Pressure canning basics

01 — THE PH LINE

Why pressure matters for low-acid foods

The single most important concept in home canning safety is the pH 4.6 line. Foods with a pH below 4.6 (high-acid foods like fruits, pickles, and tomatoes with added acid) can be safely processed in a boiling-water canner. Foods with a pH above 4.6 (low-acid foods like vegetables, meats, poultry, and seafood) must be pressure canned.

The reason is Clostridium botulinum. This bacterium's spores survive boiling water temperatures (212 degrees F at sea level) but cannot grow in acidic environments. In low-acid foods processed without pressure, spores can survive, germinate in the sealed anaerobic jar, and produce botulinum toxin. The toxin is odorless, colorless, and lethal in microscopic amounts.

A pressure canner reaches 240 degrees F at 10 pounds of pressure at sea level. This temperature destroys C. botulinum spores. No amount of extra time in a boiling-water canner can reach this temperature.

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02 — ALTITUDE ADJUSTMENT

The most common canning safety error

Water boils at lower temperatures as elevation increases. At 5,000 feet, water boils at about 203 degrees F instead of 212 degrees F. This means the pressure inside a canner at a given gauge reading produces less heat at altitude. To compensate, you increase the pressure, not the processing time.

Weighted-gauge canners offer only three settings: 5, 10, and 15 pounds. The USDA's Complete Guide to Home Canning requires moving from 10 to 15 pounds above 1,000 feet. There is no in-between setting.

Dial-gauge canners allow finer adjustment. The USDA table increases pressure by 1 pound for each 2,000-foot band: 11 pounds at 2,001-4,000 feet, 12 pounds at 4,001-6,000 feet, and so on up to 15 pounds above 8,000 feet. Dial gauges should be tested annually for accuracy at your local extension office.

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03 — THE COMMON MISTAKES

Where people go wrong

Using untested recipes. Only use recipes tested by the USDA, university extension services, or the National Center for Home Food Preservation. Blog recipes, family recipes, and old cookbooks may use outdated processing times or unsafe methods.

Confusing water-bath and pressure-canning adjustments. For water-bath canning, altitude is compensated by adding processing time. For pressure canning, altitude is compensated by increasing pressure. Mixing these up is dangerous.

Omitting the venting step. Before pressurizing, the canner must vent steam for 10 minutes to expel air. Air trapped in the canner reduces the actual temperature reached at a given pressure reading.

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