The process of canning involves placing foods in containers and heating them to a temperature that destroys microorganisms that spoil food quality. During this heating process air is driven out of the container and, as it cools, a vacuum seal is formed. This seal prevents entry of air which can bring with it contaminating microorganisms.
 Why should I be aware of this?
If practiced with proper care, canning is an important and safe method for preserving food. Canned foods have their advantages and disadvantages. According to many nutritionists, many canned foods are as nutritious as fresh or frozen food. An opposing school of thought believes that the amounts of nutrients that are sensitive to heat (examples: folic acid and vitamin C) are substantially reduced during the canning process.
For instance canned tomatoes are said to have as much lycopene as fresh ones. Lycopene is carotenoid pigment that acts as an antioxidant and protects the body from the so-called "free radicals" that cause cancer. Canned purple plums and pumpkin contain more vitamin A than the fresh varieties and canned sardines and salmon contain calcium-rich edible bones.
 Canned food and health
A study on canned fruits and vegetables conducted by the University of Illinois Department of Food Science and Human Nutrition, found that they provide as much dietary fiber as fresh and frozen food and even help to combat the risk of heart disease and some types of cancer. 
Key findings from the nutrition study included:
- Many canned fruits and vegetables, such as green and yellow vegetables, sweet potatoes, carrots, peaches, pumpkin and apricots are high in vitamin A and related carotenes, antioxidants that provide essential protection for the body's cells. Canned tomatoes contain an important carotenoid called lycopene, which other studies indicated, helps prevent prostate cancer. In fact, some analyses show lycopene is more effective when eaten after heating or canning the tomatoes.
- Canned salmon was found to be higher in calcium - a vital nutrient needed to maintain strong bones and teeth - than fresh or frozen salmon.
- There are also healthy recipes with canned food, such as spaghetti sauce recipes made with canned tomatoes, which provide more fiber, potassium, vitamin A, vitamin C, calcium and iron than the same recipes using fresh tomatoes.
- Many canned fruits and vegetables, particularly varieties of beans, are a valuable source of soluble fiber, which other research has shown reduces blood cholesterol and gastrointestinal problems that increase the possibility of certain cancers.
Most of the vitamin C in the food is retained after canning and remains stable during the one- to two-year shelf life of the product. Significant sources of vitamin C include canned apricots, asparagus, oranges, grapefruit, pineapple, strawberries, spinach and tomatoes.
Folate is an important nutrient that helps to regulate blood pressure and kidney functions.
 All about canned food
As soon as they are plucked, fresh foods begin losing vitamins. And when they are left in the warehouses or in transit for long periods of time before they reach the markets, the damage is worse
Though fresh fruits and some vegetables need time to ripen, they are often harvested much earlier. Canned foods are harvested at their peak of ripeness and normally cooked and processed at the source within hours, thus preserving more vitamins than their fresh counterparts.
Today thousands of products are available in a canned state, adding convenience to our busy lifestyle. The sodium content in commercially-canned foods has been significantly reduced, up to 40% over old canning methods.
Most canned foods are also now available in low-salt, no-salt, low-sugar, and no-sugar preparations for those with special dietary needs and/or those who want a more natural flavor.
Food can be canned safely by using the following two methods:
 Boiling water bath method
This is a safe method for canning tomatoes, fruits, jams, jellies, pickles and other preserves. In this method, jars of food are heated completely covered with boiling water (212°F at sea level) and cooked for a specified amount of time.
The canner used for this is a large cooking pot, which has a tight fitting lid and a wire or wooden rack which prevents the jars from touching each other. The content is processed evenly as the rack allows the boiling water to flow around and underneath jars. As the jars are kept apart there is no risk of bumping against each other and cracking.
To ensure proper heating of all jars, the diameter of the canner should not be more than 4 inches wider than the diameter of your stove's burner.
 Pressure canner method
For vegetables, meats, poultry and seafood pressure canner is the only safe method of preserving food. Food is placed in a special pressure cooker in 2 to 3 inches of water and heated to a temperature of at least 240° F. Though the bacterial cells are killed at boiling temperatures, they can form spores that can withstand these temperatures. The spores grow well in low acid foods, in the absence of air, such as in canned low acidic foods like meats and vegetables. These spores can be destroyed only by pressure cooking the food at such a high temperature
This is a specially-made heavy pot with a lid that can be closed steam-tight. The lid is fitted with a vent (or pet-cock), a dial or weighted pressure gauge and a safety fuse. Newer models have an extra cover-lock as an added precaution.
 Low acidic food
Low acidic foods include:
- dairy products
- all vegetables
 High acidic food
High acidic foods can be safely canned using the boiling water bath method. They include:
- properly pickled vegetables
Certain foods like, tomatoes and figs, that have a pH value close to 4.6 need to have acid added to them in order to use the water bath method. This is accomplished by adding lemon juice of citric acid.
 What can I do?
 Methods for canning foods at home
There are various safe methods available today for home canners to simplify and safely preserve higher quality foods.
- When processing foods using the open kettle method, jars should still be sterilized.
- When you cook the foods in the jars, the jars do not need sterilization, but should be thoroughly washed.
- Since heat penetration of items such as peas, corn, lima beans and most meats is low, these foods should be packed loosely. As fruits and berries tend to shrink, they should be packed solidly
- Do not use preservatives or any other type of artificial chemical substance iin the canning
- After canning store the products in cool or cold and dark locations. Bacteria can be formed due to excessive heating, specially during the summer months
- Make sure that the seal is intact. Canned foods will remain safe indefinitely, as long as the seal is intact and they have been properly processed.
- After canning your food, tap the top; you should hear a ringing note. If food is touching the top, this may not occur, but as long as the top does not move up and down, the food does not have to be reprocessed.
- Do not worry if sometimes black deposits form at the bottom of the lid,. If the cans are properly sealed they are probably caused by tannins in the food or by hydrogen sulfide released by the foods during their processing.
- Cloudy liquid in the jar is an indication that the food is spoiled.
- When canning jellies or jams, it makes no difference whether you use cane or beet sugar. They are both the same.
- Pure apple cider with four to five percent acidity is the best vinegar to use when pickling.
- The outside of jars should be wiped with vinegar before storing .This reduces the risk of mold forming on any food that was not cleaned off well.
- To avoid hard water deposits on sealers, add vinegar to the water bath when canning.
- Jars of frozen fruits should be thawed in the refrigerator. This will allow the fruit to absorb the sugar as it thaws.
- Jelly jars should have a small piece of string placed on top of the wax before sealing the jar. This will make it easier to remove the wax.
There is a common myth is that canning is difficult, costly and time consuming. On the contrary it is an easy, inexpensive, and a timesaving way to have fresh food anytime you want or need it. The purchases you make are one-time costs that save you money in the long run. Only by replacing the lids, the majority of your canning equipment, such as the canner, jars, and rings can be reused repeatedly.
- Canning Basics for Preserving Food
- Getting the Best Out of Canned Foods
- Food Canning Facts
- Learn a little about Preserving the Harvest
- ↑ UNIVERSITY OF ILLINOIS NUTRITION STUDY 1997