General Canning Information

How Canning Preserves Foods

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.

This document was extracted from the "Complete Guide to Home Canning," Agriculture Information Bulletin No. 539, USDA (Revised 1994).

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