Time, Temperature, and Pressure in Canning Foods (2024)

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Determinants of Time, Temperature, and Pressure

Food Acidity

• Foods are classified as high or low acid based on their pH, a scientific measurement.
• Higher pH numbers represent lower acidity in the food. Lower pH numbers represent higher acidity in the food. The dividing line for acidity in foods is 4.6.
• Foods with a pH of 4.6 or lower are considered high-acid foods. This includes most fruits and pickled products.
• Foods with a pH of 4.6 or above are low-acid foods which include vegetables, meats, and combination foods such as soups.
• High-acid foods may be safely canned in boiling water or atmospheric steam because their acidity will not support the growth of spores that cause botulism.
• Low-acid foods must be processed at higher temperatures in a pressure canner to destroy C. botulinum spores that can produce a deadly toxin under the right conditions.
• Basically, the more acidic the product—the shorter the processing time and the lower the temperature that is needed. For example, most pickles and relishes are processed in a boiling water bath (212°F) for 10 to 20 minutes, but canned meat is processed for 75 (pints) to 90 (quarts) minutes in a pressure canner (240°F). High-acid fruits take longer than pickles but less time and heat than meats.

Temperature

• Water boils at 212°F. It does not matter how long water boils; it will still only reach a temperature of 212°F—water will evaporate but that does not change the temperature.
• However, if you add pressure to that water in a closed container, you increase the temperature above the boiling point of water at sea level (212°F). The more pressure you add, the more heat you can add to raise the temperature even higher.
• 240°F is the temperature needed to destroyClostridium botulinumspores. While high-acid foods will not support the germination of these spores, they will reproduce in low-acid foods. Therefore, vegetables, meats, soups, and other low-acid combination foods must be processed in a pressure canner for safety.

Altitude Lowers Temperature

Water boils at 212°F at sea level. It boils at a lower temperature as altitude increases. As a result, you need to increase the time or the temperature to kill microorganisms as altitude increases.

Canning recipes have been designed so that time is increased for canning foods in a boiling water canner or an atmospheric steam canner.

• For fruits, this is usually 5 additional minutes for altitudes between 1,001 and 3,000 feet, 10 additional minutes for altitudes between 3,001 and 6,000 feet, and 15 additional minutes at altitudes over 6,001 feet.

In a pressure canner the time remains the same, but the pounds of pressure is increased.

• For canning vegetables and other low-acid foods in a weighted gauge pressure canner, the food is processed at 15 pounds pressure at altitudes above 1,000 feet.
• In a dial gauge canner, the pressure is not increased between 1,001 and 2,000 feet—use 11 pounds pressure. At altitudes of 2,001 and 4,000 feet, process at 12 pounds pressure, at altitudes of 4,001 and 6,000 feet process at 13 pounds pressure, and at altitudes of 6,001 and 8,000 feet process at 14 pounds pressure.
• Do you know the altitude where you live?

Inside the Pressure Canner

Two things that affect processing inside a pressure canner are air inside the canner and inaccurate dial readings. It is necessary to vent the canner for 10 minutes to drive the air out of the canner before applying the pressure regulator or weight because air will transfer heat more slowly into the jars than steam. If air is not removed from the canner, the jars may not reach the required temperature in the allotted time. If a dial registers high, the temperature inside the canner will be lower than what you expect. This lower temperature could allow dangerous bacteria to survive processing. Have dial gauges checkedfor accuracy each year by your local Penn State Extension office.

Hot is Better

• Jars filled with hot food should always go into the canner hot.
• That is why directions recommend filling one jar at a time, sealing it, and placing it in the canner before filling the next jar.
• Only prepare one canner load at a time.
• As food sits in jars waiting to go into the canner the temperature decreases as the food cools.

Remember

Follow scientifically tested recipes to determine the process time and whether to water bath or pressure can the food. Penn State Extension Let's Preserve series, USDA Complete Guide to Home Canning and the National Center for Home Food Preservation provide reliable sources of tested recipes.