Induced Heat Resistance in E. Coli
Escherichia coli O157:H7 bacteria that get only a sublethal dose of heat can become more heat resistant than bacteria that are not so exposed, report Agricultural Research Service scientists in Wyndmoor, Pennsylvania.
The microbiologists at the ARS Eastern Regional Research Center (ERRC) say the finding reiterates the continuing need to adequately cook food to kill E. coli O157:H7 and other food-poisoning microorganisms, or pathogens. Cooking remains the primary means to kill these organisms in foods.
"Our increasing understanding of the wide range of factors that can affect pathogens' thermal resistance indicates the need for a standard way to measure that resistance," says Vijay K. Juneja, who conducted the study in ERRC's Food Safety Research Unit.
Juneja and colleagues subjected beef gravy samples containing E. coli O157:H7 to 114.8oF for 15 to 30 minutes, heat-shocking the bacteria at a temperature not quite sufficient to kill them. Then they cooked the gravy to a final internal temperature of 140oF.
The results: The pre-heated E. coli survived longer (a 1.5-fold increase in heat resistance) than other E. coli not subjected to the sublethal heat. The increased thermotolerance lasted for at least 48 hours.
Therefore, says Juneja, food processors should realize that bacteria will not be killed in foods that are heated slowly to the final cooking temperatures normally used. Heat-shocking conditions may occur in minimally processed, refrigerated, cook-in-bag foods such as filled pasta products (ravioli, tortellini, canneloni, etc.), moussaka, lasagna, and chili con carne. The slow heating rate and low heating temperatures used in preparing these foods expose potential pathogens to conditions similar to heat shock--which could make them more heat-resistant.
This induced heat-resistance could also be a concern in meat products kept on warming trays before final heating or reheating, or when equipment failure interrupts the cooking cycle during processing.
Juneja says that traditional research methods to determine if heat kills pathogens are cumbersome because of lengthy sample preparation times and nonuniform heating. He and colleagues used a submerged stainless-steel coil-heating apparatus that allows quick temperature control by a thermostat, eliminating the customary problems.--By Doris Stanley, Agricultural Research Service Information Staff.
"Induced Heat Resistance in E. Coli" was published in the July 1998 issue of Agricultural Research magazine. Click here to see this issue's table of contents.