Canning At Various Altitudes
The summer garden provides a surplus of fruits and vegetables and preserving the fruits of our labors is essential. But much of the vegetables are considered low-acid, with the exception of tomatoes, and therefore much care is needed to safely preserve the surplus. Clostridium botulinum is the bacteria of greatest concerns in home-canned foods. The spores, found in the soil and water, are harmless to us as long as oxygen is available. 
The spores reproduce bacteria cells which can produce the harmful toxin that causes botulism when grown in an air-free environment containing moisture and low-acid foods. The toxin produced affects the nervous system and can cause death. In order to kill the spores, low-acid foods must be heated long enough at a temperature of 240°F. This is only achieved using a pressure canner. Microorganisms on high-acid foods will be killed at 212°F, the temperature achieved by a boiling water bath.
The Pressure Canner
The pressure canner (not a pressure cooker) must be in good working order; seals and gauges should be checked before starting the process. Also, the University of Minnesota Extension recommends using only current (1994 or newer) processing times and methods (see Resources below). You can watch a short but informative presentation on Botulism for a quick and important lesson on safety in canning low-acid foods.Altitude Matters
Most processing times have been developed at altitudes between 0 to 1000 feet and must be altered if you live above 1000 feet. The extra time is needed because water boils at lower temperatures as the elevation increases, so you need to boil longer to kill the microorganisms. Not only will the processing time need to be changed but the pressure achieved by the canner.
Canning Low-Acid Foods
So what are the low-acid foods and how do you determine their acidity? Well a general rule of thumb is that foods below pH 4.6 are acidic and considered safe to process in a water bath. Fruits and vegetables such as tomatoes, apples, plums, blueberries are considered acidic. Some tomatoes can vary in acidity and an acidifying agent such as lemon juice or vinegar should be added. A simple and fast way to determine the pH of the food is to test the juice with litmus paper. The color of the paper will change according to the pH of the juice. This is only a rough guide, but is especially useful for vegetables that may straddle the 4.6 pH fence, like tomatoes. Low-acid foods, higher than pH 4.6, are most vegetables, meats or mixes such as spaghetti sauce, and will need to be processed using a pressure canner. There are some good reliable resources on preserving and canning available, however, the exact processing directions and times should be followed for safety.
Enjoy preserving and canning the fruits and vegetables of your labor, safely!
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