Canning is a method of food preservation in which food is processed and sealed in an airtight container, such as jars or cans. The canning process involves heating the food to a temperature that destroys microorganisms that could be a health hazard or cause the food to spoil. Canning also inactivates enzymes that could cause the food to spoil. Air is driven from the jar during heating, and as it cools, a vacuum seal is formed, which prevents air from getting back into the product and bringing with it microorganisms to recontaminate the food.
Canning provides a shelf life that typically ranges from one to five years, although under specific circumstances, it can be much longer. A freeze-dried canned product, such as canned dried lentils, could last as long as 30 years in an edible state. However, to safely home can foods and prevent food-borne illness, research-based canning methods must be followed. Botulism is the most commonly associated food-borne illness with home-canned foods, and food-borne illnesses related to the consumption of home-canned foods are often linked to the person.
The canning process has evolved over time, and research has enabled home canners to simplify and safely preserve higher quality foods. Knowing why canning works and what causes food to spoil underscores the importance of following directions carefully. Acid foods such as fruits can be processed or canned in boiling water, but low-acid vegetables and meats must be canned in a pressure canner at 240° F (10 pounds pressure at sea level). Tomatoes are on the borderline between low and high acid. They can be canned in boiling water, but acid must be added to them to increase the acidity. In pickling, low-acids foods such as vegetables are acidified by adding vinegar.
