How to Tell If Food Has Gone Bad—and What Expiration Dates Actually Mean, According to Food Scientists

We've all been there: You're gathering ingredients to cook a meal, only to reach for a packaged food and realize it's past the labeled date. And if you're like most people, you'll likely toss it straight in the trash, assuming the food is spoiled and unsafe to eat.

But as it turns out, that probably isn't the case. In fact, spoilage and expiration dates aren't related, so you'll need to rely on other factors to determine if your product has actually gone bad. Ahead, food science professionals explain the tell-tale signs that food has spoiled, plus when you should actually follow expiration dates.

The Truth About Expiration Dates

According to the U.S. Department of Agriculture, there are no uniform standards for labeling food expiration dates in the United States. That's why food dates appear with myriad labels, including "best if used by," "sell by," and "use by." Additionally, contrary to popular belief, these dates do not indicate safety. Instead, they refer to the date a product's quality will become less ideal, says Bryan Quoc Le, Ph.D., a consultant and author of 150 Food Science Questions Answered. "After this date, the flavor, crispness, or performance of an ingredient may decline," adds Mitzi Baum, MSc, food safety instructor at Michigan State University and CEO of STOP Foodborne Illness.

In other words, even if the date on a food product has come and gone, it doesn't necessarily mean the item is rotten. Rather, it means the food is past its prime and less fresh than when you first bought it. But thanks to your sense of smell, sight, and touch, there are other ways to determine if the food has gone bad.

Signs of Spoiled Food

Although the exact signs of spoilage can vary greatly, most foods develop unpleasant characteristics that will likely deter you from adding it to your plate. "Depending on the food, if it has gone bad, it may have a moldy odor, sour taste, [and] spots or blemishes," explains Baum. The color, texture, and firmness of the food might have also changed.

Meats

Spoiled meats may be slimy, fuzzy with mold, or sticky. Beef and pork may emit an unpleasant odor and turn brown due to oxygen exposure, while poultry, like chicken and turkey, may also not smell fresh, says Baum. The wing tips of poultry might also turn an off-color (anything but pink, essentially), she adds.

Dairy Products

Spoiled dairy products (like milk or yogurt) will have a sour smell, slight yellow coloring, and lumps due to the precipitation of milk proteins, says Le. For milk in particular, the bad odor may be more prominent at the top of the jug or carton, as this part gets more exposure to oxygen and microorganisms, he says.

Butter can become rancid and develop an unpleasant flavor if it's left at room temperature for more than two days, notes Baum. It might also turn slightly brown, though this can be difficult to see in the yellow coloring of the butter, says Le.

In cheese, typical signs of spoilage are green and white mold and pink yeast slime. Soft cheeses with mold should be tossed, but hard cheeses like cheddar can still be eaten if you cut at least an inch around and below the mold spot (and avoid touching the mold with the knife), says Baum.

Eggs

If you see exterior damage to the shell of the egg, such as cracks or a slimy or powdery film, do not use the egg. It is likely contaminated by bacteria or mold. Another sign that an egg has spoiled? If you notice a strong smell with a sulphuric, gassy, or sour edge when you crack it—eggs should always have a neutral smell. And according to the USDA, if the egg white has pink or iridescent color, this is an indicator of spoilage due to Pseudomonas bacteria.

Flour

"Flour will not spoil unless it's stored in an environment that's hot, humid, or exposed to bugs or rodents," explains Baum. If it does spoil, flour (especially whole wheat flour) will develop a rancid odor.

Dried Pasta and Rice

According to Baum, dried pasta and rice also won't spoil or become unsafe to eat as long as they're stored in a cool, dry environment free of pests. This is true regardless of the expiration date—which, again, indicates peak freshness rather than safety. Should these products be stored improperly, they may have an off flavor and stale odor.

Is It Safe to Eat Spoiled Food?

You might be surprised to learn that spoiled food isn't automatically unsafe (though it can certainly make your meal unappealing). According to Baum, the aforementioned signs of food spoilage are not caused by the same bacteria and viruses that cause foodborne illness. In general, the latter doesn't affect the smell, taste, and appearance of food, which explains why you can get food poisoning after eating food that looks fresh and unsuspecting. Likewise, it's possible to eat spoiled food and live to tell the tale—but understandably, you might prefer to pass on the practice.

When to Follow Expiration Dates

If you're unsure when to follow an expiration date, consider whether quality or safety is your most current priority.

Quality

If quality is the goal, stick to the date on the packaging for optimal freshness, flavor, and texture. Following the listed date is particularly important when using products that perform a specific purpose. For instance, when using baking powder—a leavener in baked goods—freshness is key for a successful recipe.

Safety

If you're concerned about the safety of the product, you can place less stock on the expiration date and consider other factors, like when it was opened, how it was handled, and how it was stored. Also, follow these guidelines:

• Leftovers should be promptly refrigerated at 40 degrees Fahrenheit and eaten after three to four days, says Le.
• Food should not be left unrefrigerated for more than two hours; after that, foodborne illness-causing bacteria can quickly grow.

Ultimately, so long as the food is handled properly, it's generally safe to eat past the expiration date, according to the USDA.