Canning season is here, so it is a good time to review the canning of low-acid foods, especially green beans, a food that is commonly canned incorrectly. Low-acid foods are those with a pH above 4.6. The pH of the food is important, as those with a pH above 4.6 allow for Clostridium botulinum to grow in canned foods and produce the botulinum toxin. Meat, seafood, legumes, vegetables — including green beans, corn, carrots, and squash — and mixtures of these foods all have a pH above 4.6. Many other foods, such as tomatoes and white-fleshed peaches, often have a pH above 4.6.
Low-acid foods must be properly processed in a pressure canner because Clostridium botulinum forms spores, a resistant form of the bacteria. The spores survive boiling water temperatures of 212 degree Fahrenheit and can only be killed at higher temperatures around 240 degrees. These temperatures can only be reached by using a pressure canner. If low-acid food is not processed in a pressure canner, bacteria will begin to grow after the food has cooled producing toxin in the oxygen-free environment.
Alternatively, low-acid foods can be acidified through the addition of acid, which prevents bacteria from growing.