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Shoppers select fresh produce at the Heart of the City Farmers' Market in San Francisco's United Nations Plaza. Photo: Sam Wolson, Special To The Chronicle

Americans throw away 40 percent of the food they buy, often because of misleading expiration dates that have nothing to do with safety, said a study released Wednesday by Harvard University Law School and the Natural Resources Defense Council, an environmental group.

"The date labeling system is not a system at all," said NRDC staff scientist Dana Gunders, co-author of the report, the first to assess date labeling laws nationwide.

The report said 90 percent of Americans toss good food into the garbage because they mistakenly think that "sell by," "best before," "use by" or "packed on" dates on food containers indicate safety. one-fifth of consumers, the report said, "always" throw away food based on package dates.

In fact, "sell by" dates are used by retailers for inventory control. "Best before" or "use by" dates usually reflect manufacturer estimates of peak quality. While some labels are intended to indicate freshness, none of them reflects edibility or safety, said Ted Labuza, a food science professor at the University of Minnesota who collaborated with the authors.

"If food looks rotten and smells bad, throw it away, but just because it reaches a certain date does not mean the food is unsafe," Labuza said. "I don't know of any food poisoning outbreak that came from people eating food that was past its shelf-life date."

The report estimated the value of food tossed away at $165 billion a year.

Greenhouse gases

Food waste is a big source of greenhouse gases. As food decomposes in landfills, it releases methane, which is 21 times more potent than carbon dioxide. Agriculture itself is a leading source of emissions.

Wasting food also squanders vast quantities of water, land, fertilizers, petroleum, packaging and other resources that go into producing it. About a quarter of all fresh water used in the United States goes into the making of food that is thrown away, the report said.

San Francisco collects 600 tons of food scraps each day, or about half the city's compost waste, said Robert Reed, a spokesman for Recology, the San Francisco company that runs the city's composting and recycling operation. (The rest is from yard trimmings.)

Emily Broad Leib, director of Harvard's Food Law and Policy Clinic and co-author of the report, said a welter of state laws and voluntary labeling regimes came into being during the 1970s, after Congress failed to devise a uniform standard.

Doing it differently

As a result, 41 states have some kind of freshness labeling law, while nine have none. The patchwork makes little sense.

California requires dates on dairy products and shellfish, but doesn't restrict sales after the dates are expired. New Hampshire requires dates on cream but not milk. New York has no label requirement, but all six of its neighboring states do.

"Consumers treat the dates as meaningful," Broad Leib said, when in reality the labels may impart a false sense of security.

Labuza said 80 percent of labels that indicate freshness "are guesses from consumer questionnaires" or are based on competitor practices.

Microbes, which cause food to taste, smell or feel bad, often show up before pathogens, he said, which means food is likely to become unappetizing before it makes a person sick. Food poisoning comes from pathogens that can enter the food chain at any point, from field to kitchen counter, Labuza said. Careless handling by consumers or businesses who leave food sitting in a hot car or loading dock are often culprits.

The authors said the Food and Drug Administration and the Department of Agriculturehave the authority to impose a standardized rule. They recommended establishing clear date labels for quality and safety, discontinuing quality-based dates on shelf-stable packaged products, and promoting "freeze by" dates to encourage freezing.

San Francisco's Board of Supervisors began requiring composting in 2009, with a goal of zero landfill waste by 2020. Reed said San Francisco's food scraps are a nutrient-rich mix of "Chinese food, coffee grounds, cantaloupe skins, pasta, fish bones" and the like that is composted and used in vineyards to grow carbon-sequestering sweet peas, mustard and other cover crops between vine rows.

Environmental benefit

He said scientists have shown that the United States could offset more than 20 percent of its carbon emissions by increasing such practices.

Farmers' markets, where most of the food for sale does not have expiration dates, allow growers "to sell good-quality products that might not meet size, shelf life or other criteria imposed by retailers," the report said.

Gunders said farmers' markets "are a great example of people getting to know their food and deciding for themselves if it's good - instead of just trusting a label."

Scoma's restaurant at Fisherman's Wharf served as a test for the composting program a decade ago, said buyer Kelly Bennett. The restaurant now diverts 95 percent of its waste from landfills, and by volume half of that is food scraps.

Scoma's chefs use cooking scraps in soups and stocks, and analyze returned plates where food went uneaten to see what went wrong, Bennett said. Despite the restaurant's large portions, Bennett said, most people "do a very good job of cleaning their plates."