Acidity, Heat and Pressure

• Foods canned in a water bath become hot enough for food safety, in general terms, but there's one glaring exception. That's a heat-resistant bacterium called Clostridium botulinum, which secretes the lethal botulin toxin. It can't flourish in the high-acid environment of canned fruits, pickles or many tomato products, but it's perfectly at home in your corn. To protect yourself from botulism, your corn must reach a temperature of 240 to 250 degrees Fahrenheit, well above the boiling point of water. To reach that temperature you'll need a pressure canner, which -- by containing the expansion of the water vapor as it boils -- creates a high-pressure, high-temperature canning environment.

Packing the Corn

• You'll need approximately 20 pounds of whole corn to fill your canner with nine 1-pint jars. Strip the husks and silk, and blanch your ears of corn in boiling water for three minutes to firm them up. Once they've cooled enough to handle, cut the kernels from the ears into a saucepan. Add 1 cup of boiling water and a pinch of salt for every 2 cups of kernels, and simmer them for five minutes. Pack the corn loosely into sterilized Mason jars, topping them up to within an inch of the top with your cooking liquid. Alternatively, for a fresher flavor, you can pack the raw kernels into your jars and top them up with boiling water. Cover the jars with fresh, never-used lids and screw the rings finger-tight to hold them in place.

Preparing the Canner

• Pressure canning starts with a careful safety inspection of the canner. The lid's gasket must be free of debris or cracks, or it won't seal properly. The pressure gauge and steam valves must be free of food residue or -- if you live in a hard-water area -- built-up scale. Once you're satisfied that the canner is safe to use, run 3 to 3 1/2 inches of water into the bottom and insert the canner's rack. Center it over your burner and preheat it until it's too hot to touch but not nearly boiling. Fill the canner with jars of corn. Open the lid's vent or remove the counterweight, depending on your canner, and lock the lid in place.

The Canning Process

• Bring your canner to a boil. Once you see a steady, V-shaped plume of steam escaping or "venting" from the lid, start a 10-minute timer. At the end of those 10 minutes, close the vent -- if it's a gauge-style canner -- or replace the counterweight if it's a rocker-style canner. When the gauge displays the correct pressure, or your rocker is seesawing as described in the owner's manual, begin timing your corn. At sea level, you'll need to process pints of corn for 55 minutes at 10 to 11 psi, or quarts for 85 minutes. If you live at an altitude of 1,000 feet or higher, bring a rocker canner to 15 psi for the same length of time. In a gauge canner, increase the pressure by 1 pound for every 2,000 feet above sea level.

Final Details

• You'll need to adjust the burner's heat to keep your canner at the correct pressure. It must stay at that pressure continuously, so if it dips too low you'll need to bring it back up to pressure and start timing your 55 minutes all over again. Once the time is up, remove your canner from the heat and let it cool naturally until the pressure releases. Most modern models have a lock that will release once the pressure subsides. Remove the lid carefully, avoiding the steam, then lift out the jars onto a rack or towel. Leave the corn to cool overnight, then check each jar for a good seal. Those that didn't seal should be refrigerated and used immediately, and the rest stored in a cool, dark place for up to two years.