Seventy-one Celsius is the recommended temperature for cooking hamburgers to kill any bacteria they might contain.
Now research indicates 71 C is not enough to kill some forms of E. coli, including some that are dangerous to humans.
It opens the door to new worries about food safety.
Lynn McMullen, a food biologist and professor in the University of Alberta’s agriculture, food and nutritional science department, said she and her research colleagues estimate about two percent of all E. coli strains, both pathogenic and non-pathogenic, are resistant to heat. Studies are ongoing.
“That’s one of the pieces of information that we haven’t got, is what does it take to kill,” McMullen said.
“What we do know is, when we found the organisms … it was in a generic, non-pathogenic E. coli, but we have since found the same genetics in pathogenic strains.
“It is present in pathogenic strains of E. coli. Nobody had linked that to the heat resistance of these organisms.”
The same strains are also resistant to pressure. McMullen said exposure to 600 megapascals for 15 minutes was not enough to kill them.
McMullen and food microbiologist Michael Ganzle have been exploring the hardy E. coli strains since 2008.
Their paper published in the Sept. 9, 2015, Frontiers in Microbiology journal referenced a study in which “highly heat resistant E. coli are recovered in high numbers from inoculated beef patties that are cooked medium rare and even survive in burger patties that are cooked well done, corresponding to an internal temperature of 71 C.”
McMullen said she has been working with Dr. Linda Chui of the U of A’s medical immunology department, who has found that some E. coli strains responsible for human illness have the same genetics as the heat resistant types already identified.
“We know it is a problem. We know it’s out there. We know from looking at genome databases it’s in approximately two percent of all E. coli.”
“What we don’t know is what it takes to kill it.”
She said experiments continue, but in the meantime she recommends continued use of the 71 C rule and the use of a meat thermometer to ensure that internal temperature is reached in meat.
“There’s a real resistance, but it really is the only way to safely cook grilled meats, is to know what temperature. I know that 71 is not probably sufficient.”
McMullen and her colleagues are also trying to determine the prevalence of heat-resistant E. coli in the food system by working with Health Canada in Ottawa and Agriculture Canada in Lacombe, Alta.
Ron Davidson, a director with the Canadian Meat Council, said the research does raise questions.
“We do try to produce meat that’s free of E. coli and there is a lot of testing done on it, but up until now, we’ve always been able to say that the heat will kill E. coli,” he said.
Davidson said the meat industry has procedures that result in the lowest possible presence of bacteria on meat products, and the 71 C cooking temperature recommendation “is an effective and approved method of inactivating any residual bacteria.”
Health Canada has announced plans to recommend irradiation as an option for ground beef, adding it to the list of other foods for which irradiation is already approved.
The department said it plans to post the recommendation to Canada Gazette sometime this month, opening it to a public comment period.
McMullen said she doesn’t know if irradiation will kill the heat and pressure-resistant E. coli.
“I suspect irradiation would destroy the DNA because it is a different mode of action, but we haven’t tested it, so I can’t say for sure.”
Davidson said the meat industry has supported meat irradiation for years as an additional food safety option.
“We’re very much supportive of this and we recognize that it’s an option and that it would be labelled, so people would have the choice,” he said.