It’s a familiar scene in my house. My fridge is jammed, I need space, and so I purge, sorting through the jams and dressings and leftovers to determine which have expired and are, therefore, inedible. But it turns out those dates stamped on so many of our foodstuffs — “best before,” “sell by,” “use by” — have nothing to do with food safety. Rather, they are determined by lab tests that estimate when a food tastes its best. Most of us are unlikely to notice any difference.