Organisms that cause food spoilage–molds, yeasts and bacteria–are present everywhere in the air, soil and water. Enzymes that may cause undesirable changes in flavor, color and texture are present in raw vegetables.
When vegetables are canned, they are heated hot enough and long enough in the jar to destroy organisms that can make people sick in addition to spoilage organisms. This heating (or processing) also stops the action of enzymes that can spoil food quality.
Pressure canning is the only safe method of canning all vegetables (except tomatoes). Jars of food are placed in a pressure canner which is heated to an internal temperature of at least 240°F. This temperature can be reached only in a pressure canner.
The Clostridium botulinum microorganism is the main reason pressure canning is necessary. Though the bacterial cells are killed at boiling temperatures, the spores they form can withstand very long boiling. In the soils, these bacteria are naturally found in the spore form. The spores grow out well in moist, low-acid foods in the absence of air, such as in canned low acid foods (vegetables and meats). When this happens, the spores change to growing bacterial cells which produce the deadly botulinum toxin (poison). This growth and toxin formation can occur without any noticeable signs of spoilage in the sealed jar.