The safety of home-canned foods depends on the pH, or acidity, of the product and the heat process that is used. Before any changes are made to a tested recipe, it is important to understand the impact the change will have on either the pH or the processing time and temperature.
Acid foods, such as many fruits and pickled products, have a pH of 4.6 or below and may be safely canned in a boiling water bath or an atmospheric steam canner* at roughly 212 F. The acid in the recipe will prevent the germination of spores of Clostridium botulinum. When canning low acid foods (those with a pH above 4.6), high temperature is needed to destroy the spores of Clostridium botulinum. The process temperature when canning low-acid foods ranges between 240 F and 250 F for sufficient time to destroy botulinal spores.
Some factors that impact the processing time and temperature when canning include:
- Density or thickness of the food. Many ingredients such as sugar, salt or starch, or even the type of ingredients (starchy lima beans versus non-starchy green beans) may impact the viscosity (thickness) of a food and alter the rate of heat penetration.
- Size, shape and temperature of food pieces. It takes longer for acid to penetrate into a whole cucumber versus a cucumber slice. Heat penetration is different in a jar of raw-pack versus hot-packed meat.
- Size and shape of jars or containers. Both the size and the shape of the jar impact the rate of heat penetration into the container. Do not use a jar size larger than what is recommended in recipes. You may use a smaller size. For example, you may use a 12-ounce jar in place of a pint (16-ounce) jar and use pint processing recommendations.
*Steam Can It Right! Guidelines for Safely Using a Steam Canner for Home Food Preservation. North Central Food Safety Extension Network (NCFSEN).