Does Cooking Kill E Coli

The Curious Cook

Bending the Rules on Bacteria

Aug. 23, 2011

PEPPERED as we are by government warnings about the potential health hazards of eating and drinking just about everything, it was refreshing (and perplexing) to see a widely respected food writer assert recently that "people are unnecessarily afraid of bacteria" in the kitchen.

In April, Michael Ruhlman, author of "Ratio" and "The Elements of Cooking" and co-author of books by Thomas Keller and other chefs, said on his blog that he likes to make chicken stock and leave it out on the stovetop all week, using portions day to day to make quick soups and sauces.

But what about the harmful microbes that could grow on foods if they were not kept either chilled or hot? "Once your stock is cooked, it's safe to eat," Mr. Ruhlman wrote. "If there were bad bacteria in it, you'd have killed them." After the stock has cooled, simply reheat it, he continued, and "any bacteria that landed there and began to multiply will be dispatched well before the stock hits a simmer."

Sounds plausible, and Mr. Ruhlman and his family are alive and well. But after checking with an independent expert on food safety, I wouldn't follow this recipe without slapping a biohazard label on my stockpot.

The Food and Drug Administration sets regulations for commercial food production. These specify that cooked foods should sit out at temperatures from 41 degrees to 135 degrees, the range in which bacteria can grow and multiply, for no more than four hours.

Guidelines for the consumer and home cook, which come from the Department of Agriculture Food Safety and Inspection Service, are even stricter. The current brochure, "Keep Food Safe! Food Safety Basics," on the U.S.D.A. Web site, says not to leave prepared foods in the bacterial growth zone for longer than two hours. And if it's a 90-degree summer day, cut the two hours to one.

Mr. Ruhlman's stock spends days in the bacterial growth zone, and he happily makes it into chicken soup for his children.

I'll admit to violating the guidelines in my own stock-making, though by a few hours, not days. When I cook a roast for dinner, I use leftover scraps and bones to start the stock, simmer it while I clean up, and take the pot off the heat right before I go to bed. At that point it's too much trouble to cool the hot stock so it won't warm up its neighbors in the refrigerator. Instead, I cover the pot, leave it at room temperature and reheat it in the morning, about eight hours later, before straining, cooling and refrigerating it. And my stock hasn't made me or my family ill, either.

Can I be even more relaxed about my stock-making? Or have Mr. Ruhlman and I just been lucky? For an expert opinion, I sent our recipes to O. Peter Snyder, a food scientist and veteran educator and consultant to the food-service industry, who has at times taken issue with government guidelines he considers unnecessarily conservative.

Dr. Snyder replied in an e-mail: "The process described by Mr. Ruhlman is a very high-risk procedure. It depends totally on reheating the stock before it is used to be sure that it doesn't make anyone ill or possibly kill them."

It's a basic fact that every cook should know: bacteria that cause illness inevitably end up on nearly every ingredient we cook with, and even boiling won't kill all of them.

Boiling does kill any bacteria active at the time, including E. coli and salmonella. But a number of survivalist species of bacteria are able to form inactive seedlike spores. These dormant spores are commonly found in farmland soils, in dust, on animals and field-grown vegetables and grains. And the spores can survive boiling temperatures.

After a food is cooked and its temperature drops below 130 degrees, these spores germinate and begin to grow, multiply and produce toxins. One such spore-forming bacterium is Clostridium botulinum, which can grow in the oxygen-poor depths of a stockpot, and whose neurotoxin causes botulism.

Once they've germinated, bacteria multiply quickly in nourishing stock. They can double their numbers every 90 minutes at room temperature, every 15 minutes at body temperature. A single germinated spore can become 1,000 bacteria in a matter of hours, a billion in a few days.

As Dr. Snyder put it, "After sitting on the stove and growing bacteria for two or three days, Mr. Ruhlman's stock almost certainly has high levels of infectious Clostridium perfringens cells, or Clostridium botulinum or Bacillus cereus cells and their toxins, or some combination thereof."

Why has the Ruhlman family survived? Because Mr. Ruhlman boils the stock before he serves it, Dr. Snyder wrote. Any active bacteria are killed by holding the stock for a minute at 150 degrees or above, and botulism toxin is inactivated by 10 minutes at the boil.

