However, the safety depends chiefly on how the product is handled once it leaves the store. If it's refrigerated, it should be good for a couple of days, according to the agency, but if it smells bad, or if it looks like it has spoiled, toss it, or better yet, return it to the store.
There is no federal requirement to label expired foods, except for infant formula and some baby food, contributing to inconsistencies in the marketplace.
At least 20 states require some dating, but selling expired foods, even at a discount, is rarely prohibited. And different foods are labeled in different ways, ranging from "sell by," to "best if used by," to "use by."
And canned goods, which should be good for two to five years, according to the USDA, frequently carry an encoded expiration date that means nothing to the consumer.
Some expired foods are sold at a discount, which Lewis's researchers call a "manager's special," so at least the buyer knows the quality may be slightly less. But is it safe?
Lewis isn't real sure. "I'm still not 100 percent convinced," she said.
"We don't think people are making informed choices because the information is so hard to find," she added.
She's particularly concerned about residents in poorer neighborhoods who may be especially vulnerable.
"The consequences may be greater in south Los Angeles because of a low health status," she said. "If people are already compromised by chronic disease and other kinds of things, the consequence of eating expired food could be greater."
But this isn't just a problem for the inner city. Consumers Union researchers found 72 items past their "sell by" date in supermarkets in Connecticut, New York, Ohio, Tennessee and Texas. They found no expired foods in markets in Michigan and Washington.
By the way, "sell by" indicates the last day a product should be sold. "Use by" indicates the last day the product is likely to have its full flavor and quality.
Lewis and her colleagues have met with representatives from all five of the supermarkets where expired foods were found. When they laid out their findings, the reactions ranged from "defensive" to "challenging the data," she said. No one from any of the markets seemed shocked, she added.