Canning basics 6: Selecting the correct processing time
In addition to the acidity of the food and the heat resistance of the microorganism, the time required for sufficient heat to penetrate all parts of the food in the jar must be considered. Heat is transferred from the outside of the jar through the food and thus is affected by:
- The size and shape of the container. Smaller jars heat faster than wider or taller jars.
- Amount of liquid. Food containing a large amount of free liquid heats much more quickly than a more solid product.
- Piece size. Smaller pieces of food (corn, peas) heat much more quickly than large chunks.
- Amount of fat. Fat insulates the food and slows heat transfer.
- The type of heating medium being used. Wet steam heats faster than dry air.
The many factors involved make it impossible to estimate the correct processing conditions for any food product. This is especially true for items which are mixtures of food with differing water content, piece size, fat content, or acidity as well as types and numbers of microorganisms present. The establishment of a correct, safe process requires laboratory research by trained scientists.
Individuals who can food at home and do not use methods approved by Minnesota Extension, other state extension services, United States Department of Agriculture (USDA), or other reputable sources (Kerr, Ball, etc.) are taking a great risk with the health of those who eat the canned product.
When canning in boiling water, more processing time is needed for most raw-packed foods and for quart jars than is needed for hot-packed foods and pint jars.
The food may spoil under any of these conditions:
- you fail to add process time for lower boiling-water temperatures at altitudes about 2,000 feet
- you process for fewer minutes than specified
- you cool jars in cold water
- you fail to exhaust canners properly
- you process at lower pressure than specified
- you cool the canner with water
Process times for 1-1/2 pint and pint jars are the same, as are times for 1-½ pint and quart jars. For some products, you have a choice of processing at 5, 10, or 15 PSI. In these cases, choose the canner pressure (PSI) you wish to use and match it with your pack style (raw or hot) and jar size to find the correct process time.
To destroy microorganisms in acid foods, processed in a boiling-water canner, you must:
- Process jars for the correct number of minutes in boiling water
- Cool the jars at room temperature
To destroy microorganisms in low-acid foods processed with a pressure canner, you must:
- Process the jars for the correct number of minutes at 240°F (10 PSI) to 250°F (15 PSI) as the method indicates.
- Allow canner to cool at room temperature until it is completely depressurized.
Canning basics series
Reviewed by Suzanne Driessen 2012