At the University of Az Cooperative Extension offices, we occasionally have workshops to teach about canning methods. There are two accepted methods for canning foods at home: the boiling-water method and the pressure canning method, which can achieve higher processing temperatures. Knowing which canning method to use is determined by the type of food being canned. For the purpose of canning, foods are divided into two categories: acid foods and low-acid foods. When canning acid foods, the boiling-water method is safe. When canning low-acid foods, a pressure canner must be used.
Acidity is the first consideration when canning. The boiling water canner is used with acidic foods (pH 4.6 or below). A pressure canner is used for low-acid foods (pH above 4.6).
Acidity can be natural, as in most fruits, or added as in pickled foods. Low-acid canned foods contain too little natural acidity to prevent the growth of heat-resistant, spore-forming bacteria. The acidity level in acid foods inhibits the growth of these bacteria.
Foods with adequate acid are processed in a boiling water canner. They include most fruits, pickles, fully fermented sauerkraut, jams, jellies, marmalades and fruit butters. Although tomatoes usually are considered an acid food, some have pH values slightly above 4.6. Tomatoes require acidification with lemon juice, vinegar, or citric acid before canning. Figs and Asian pears also require acidification before processing in a boiling water canner.
Low-acids foods have a pH above 4.6. These include meats, poultry, fish seafood, and all fresh vegetables except for most tomatoes. Most mixtures of low-acid and acid foods have pH values above 4.6, unless the canning recipe contains sufficient vinegar or lemon juice to reduce the pH below 4.6. Tomato and chili salsas are a mixture of low-acid and acid foods and must have sufficient amount of vinegar or lemon juice added to safely use the boiling water method.
Heat processing destroys pathogenic and spoilage microorganisms sufficiently to allow long-term storage of food at room temperature.
• Boiling water temperatures are sufficiently high for canning foods that are naturally acid enough and those that have been properly acidified.
• Pressure canning is required to safely process foods that may support the growth of spore-forming bacteria.
• The canning process vents oxygen from the jar, creating an airtight vacuum seal upon cooling.
• Absence of oxygen prevents the growth of molds and some yeast, but invites the growth of anaerobic bacteria.
Additional ingredients such as salts, sugars, and acids are secondary measures beyond heat that can enhance the safety of home-canned foods.
Resources:
• The University of Arizona, Cooperative Extension in Apache County at 928-337-2267
• The University of Arizona, Cooperative Extension in Navajo County at 928-524-6271
• National Center for Home Food Preservation https://nchfp.uga.edu/how/freeze.html
• The USDA Complete Guide to Home Canning can be downloaded at http://www.uga.edu/nchfp/publications/publications_usda.html
It is sold in print form by Purdue Extension: The Education Store, located at https://mdc.itap.purdue.edu/item.asp?item_number=AIG-539
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