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.
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.