Everyone has had that moment where they grab a can of food from the back of the cupboard and wonder if it's still good. How long has it been there? And how long is too long? Does canned food ever expire?
These are good questions, and since eating spoiled food can make you and your family ill, it's good to know what you're doing before you reach for that can opener.
How Does Canning Work?
Canning, a food preservation technique, involves heating food in a container to kill bacteria which produces a vacuum effect removing the oxygen from the container. Without oxygen, bacteria can't survive and reproduce. So as long as the can remains sealed, canned food is safe to eat. Indeed, in one case, 100-year-old canned foods found in a shipwreck were tested and showed no signs of microbial growth.
Food Quality Vs. Food Safety
If properly canned foods will last indefinitely, what do the so-called expiration dates printed on commercially canned foods mean?
It turns out that those dates, commonly known as use-by or sell-by dates, have nothing to do with food safety. Instead, they indicate the dates by which the manufacturer estimates the food will have lost quality, freshness, or nutritional value. So, though the cans from the shipwreck showed no signs of bacterial spoilage, they lost their fresh smell and appearance and much of their nutrients, particularly their vitamin content (although protein and mineral content remained unchanged).
In other words, canned foods that have passed their use-by dates are still safe to eat, but they might not smell or taste as good, and they may have lost some of their nutrients.
High Acid Vs. Low Acid Foods
If you are familiar with food preservation techniques in general, you probably know acid is one of the factors controlling the growth of the bacteria causing food spoilage and food poisoning. The more acidic the food, the less it will sustain bacterial growth. In terms of spoilage, acid is generally a good thing and acids are sometimes added to canned foods as an additional preservative.
When it comes to shelf-life, high acid foods like canned tomatoes, fruits, juices, and pickled foods, actually have a shorter shelf-life, as measured by their use-by dates, than low acid foods like canned meats, pastas, vegetables, soups and stews.
Low-acid canned foods generally have a shelf-life of 2 to 5 years, whereas high-acid foods have a shelf-life of only 12 to 18 months.
When it comes to home-canned foods, the guidelines are even stricter. The USDA considers home-canned foods to have a shelf-life of 12 months.
How to Store Canned Foods
It is best to store canned foods in a cool, dry place like a pantry or in cupboards not subject to extreme temperatures like over or next to a stove. Extremes of temperature and moisture, respectively resulting in heat and rust, can effect the quality and sometimes the safety of canned foods.
Once canned food is opened, any unused portions should be, first, transferred into a storage container, covered, and then stored in the refrigerator. High-acid foods will last 5 to 7 days in the fridge, while low-acid foods will last 3 to 5 days.
Does Canned Food Go Bad?
Although it is rare, commercially canned food can definitely go bad. Usually when this happens, there are obvious signs, like leaking, dents, rust or bulging.
Small dents in cans are fine, but if it is deep enough for you to be able to lay your finger into it, that's too deep and the can should be discarded. A deep dent can produce a crease in the can which can cause air and bacteria to get in and trigger food spoilage.
The same is true for rusty cans. If you can wipe the rust off the can, it's fine, but in the case of severe rust, it's possible that the walls of the can could have corroded so much that bacteria could enter. So if you can't wipe the rust off the can, throw it away.
As mentioned, the canning process removes the oxygen to prevent the growth of bacteria. The one bacterial exception is the botulism toxin. It is produced by a spore that can survive without oxygen, like in a vacuum sealed can. The botulism toxin can cause potentially fatal illness. Fortunately, it's extremely rare.
The good news about botulism bacteria is it produces gas causing cans to bulge. This is a great indicator of a possible botulism infection. Overall, whatever the cause, bulging cans are a sign that the food in it is not safe and the can should be discarded without opening.
Canned foods that have been exposed to extremes of temperature, either hot or cold, could also suffer from quality and safety issues. Cans that are exposed to temperatures higher than 100 F can be subject to spoilage. Likewise, cans that have frozen and then thawed can potentially be unsafe. Moreover, when a can freezes, its contents can swell up, and since swelling could be a sign of botulism contamination, those cans should be discarded.
Even if the can is in good shape, if the food inside looks or smells bad, don't risk it. Also, please do not taste the contents to check their safety. When it comes to eating canned foods, it is best to remember the invaluable adage "when in doubt, throw it out."