Water Bath Canning and Pressure Canning: Explained

Image Gallery Don't let a little steam and boiling water scare you. Pressure canning and water bath canning are safe methods to preserve your food. Learn how with this excerpt from the USDA Complete Guide to Home Canning. PHOTO: FOTOLIA/JAMES STEIDL

If you’re new to canning, deciding between water bath (boiling-water) canning and pressure canning can be a difficult task. Water bath canning seems easier, and works for fruits and pickles, but can’t be used for low-acid foods such as red meat and vegetables (except most tomatoes). Pressure canning, on the other hand, can be intimidating to novices, particularly if you’ve heard the old tales of exploding canners. Fortunately the United States Depart of Agriculture publishes its Complete Guide to Home Canning, a detailed guide for canning newbies and veterans alike. In the following excerpt, the USDA gives helpful explanations of water bath and pressure canning, how to perform each while avoiding common pitfalls, as well as which foods require which method. For more canning tips, check out Jackie Clay’s article The ABCs of Canning and Roberta Bailey’s Learn to Can for Homegrown Flavor. Before you know it, there will be nothing you can’t can.

Food Acidity and Processing Methods 

Whether food should be processed in a pressure canner or boiling-water canner to control botulinum bacteria depends on the acidity of the food. Acidity may be natural, as in most fruits, or added, as in pickled food. Low-acid canned foods are not acidic enough to prevent the growth of these bacteria. Acid foods contain enough acid to block their growth, or destroy them more rapidly when heated. The term “pH” is a measure of acidity; the lower its value, the more acid the food. The acidity level in foods can be increased by adding lemon juice, citric acid, or vinegar.

Low-acid foods have pH values higher than 4.6. They include red meats, seafood, poultry, milk, and all fresh vegetables except for most tomatoes. Most mixtures of low-acid and acid foods also have pH values above 4.6 unless their recipes include enough lemon juice, citric acid, or vinegar to make them acid foods. Acid foods have a pH of 4.6 or lower. They include fruits, pickles, sauerkraut, jams, jellies, marmalades, and fruit butters.

Although tomatoes usually are considered an acid food, some are now known to have pH values slightly above 4.6. Figs also have pH values slightly above 4.6. Therefore, if they are to be canned as acid foods, these products must be acidified to a pH of 4.6 or lower with lemon juice or citric acid. Properly acidified tomatoes and figs are acid foods and can be safely processed in a boiling-water canner.

Botulinum spores are very hard to destroy at boiling-water temperatures; the higher the can­ner temperature, the more easily they are destroyed. Therefore, all low-acid foods should be sterilized at temperatures of 240 to 250 degrees Fahrenheit, attainable with pressure canners operated at 10 to 15 PSIG. PSIG means pounds per square inch of pressure as measured by gauge. The more familiar “PSI” designation is used hereafter in this publication. At temperatures of 240 to 250 degrees, the time needed to destroy bacteria in low-acid canned food ranges from 20 to 100 minutes. The exact time depends on the kind of food being canned, the way it is packed into jars, and the size of jars. The time needed to safely process low-acid foods in a boiling-water canner ranges from 7 to 11 hours; the time needed to process acid foods in boiling water varies from 5 to 85 minutes.