Induced Heat Resistance in E.
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
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.
Vijay K. Juneja is in the USDA-ARS
Food Safety Research Unit, Eastern
Regional Research Center, 600 East Mermaid Lane, Wyndmoor, PA 19038; phone
(215) 233-6500, fax (215) 233-6406.
"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.