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RENEE MONTAGNE, HOST:

This is MORNING EDITION from NPR News I'm Renee Montagne.

DAVID GREENE, HOST:

And I'm David Greene.

Well, if you had a Christmas feast last night, you might be waking up to a fridge full of leftovers. So we thought it would be helpful to talk about how long that food will be good. I mean if you eat that cranberry sauce after the expiration date, will something really go terribly wrong?

NPR's Dan Charles has some answers.

DAN CHARLES, BYLINE: In Livermore, California, there's a scientific establishment called the National Food Lab, or as people in the food industry call it, the NFL. At the NFL, they put food on shelves for days or weeks or even years to see how it holds up. And then from time to time they'll take some of that food down off the shelf, whether it's bagged salad greens or breakfast cereal.

Gina Roberts, who's vice president for business development at the NFL, says they give the food to a highly trained panel of experts who check its taste and smell and texture.

GINA ROBERTS: You would think that everybody can taste and smell food, but some of us are much better at it than others. And those people get paid to eat food all day long.

CHARLES: So how many of these food tasters do you have on staff?

ROBERTS: You know, we have 40.

CHARLES: Forty of them.

ROBERTS: They are probably the most fit bunch of the group.

(LAUGHTER)

ROBERTS: Because we don't eat the food product, we expectorate it, which is a really fancy college word for spit it in a cup.

CHARLES: The experts give the food number grades. The numbers go down as the food gets older; bread gets stale, salad dressings can start to taste rancid.

John Ruff, who's president of the Institute of Food Technologists in Chicago, says the companies that sell this food take a look at those grades and they say: This is where I draw the line.

JOHN RUFF: If the product was designed to be a 7 when it was fresh, you may choose that at 6.2 it's gotten to the point where I don't want it to be on the market anymore.

CHARLES: That's how they come with those dates you see on practically everything in the store. Ruff says companies put expiration dates on food to protect the reputation of their products.

RUFF: If its 6.0, would most people still find it reasonably good? Absolutely, but companies want people to taste their products at their optimum, because that's how they maintain their business and their market share.

CHARLES: This all organized and carried out by food companies. There's no federal law that requires dates on any food except for infant formula, although some states do require dates on milk or meat.

Also these dates don't really tell you anything about whether food is safe. John Ruff says food is generally safe to eat long after its sell-by date. In fact, even meat or milk that's starting to spoil is not necessarily dangerous.

RUFF: You're not going to eat it because of the smell and you probably wouldn't particularly like the taste, but that still is unlikely to cause you illness.

CHARLES: It's not the food that sat on the shelf too long that will make you sick, Ruff says. It's the food that got contaminated with salmonella or listeria or certain kinds of E. coli bacteria, or the hamburger that you didn't cook long enough. But that food might not smell bad - it might have arrived in the store just yesterday.

RUFF: In 40 years in eight countries, if I think of major product recalls and food poisoning outbreaks, I actually can't think of one that has been driven by a shelf-life issue.

CHARLES: Canned food in particular can stay safe for a really long time. In 1974, scientists at the National Food Processors Association in Washington, D.C. got their hand on several very old cans of food. Some were over a hundred years old. They'd been recovered from a sunken steamboat buried in river silt near Omaha. Also...

JANET DUDEK: There was a can of corn that was found in someone's basement - I believe in California.

CHARLES: That's Janet Dudek, one of the scientists who got to analyze this old food. Dudek's assignment was the corn, which was 40 years old. When they opened the can, she says, it looked and smelled just like canned corn. It also had plenty of nutrients left. So did the century-old canned oysters, tomatoes and red peppers from the sunken steamship.

Dudek says as far as she knows, nobody actually tasted this food. That just wasn't done in the lab, she says. But they probably could have.

DUDEK: It would have been safe to eat them. If the can itself maintained its integrity, it probably would have been safe to eat.

CHARLES: When food in supermarkets passes its sell-by date, though, it gets swept off the shelves. Often it's donated to food banks. But if you discover such food in your pantry at home, John Ruff, from the Institute of Food Technologists, says there's usually no reason to throw it out. If it's meat or milk, just smell it. If it smells bad, he says, sure, don't eat it. But otherwise don't worry. Ruff says, I've certainly opened packages of food that were five years old.

Dan Charles, NPR News.

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