But quickly reheating a contaminated stock just up to serving temperature won't destroy its active bacteria and toxins, and the stock will make people sick.

"If Mr. Ruhlman ever has a cup of his three-day-old stock without thoroughly boiling it first, he will probably only do it once," Dr. Snyder wrote. "It is irresponsible of any cook to prepare food in a way that actually creates a new and significant hazard, even though the hazard may be eliminated in a later step."

Safety is one problem with keeping a stock at room temperature. Flavor is another. A reboiled three-day-old stock may be safe to eat, but it is now seasoned with millions to billions of dead bacteria and their inactivated toxins. It's conceivable that they might add an interesting flavor, but more likely that the bacteria have feasted on the stock's sugars and savory amino acids, the air has oxidized and staled the fat, and the stock has become less tasty.

I spoke with Mr. Ruhlman about Dr. Snyder's analysis of his stovetop-stored stock. "I agree that I should have been clearer about the importance of the 'kill step,' a good 10 minutes at the boil," he said. "And certainly to make the freshest, cleanest stock, it's always best to strain, cool and chill it as rapidly as possible."

What about my lazy method of letting stock cool overnight, then reboiling and refrigerating it first thing in the morning? Dr. Snyder gave it a pass because it would spend only a few hours below 135 degrees, not enough time for the bacterial spores to germinate, start growing and reach hazardous numbers.

Like meat stocks, all moist cooked foods are susceptible to being recolonized by survivalist bacteria. (Baked goods are generally too dry for bacteria; they're spoiled by molds.) That's why we should avoid leaving cooked foods out at room temperature for long when we're preparing for a party or holiday feast (or enjoying their lazy follow-ups), or having a picnic, or packing lunch boxes for young children, who along with the elderly and ill are more vulnerable. It's best to keep moist lunch items either cold or hot, surrounded by cold packs or in a thermos.

What are the actual odds of getting sick from casual food handling at home? No one really knows. There are many variables involved, and only a small fraction of illnesses are reported, even to a family doctor, since they're usually brief. But one unambiguous and heartbreaking story can bring home the value of handling food carefully.

In 2008, a 26-year-old Japanese mother in the Osaka region shared a meal of leftover fried rice with her two children, ages 1 and 2. She had prepared and served the rice the day before and kept it at room temperature.

All three became ill 30 minutes after eating the leftovers, and were hospitalized. Both children lost consciousness, and the youngest died seven hours after the meal. Pathologists later reported in the journal Pediatrics that the rice contained a very common spore-forming bacterium, Bacillus cereus, along with a heat-resistant toxin that the bacterium tends to make on starchy foods, and that can cause vomiting even after being heated to the boil.

It may be true that most cases of food-borne illness aren't that serious, and that most reported cases can be traced to foods that were contaminated during their production or processing. But it is also true that one simple mistake at home can be fatal.

Even though I know this, I tend to discount specific government guidelines because they seem to change arbitrarily, and they don't seem workable in real life. This is true of the latest U.S.D.A. numbers. It's unrealistic to expect home cooks to chill or reheat or discard dishes every two hours during a dinner party, or every hour at a summer barbecue.

Dr. Snyder agreed that official pronouncements on food safety can be inconsistent and self-defeating. "The F.D.A. Food Code is very conservatively written," he wrote. "Four hours after it's cooked is plenty fast enough to get food into the refrigerator." And slow enough to relax and enjoy the meal.

Dr. Snyder added that it's safest to cool leftovers uncovered and in a mass no thicker than two inches, so they cool through quickly. If they're still hot, start the cooling on the countertop. When the container is no longer hot to the touch, put it in the refrigerator, and cover it once the food is good and cold.

My own everyday approach to safety is to try to keep cooked foods either hot or cold until I'm ready to serve them, get leftovers in the fridge during the pause before dessert or soon after, and reheat leftovers that need it until they're boiling or steaming.

This set of habits isn't dictated by an unnecessary, pleasure-killing fear of microbes. It simply acknowledges their inevitable presence in my kitchen, and the fact that both my food and anyone who eats it will be better off if the care I give it doesn't end with the cooking.