How to Tell When Leftovers Go Bad

To toss or not to toss: Exactly when leftovers become trash has fueled arguments of couples, roommates and co-workers since the dawn of the refrigeration.

Does moldy bread go in the trash, or just get a trim around the green spot? Can Sunday's leftovers be Friday's meal? What about that day-old ground beef?

While scientists have developed methods to detect spoilage -- for example, sensors that go off when milk changes consistency or a polymer to detect bacteria growth in meat -- until these are available on a mass scale, food science and safety experts have some tips.

Deadly and Invisible

First tip: slimy, stinky, spotty or chunky changes in food don't mean very much in terms of safety.

"It may not taste good, that doesn't mean it's going to make you sick," said Michael Doyle, director of the Center for Food Safety at the University of Georgia in Griffin.

Doyle said there's a difference between what food scientists call spoilage bacteria and pathogens.

Spoilage bacteria make themselves known by way of slimy films on lunch meat, soggy edges on vegetables or in stinky chicken. But "there's a big difference between spoilage and what's going to make you sick," Doyle said. "Often spoilage bacteria will outgrow the harmful bacteria and protect [the food]."

The pathogens that do make you sick are odorless, colorless and invisible. The consumers sickened in the e-coli contaminated beef recalled from Whole Foods this month likely could not smell, see or taste the bacteria. Salmonella is invisible, too. "Even if you put it under the microscope, you couldn't tell it's salmonella [bacteria]," Doyle said.

Catherine Donnelly, professor of nutrition and food sciences at the University of Vermont in Burlington, said, "That's the problem: it's that you really can't tell."

But that doesn't mean Donnelly and other food safety experts think consumers are simply at the mercy of farms and slaughter houses.

It's Getting Hot in There

Because consumers can't use the looks-OK, smells-OK, is-OK mantra for safety, Donnelly has some other advice. "Do you know the temperature of your refrigerator? Most people don't," Donnelly said.

For leftover food to be safe, it must be kept in what Donnelly calls the rule of four: no more than four days at 40 degrees Fahrenheit or 4 degrees centigrade. (Freezing fresh food at zero degree Fahrenheit will keep it safe indefinitely.)

"About 25 percent of the refrigerators in the country are operating at a temperature that can make food unsafe," says Donnelly, citing a study commissioned by the Federal Transit Administration. "Here we're using the refrigerator as a food safety device and most people have no clue, no idea what temperature it should be."

Temperature can slow or stop bacterial growth of either the pathogens or the spoilage variety. Forty degrees Fahrenheit buys people three days for safety with raw chicken and ground beef, three days with cuts of beef and lamb and four days for leftovers.

Allowing anything to go above the cold 40 degrees along the way from store to frying pan can make the difference between illness and safety.

"If it's contaminated and then you further abuse it temperature-wise, then you're at risk," Donnelly said. "In heat, low levels of contamination can just go really wild."

That means not leaving groceries in a hot car for hours during other errands. It also means changing doggy bag habits.

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