Originally written by Hope Kleine, former SDSU Extension Health Education Field Specialist.
Astonishingly, food preservation has permeated every culture at nearly every moment in time. Ancient man used his climate to determine his food preservation method, such as using the sun to dry out his food, or the ice to keep food frozen. In the present, many continue home food preservation to keep home grown taste through all seasons with safer techniques than those who preceded us.
Water bathing and pressure canning are two common ways to preserve foods by canning. These techniques use heat processing to preserve foods, and which technique you use depends on the acidity of the food. If you are canning a high acid food, you will use the water bath canning method. If you are canning a low acid food, you will use the pressure canning method. Acidity may be natural, as in most fruits, or added, as in pickled food. While low-acid canned foods contain too little acid to prevent the growth of C. botulinum, high-acid foods contain enough acid to block its growth or destroy them more rapidly when heated.
When to Water Bathe
Water bath canning is used for preserving foods with high acid levels such as most tomatoes, pickles, relishes, jams, jellies, and marmalades.
The term pH is a measure of acidity; the lower its value, the more acidic the food. High acid foods have a pH of 4.6 or lower. They include fruits, pickles, sauerkraut, jams, jellies, marmalades, and fruit butters. Although tomatoes are usually considered a high acid food, some have pH values slightly above 4.6, therefore, in the process of preserving tomatoes, they must be acidified to a pH of 4.6 or lower with lemon juice, vinegar, or citric acid prior to processing.
When to Pressure Can
Pressure canning is used to can foods with low acid levels including red meats, seafood, poultry, and low acid vegetables such as okra, carrots, green beans, asparagus, and spinach.
Due to these foods having a natural acidic level that is too low to prevent growth of the heat-resistant spore-forming bacteria (C. botulinum), these foods must be canned in a pressure canner to be safe and destroy vegetative and spore forming pathogens. The United Stated Department of Agriculture (USDA) endorses pressure canning as the only safe method for canning these low acid foods.