How to Tell When Leftovers Go Bad

Aug. 12, 2008— -- 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.

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.

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.