Have you ever examined a container of leftover food wondering if it’s safe to eat? In general, it’s a good idea to follow the 7-day rule (eat or toss leftovers within seven days).
You can also watch for five common signs that food has spoiled or become contaminated.
1. Dents, bulges, or holes in canned food
Clostridium botulinum is a type of bacteria that thrives in oxygen-deprived environments. That includes food in cans and jars, baked potatoes wrapped in foil, and even honey. This nasty bug can cause a disease named botulism.
Dents, bulges, and holes in canned food are a sign that botulism bacteria may be present. Food in improperly sealed jars can also contain Clostridium botulinum. If you notice any of these signs in canned or jarred food, don’t eat it!
You can prevent Clostridium botulinum from growing on baked potatoes by removing the foil as soon as the potatoes are done cooking. Never store baked potatoes wrapped in foil.
For more information about botulism, check out this article written by our in-house food scientist.
2. Visible spots of mold
Mold typically looks like white, black, gray, or green fuzz growing on food. In most cases, you should throw away food items with visible spots of mold. In general, you can’t just cut out the mold and call it good. That’s because mold isn’t limited to the furry spots you see — it also has tiny threadlike roots. These roots are very difficult to see, which means they may have contaminated much more of your food than you realize.
There are a few exceptions to this rule:
- Hard, cured meats — If you see mold on a hard, cured meat that is stable at room temperature, like salami or dry-cured ham, you can simply scrub off the visible mold and eat the meat as usual. Cured meats are too salty or dry to allow the mold roots to grow, so any mold growth is contained on the surface.
- Blocks of hard cheeses — Parmesan is a good example. Cut at least 1 inch around and below visible spots of mold and throw away those pieces. The rest of the cheese block is safe to eat. Just be careful not to touch the mold with your knife, or you could accidentally contaminate the rest of the cheese. Warning: this does not apply to hard cheeses that have been crumbled or shredded. Throw away moldy crumbled or shredded cheese.
- Firm fruits and vegetables — This category includes items like cabbage, carrots, and some pears. Like dealing with mold on hard cheese blocks, cut at least 1 inch around and below the visible spots of mold. To prevent cross-contamination, do not touch the mold with your knife.
For more information about dealing with mold, check out our online Food Manager course.
3. Color changes
If you notice a food item has turned an unusual color, that’s a red flag that something may be wrong. Color changes often happen in tandem with sign number 4, unusual odors or textures.
4. Unusual odors or textures
Take a closer look at food with an “off" smell or texture. Consider deli meat as an example — when it goes bad, its color and smell start to change. It may also become sticky, slimy, or moldy.
5. Evidence of pests
Pests carry germs, which can get on food and make you sick. Some common clues that you have pests include:
- Cockroach egg casings (small black, brown, or dark red shells)
- Strong oily odor (sign of cockroaches) or smell of ammonia (sign of rodents)
- Droppings
- Flies buzzing around regularly or in large numbers
- Rodent nests
- Gnaw marks on food containers
Throw away any food items you think may have become contaminated by pests. Then, thoroughly clean the area where the food was stored.
For tips on preventing pests, see our article about pest prevention.
How strict are food expiration dates?
Would it surprise you to learn that expiration dates are more about food quality than food safety? That means most foods are safe to eat even after they’re expired. However, depending on how long they’ve been expired, they may not taste the best.
Commercial food establishments and grocery stores must strictly avoid using or selling expired food. But at home, you can decide for yourself whether to use an expired item.
For more information about expiration dates, read our article “Beyond the Food Expiration Date."
What about frozen food?
Freezer temperatures (0°F or below) stop germs from growing in food (as opposed to refrigerator temperatures, which only slow growth). Technically, food kept in the freezer is safe to eat indefinitely.
However, that doesn’t mean it maintains the same quality. The longer food stays frozen, the more its quality tends to decrease. That includes changes in taste and texture.
Learn more in the USDA’s guide to freezing food.
—Jessica Pettit
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