Here is the scoop from the land of 10,000 lakes university... Has some pretty good explanations.... there are others on the web.... I usually read three to make sure the info they put out is correct.... You never know when a typo will make you ill or worse.......
http://www.extension.umn.edu/distribution/nutrition/components/dj0516section1.html
Food Acidity and Processing Methods
Whether food should be processed in a pressure canner or a boiling-water bath to control botulism bacteria depends on the acidity in the food. Acidity may be natural, as in most fruits, or added, as in pickled food. Low-acid canned foods contain too little acidity to prevent the growth of these bacteria. Acid foods contain enough acidity to block their growth, or destroy them more rapidly when heated. The acidity level in foods can be increased by adding lemon juice, citric acid, or vinegar.
The term "pH" is an index of acidity. The lower its value, the more acid in the food. Low-acid foods have pH values higher than 4.6. They include red meats, seafood, poultry, milk, and all fresh vegetables except for most tomatoes. Most mixtures of low-acid and acid foods also have pH values above 4.6 unless enough lemon juice, citric acid, or vinegar is included to make them acid foods. Acid foods have a pH of 4.6 or lower. They include fruits, pickles, sauerkraut, jams, jellies, marmalades, and fruit butters.
Although tomatoes usually are considered an acid food, some are now known to have pH values slightly above 4.6. Figs also have pH values slightly above 4.6. Therefore, if they are to be canned as acid foods, these products with unknown pH must be acidified to a pH of below 4.6 with lemon juice or citric acid. Properly acidified tomatoes and figs are acid foods and can be safely processed in a boiling-water bath. Processing acid foods at boiling water temperatures will destroy yeast and molds, the most common forms of spoilage microorganisms in these foods. Heat-sensitive bacteria are also killed. Those that are heat resistant, such as C. botulinum spores, are prevented from multiplying because of the high acid conditions of the food.
Botulism spores are very heat resistant. They may be destroyed at boiling water temperatures, but extremely long times are required. The higher the canner temperature, the more easily and quickly they are destroyed. Therefore, all low-acid foods should be sterilized at temperatures of 240° to 250°F, attainable with pressure canners operated at 10 to 15 PSI. PSI means pounds per square inch of pressure as measured by a gauge. At these temperatures, the time needed to destroy bacteria in low-acid canned food ranges from 20 to 100 minutes. The exact time depends on the kind of food being canned, the way it is packed into jars, and the size of jars. The time needed to safely process low-acid foods in a boiling water canner ranges from 7 to 11 hours. Such long processing times are not researched and are
not recommended. Losses in nutrients and quality would be unacceptable. The time needed to process acid foods in boiling water varies from 5 to 85 minutes.
Acidity of foods helps determine the type of heat processing or home canning required for safe preservation.