Consider interaction among hurdles.

Hurdles are frequently combined to minimize the impact of processing on product quality and to improve the safety of ready-to-eat foods. Traditional hurdles used in various combinations include mild heat, acid, salt and modified atmospheres. A synergy generally occurs among these hurdles to enable them to do their job.

New-generation technologies such as high-pressure processing and irradiation can benefit from the hurdle concept. Adding bacteriocins, for example, improves the lethality of high-pressure processing, and makes this technology more feasible and potent for today's applications.

Unlike conventional preservation factors, the new technologies target a limited number of loci in microbial cell. So, carefully integrating these technologies with conventional hurdles is crucial to the success of many combined treatments. Understanding the mechanism of action of these newer preservation methods should help us design effective process combinations.

Recent findings at Ohio State University, for example, show that a pulsed electric field (PEF) decreases the numbers of molecular chaperones--certain types of heat shock proteins--in bacterial cells for a short period of time after they are treated. When heat was applied to PEF-treated cells, a greater lethality was observed by investigators. But applying heat before PEF processing resulted only in limited additive lethal effect.

In another instance, gamma irradiation generates free radicals that break bacterial DNA strands. Combining radiation with antimicrobials that function as free-radical scavengers decreases the efficacy of the radiation treatment.

On another front, mild hurdles affect a single or a limited number of targets in the cells. Microorganisms that survive these treatments are more likely to be stressed than injured or killed. Given the time and suitable conditions for responding to stress, these microorganisms may adapt to the processes intended to inactivate them. For these reasons, hurdles cannot be combined without giving consideration to the hurdle-hurdle interactions that may occur or to the physiology of the treated pathogens.

Further information. Ahmed Yousef, Department of Microbiology, Ohio State University, 376 Biological Sciences Building, 484 W. 12th Ave., Columbus, OH 43210; phone: [phone]; fax: [phone]; email: [e-mail].