It is time to delve into what I used to do for work. I spent a few years in a lab that tested the quality and safety of several different foods. Most of the foods were canned so I learned a lot about canning. The canning process is brilliant. Canned foods will never match what is pulled straight from a garden or cooked fresh, but they are handy to have around. The convenience of having food that can sit on a shelf for months or years and still taste good when opened is hard to beat.
Definition
Canning is a method of preserving food by applying heat until the food and its container are commercially sterile. Commercial sterilization destroys all pathogenic and spoilage microorganisms. Even though it is called canning, “canned” foods can be packaged in any container that can withstand the heat treatment: metal cans, glass bottles, laminated pouches, paperboard containers, plastic containers, etc.
Brief Canning History
Napoleon needed a way to transport food with his army without having it go bad. In 1795 he offered a 12,000 francs reward to anyone who invented a method to preserve food. Nicholas Appert, a French confectioner, won the award. Appert’s method was to put food into glass bottles, seal the lids with pitch, and heat.
Initially, canning was a French military secret but it did not stay that way for long. In 1810 Englishman Peter Durand patented the use of tin cans in the canning process. The canning process has come a long way in the past two hundred years but the principle is the same.
How Canning Works
Food goes bad after a period of time. To stop this from happening, the main culprits-pathogenic and spoilage microorganisms-have to be stopped. Most microorganisms need oxygen, food, and an environment with a specific temperature range, and a specific pH range. Canning manipulates oxygen, temperature, and sometimes pH to kill the bad bugs.
First cans are filled with food, then the oxygen is replaced by steam or another gas. Immediately after that the cans are hermetically sealed. (Hermetic is a fancy word for airtight.) Finally, cans are thermally processed in a giant pressure cooker called a retort. Thermal processing is the technical term for the heating step in canning. Each food has a specific thermal processing schedule that states the temperature and time for heating, just like a cake recipe has an oven temperature and cooking time.
There is a delicate balance between time and temperature during thermal processing. Cans have to be heated hot enough and long enough to kill pathogens. But how hot for how long can vary. Cans processed at a higher temperature do not need to be cooked as long.
Inside the Can
Canned foods are usually put into liquid to help the food cook more evenly.
Meats are canned, but they are not on store shelves in large quantities. The meat cooks within the can which means the fat which normally drains off during cooking is in the can. This leads to differences in flavor and texture than fresh cooked meats. Often meats are canned in products like a can of stew or chili which have sauce to cover any flavor differences.
Canned fish are typically heated twice. The fish is cooked, drained, then sealed into cans. So tuna in water is just that.
Fruits are canned in syrup. Syrup helps the fruit keep its flavor and shape. Ideally, the syrup has about the same sugar content as the fruit. If the syrup has a higher concentration of sugar than the fruit, sugar travels into the fruit and water travels out. If the syrup has a lower concentration of sugar than the fruit, sugar travels out of the fruit and water travels in. This is a phenomenon known as osmosis.
Vegetables don’t have as much of an issue with osmosis so they are canned in water. Sometimes salt is added for taste.
Nutrition in Canned Foods
Overall, the nutrional content of canned foods is slightly lower than their fresh counterparts. Some vitamins break down during thermal processing, but most nutrients survive. The nutrtion loss is similar to home cooked food.
Wrap Up
Canning is a great method of food preservation. Thermal processing kills off pathogenic and spoilage microorganisms so the food has a long shelf life. Canned foods are convienient, cost effective and comparable in nutrition. I wouldn’t live solely off of canned foods, but there are always some in my kitchen.
Sources:
- “History of the Can” by Quality by Vision. Accessed 1/08/2014
- Canned Foods: Principles of Thermal Process Control, Acidification and Container Closure Evaluation, 7th Ed. Editor: Lisa M Weddig. GMA Science and Education Foundation. 2007.
A Dash of Science 
There’s a case to be made that the rise of obesity in the US and in the industrialized world is in large part a result of technological progress in processing food. Processed food, with canning as a great example, tend to reduce food preparation costs in both money and time.
So, Napoleon started us down the path of pervasive paunchiness:-)
Very cool post! Thanks! Now I’m curious about freezing and drying foods….:-)
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You are right in part. According to economic theory, any food preservation method will lower the cost of food because it decreases the scarcity. An abundance of affordable food leads to less hunger, less malnutrition, and the possibility of obesity.
Note, a lack of food will lead to hunger, but an abundance of food will only lead to obesity if people eat more than they need. Being able to preserve food enables obesity, but it is not the cause.
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