Why Don't Canned Foods Need Refrigeration? (2024)

Last week we discussed how refrigerators work to keep your food cold (click here for a review). Keeping food cold or frozen is very important to prevent it from spoiling—if you keep milk on the counter at room temperature it will spoil in only a day or two, but if you keep it in the refrigerator it will stay fresh for at least one week. Keeping food frozen will prevent it from spoiling even longer—it should be safe to eat for years, although it may develop a rubbery texture! But what about food in a can? Canned foods can last for years, and never need refrigeration (as long as the can is sealed). How can this be?

Before we discuss why canned foods can last so long without refrigeration, we need to understand why food spoils. We’ll use milk as an example: think about the last time you opened a carton of milk that had spoiled (gone sour). It had a sour, rotten smell, and it may have looked chunky or curdled. This spoilage is caused by bacteria that grow in the milk.

Bacteria are very tiny, single-celled living things (organisms); most are just a few micrometers long (one micrometer is 1/1,000,000 of one meter)—that’s less than half the width of a human hair! These bacteria live in the milk, eating the sugars and other nutrients, and releasing the by-products of acid and other smelly compounds. If left to themselves, these bacteria will continue to grow and multiply until the milk has so much acid in it that it curdles (acid makes the proteins in milk curdle—for more information see our lesson on adding acid to milk). At this stage there are so many bacteria that the milk smells bad, too! If left at a warm temperature these bacteria will grow quickly, causing the milk to spoil in less than one day.

The reason milk keeps fresh much longer in the refrigerator is because the bacteria grow much more slowly at cold temperatures. Slowing down the growth of the bacteria prevents them from eating the sugar in the milk and reproducing, preventing spoilage. Foods that are frozen will not spoil for a very long time because freezing foods usually stops the growth of bacteria completely! This is true for other microorganisms that can cause food spoilage, such as yeasts and molds (if you’ve ever seen a piece of green, fuzzy bread...that’s mold!).

We know we can prevent food spoilage by slowing down or stopping the growth of microorganisms (like bacteria) in or on the food. In general, there are two ways to do this:

1. Slow down or stop the growth of microorganisms that are already there

2. Kill all the microorganisms in the food, then prevent any new ones from getting in

Canning works using this second method.

When foods are canned, they are sealed in metal cans or in jars and heated to 100°C (212°F) or higher for enough time to kill the bacteria and other microorganisms living in the food. All microorganisms in the food are now dead, and so cannot feast on the sugars or other nutrients, multiply, and cause spoilage. Because the can is sealed, no new living microorganisms can get in. This is how canned foods can sit at room temperature for over a year and not spoil (Source: Shepard)!

TRY THIS!

Here’s what you’ll need:

1. Two 1-pint canning jars, with lids

2. Two packets of yeast

3. Four tablespoons of sugar

4. Two pints clean water

5. One large pitcher

6. One small saucepan

7. A stove, cook top, or hot plate

8. One large pot or canner

Here’s what to do:

1. Clean the canning jars very thoroughly in very hot soapy water, or simply run them through a dishwasher on the sterilize cycle. Wash the lids in very hot soapy water, rinse them well, and keep them soaking in hot water.

2. In the pitcher, combine the two pints of water, two packets of yeast, and four tablespoons of sugar. Mix well until all the sugar dissolves.

3. Pour some of this mixture into one of the jars, and some into the small saucepan. Only fill the jar to 1 inch from the top of the rim! Be sure to wipe the rim of the jar clean with a paper towel before placing the lid on the jar.

4. Place the saucepan on the stove over high heat, and bring it to a boil. Once the mixture boils, take it off the heat and pour it into the other jar. Carefully place the lid on the jar, being sure to wipe the rim of the jar clean with a paper towel. Make sure the lid is on the jar tightly!

5. Place the jar with the boiled yeast mixture in the canner or pot, and fill with water until the water comes up just to the lid of the jar.

6. Bring to a boil, and keep at a boil for 10 minutes.

7. When the 10 minutes of boiling are up, carefully remove the jar from the hot water and place it at room temperature beside the jar with the un-boiled yeast mixture. As the jar cools the lid should not bounce back when pressed—this means the jar has sealed.

8. Observe the jars carefully for up to 4 hours. Watch for signs of yeast growth, including bubbles and froth forming at the surface of the water.

9. After 4 hours, remove the lids of the jars and observe the liquid carefully. Take note of the way the liquid looks, as well as how it smells.

What did you observe in the two jars? How is the boiled liquid now different from the un-boiled liquid? Did you hear or smell anything different when you opened each jar? Be sure to write down all your observations!

For an additional challenge, you could prepare these two jars again, but this time prepare a third jar without boiling, and place it in the refrigerator. How do you think these three jars would be different from each other after 4 hours?

References for further reading:

Shepard, Sue. "Pickled, Potted, and Canned: How the Art of Food Preserving Changed the World." "Canning." Simon and Schuster. New York, 2000.

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