LONDON—Everyone's done it: You reach into your fridge and pluck out something tasty that you've meaning to eat but had half-forgotten about, only to discover that its "best before" date passed a few days earlier.

What to do? If you're like many people, you let out a resentful sigh and toss it—better safe than sorry—and resolve to be more watchful, less wasteful, in the future.

But did you really need to throw away that perfectly good-looking (and good-smelling) wedge of manchego or container of ice cream?

Depending on what it was, and how well it was kept, probably not, says Emma Marsh, who leads Love Food Hate Waste, a U.K.-based group that's dedicated to tackling food waste. "More than half of