When it comes to expiration dates, food safety and food quality comprise vastly different criteria. Surprisingly, long-expired food is still considered -- by the regulators, at least -- good enough to eat. That outdated granola bar packed with soft oats or that chocolate bar now displaying a whitish bloom may not taste very good, but they can still be consumed without making you sick.
In fact, the expiration dates on your food don't have much to do with the risk of food poisoning. For example, you could buy a package of raw chicken and neglect to refrigerate it. The chicken could become bacteria-ridden within a few hours and become unsafe to eat -- no matter what the expiration date on the package says.
These dates are evidence of an inconsistent and outmoded system initially implemented as a way to address consumer concerns about food freshness. By the 1970s, consumers weren't getting their foods from a backyard garden or neighboring farm but from the market, where foods were often sold past their prime. More than 10 congressional bills were introduced from 1973 to 1975, all with a food-dating focus. The result was the patchwork of federal and state food dating regulations that are still confusing consumers today [source: Sifferlin].
In the absence of sweeping national rules governing food dating, state governments have filled the gap. Forty-one states have implemented food-dating rules with little consistency. For example, one state regulation may not allow a box of crackers to be sold after its stamped date while another state may not require an expiration date at all [source: Sifferlin].
Food scientists determine expiration dates based on observing how long it takes for a food to start losing its quality (change texture, lose flavor and so on) under proper storage conditions. As we said earlier, this doesn't mean it is unsafe to eat. It just may not look its best. For highly perishable items like ready-to-eat salads, scientists also look at how much microbial activity is present after a certain number of days, since harmful bacteria could be present before a person could detect spoilage [sources: NSW Food Authority, A Dash of Science].