Model the heat inactivation of L. monocytogenes in biofilms.

Under most circumstances, it is possible to control the development of biofilms by properly cleaning equipment and then sanitizing the surface of the equipment with chemicals. However, in some cases, usually due to poor equipment design, the biofilm may not be accessible to cleaning agents, and the best control strategy may be to apply a heat inactivation treatment to the equipment surface.

Pathogens attached to surfaces or imbedded in biofilms may have greater heat resistance than planktonic bacteria, so trying to predict how they may be inactivated by applying a heat treatment is difficult if you use data from cell suspensions. Kinetic models that require colony-forming-unit data are not readily applied to biofilm cells because it's difficult to remove these cells from a surface, and because detached cells are clumped. This clumping can cause us to underestimate cell numbers. So alternative predictive models are needed.

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

The models make it possible to predict the inactivation of L. monocytogenes in biofilms formed on stainless steel or rubber, in mixed culture with Pseudomonas spp. and with a coating of chicken fat or protein emulsion. You can use the models to adjust heating time and temperature conditions to reduce the risk of L. monocytogenes survival to a desired level. 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.

The researchers have submitted their models for publication in refereed scientific journals. One of these manuscripts has been accepted for publication in the Journal of Food Protection. The model for inactivation of Listeria biofilm on stainless steel will be available upon publication of this article.

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].