Temperature abuse allows C. perfringens to grow in cook-chill foods.

Members of genus Clostridium are Gram-positive, spore-forming rods that are anaerobic. These bacteria are ubiquitous in nature and are especially fond of soil. Under the microscope, they appear as long drumsticks with a bulge located at their terminal ends. In their active form, these bacteria secrete powerful exotoxins that are responsible for such diseases as tetanus, botulism and gas gangrene.

C. perfringens is a primary agent of foodborne illness. It often is caused by the inadequate heating or refrigeration of prepared foods. The bacterium is capable of surviving aerobic environments and temperatures as high as 100 C for more than 1 hour because it is able to form highly resistant spores.

Illness often occurs after the ingestion of at least 6 to 7 [log.sub.10] viable cells of the bacteria per gram of food. Consumer lifestyles have lead to the development of cook-chill convenience foods that are minimally processed, require only modest additional preparation and consequently are very susceptible to temperature-abuse conditions prior to adequate reheating.

USDA-ARS scientists assessed the effects and interactions of temperature (12 C to 42 C), sodium chloride content (0% to 3) and sodium pyrophosphate concentration (0% to 0.3%) on the growth of C. perfringens vegetative cells. Cyclic and static temperature abuse of precooked, vacuum-packaged, uncured or refrigerated food products for relatively long periods of time allowed C. perfringens to grow to infective levels of 5 [log.sub.10] cfus per gram.

Cooked beef had to be cooled to 7.2 C in 15 hours or less to keep the bacteria from growing from spores. Additionally, cooked beef had to be heated to an internal temperature of 62.5 C for at least 9.6 min and turkey for 7.8 min to destroy 6 [log.sub.10] cfu per gram of C. perfringens.

Researchers have found that C. perfringens cells surviving heat treatments on beef surfaces become more resistant to reheating temperatures, whereas ozone treatment reduces this resistance in surviving cells. The investigators still have not determined the duration of this effect, which would be important in cook-chill foods which are subsequently temperature-abused by the consumer.

It may be necessary to reevaluate minimal processing standards for cook-chill foods following heat-stress adaptations. Scientists are aware of the increased cause for concern over surviving cells of C. perfringens if food temperatures should allow growth again before the product is consumed. They are not completely sure whether an additional level of safety (higher cooking times and temperatures) must be incorporated to prevent survival of resistant heat-adapted survivors. Also, the exhibited effect has not been proven to last during refrigerated storage. The answer may still be to prevent temperature abuse by consumers when the product leaves the market, we're told.

Further information. John Novak, Microbial Food Safety Research Unit, USDA-ARS Eastern Regional Research Center, 600 E. Mermaid Lane, Wyndmoor, PA 19038; phone: [phone]; fax: [phone]; email: [e-mail].