Microorganisms all around us can cause food spoilage — they are in the air and soil, and on people and animals. Many microorganisms are difficult to get rid of, including Clostridium botulinum — the bacteria that causes botulism. Botulism is rare but can be fatal. Home-processed foods are often the culprit of foodborne botulism.

Clostridium botulinum comes in two forms — vegetative cells and spores. The spores produce the vegetative cells and the vegetative cells produce the toxin that makes us sick. Vegetative cells can be destroyed at boiling temperatures, but spores must be destroyed using temperatures above 240 degrees for a specified amount of time. That temperature can only be achieved in pressure canning.

Clostridium botulinum is why pressure canning is the only safe method for preserving low-acid foods. Low-acid foods include meat, poultry and dairy products, and all vegetables except tomatoes. Acid in foods like tomatoes, fruit and pickles prevents botulinum spores from becoming toxic. (Acid is added to tomatoes in canning to assure safety.)

An improperly processed jar of vegetables is a perfect environment for Clostridium botulinum. If a jar of green beans was processed in a boiling water canner, the jars would seal and the vegetative cells would possibly be destroyed — the spores would still be present. Two things that allow spores to produce are low acid and no air — exactly what the jar of green beans would have. Those conditions would allow the spores to produce vegetative cells and those cells would produce Clostridium botulinum.

People often think that if a food is spoiled, it would smell or look bad. Home-canned foods, however, can contain Clostridium botulinum and look normal. A home-canned food could contain Clostridium botulinum if:

• A low-acid food was not processed in a pressure canner at the right pressure for the right amount of time
• The gauge of the canner was inaccurate
• Ingredients were used from a recipe not tested for canning
• Proportions of foods in a tested recipe were changed

Clostridium botulinum does not grow and produce toxin at 0 degrees F. That means that freezing low-acid foods is an option. Keep in mind, however, that freezing does not destroy the bacteria, so the food should be used promptly after being removed from the freezer.