Prepared by Pamela Schmutz, HGIC Food Safety Specialist, and Susan Barefoot, Professor Emerita of Food Microbiology, Clemson University. (New 8/11.)
The acidity, or pH, of foods determines how they must be processed for canning. Acid foods such as fruits and pickles with a pH of 4.6 or lower may be canned in a water bath canner. Low-acid foods such as vegetables and meats with a pH above 4.6 must be processed in a pressure canner. Clostridium botulinum bacteria are the main reason why low-acid foods must be pressure canned to be safe. Clostridium botulinum spores can survive boiling water (212 °F) and grow in a sealed jar of low-acid food. The spores can change into the vegetative cells that produce the deadly botulinum toxin. You must use a pressure canner to raise the temperature to the desired 240–250 °F to destroy the spores during the canning of low-acid foods. Some foods, such as figs and tomatoes, may be processed as acid foods, but because they may have pH values slightly above 4.6, lemon juice or citric acid must be added before canning.
You do not need to know the pH of a food, but you must use a tested canning recipe based on the pH value of a food and other factors. In addition to pH, factors such as heat transfer properties, food particle size, food viscosity, and container size, influence the choice of an appropriate canning process. For this reason, use only tested recipes that consider all these factors. Sources of tested recipes include Clemson University’s Home & Garden Information Center at http://www.clemson.edu/hgic and the National Center for Home Food Preservation at http://www.uga.edu/nchfp/.
Considerable variation exists between varieties, condition of growing and processing methods. The pH values shown are for the edible portion of foods in their normal and natural state, unless indicated otherwise.
Fruits and Vegetables—with pH values near or above 4.6—must add acid according to tested recipes to bring pH to 4.6 or below:
Low-Acid Fruits—must pickle to lower the pH and process in a water bath canner:
Acid Fruits—may process in a water bath canner:
Meat, Poultry and Seafood—must process in a pressure canner
Low-Acid Vegetables—must process in a pressure canner or pickle to lower the pH:
|Artichokes, French (cooked)||5.60–6.00|
|Artichokes, Jerusalem (cooked)||5.93–6.00|
|Beans, pork & tomato sauce (canned)||5.10–5.80|
|Peas, Chick, Garbanzo||6.48–6.80|
|Squash, yellow (cooked)||5.79–6.00|
|Turnip Greens (cooked)||5.40–6.20|
Adapted from USDA Complete Guide to Home Canning 2009 (Bulletin No. 539)
For more information on canning foods safely, see HGIC 3025, Choose the Right Canner for Home Canning, and HGIC 3040, Canning Foods at Home.
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This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.