The high percentage of water in most fresh foods makes them very perishable. They spoil or lose their quality for several reasons:
- growth of undesirable microorganisms-bacteria, molds, and yeasts
- activity of food enzymes
- reactions with oxygen
- moisture loss
Microorganisms live and multiply quickly on the surfaces of fresh food and on the inside of bruised, insect-damaged, and diseased food. Oxygen and enzymes are present throughout fresh food tissues. Proper canning practices include:
- carefully selecting and washing fresh food
- peeling some fresh foods
- hot packing many foods
- adding acids (lemon juice or vinegar) to some foods
- using acceptable jars and self-sealing lids
- processing jars in a boiling-water or pressure canner for the correct period of time.
|Parts to a pressure canner|
|Boiling water bath canning is used for high acid foods such as tomatoes|
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Collectively, these practices remove oxygen, destroy enzymes, prevent the growth of undesirable bacteria, yeasts, and molds; and help form a high vacuum in jars. Good vacuums form tight seals which keep liquid in and air and microorganisms out.