing coupons in hot water and then testing the treated coupons for the presence or absence of L. monocytogenes. This was done by incubating the coupon in enrichment broth: trypticase soy broth with yeast extract.

The models make it possible to predict L. monocytogenes inactivation in biofilms formed on stainless steel or rubber, in mixed culture with Pseudomonas sp. and with a coating of chicken fat or protein emulsion. The models can be used to adjust heating time and temperature conditions to a desired level in order to reduce the risk of L. monocytogenes survival. For example, a heat treatment at 80 C for 16.2 minutes is required to achieve a 90% probable inactivation of L. monocytogenes on stainless steel in the presence of Pseudomonas biofilm and poultry soil.

Further information. Joseph Frank, Department of Food Science and Technology, University of Georgia, 211 Food Science Building, Athens, GA 30602; phone: [phone]; fax: [phone]; email: [e-mail].