Boiling water baths should only be used for high acid foods having a pH below 4.6, such as many fruits, pickled products, and tomatoes. Tomatoes are borderline, most having a pH of 4.2 to 4.5, but when you add large amounts of low acid ingredients such as onions, peppers, celery, garlic, corn, beans, etc., the pH rises to a level that could allow botulism spores to hatch and grow. While botulism is rare and there are usually less than 10 cases per year from home canned foods, these cases are usually related to improperly processing low acid foods.
C Botulinum spores, which are commonly present in most soils, can survive 20 hours of boiling, but are killed in only 12 minutes at 10 pounds of pressure (240°F) and 3 minutes at 15 pounds of pressure (250°F). Mind you this means the entire contents on the jar must be at those temperatures, which means the jars must be precessed for a longer time to get the center of the jar to the right temperature. There is no rule of thumb as to how long as it varies with altitude, the density of the food being canned, chunk size, thickness, etc.
Surviving spores after processing are harmless so long as they don't hatch - it is the growing bacteria that creates the toxin. In order to hatch the spores need water, food, a temperature between 38 and 140°F, a lack of oxygen, and a pH above 4.6 (low acidity). All of these conditions are present in canned low acid foods. So the name of the game is either kill the spores with pressure cooking, or prevent their hatching with high acidity, or both.
Botulinum growth does not necessarily cause the lids to lose vacuum or change the smell or taste of the product, but it usually occurs with other mold growth that does. If you choose to can such products in a boiling water bath, be sure to examine the contents carefully and do not eat the contents straight out of the jar. If botulism toxin does develop, it is easy to destroy the toxin by boiling for 20 minutes, so you should boil the contents before eating.
I processed my salsa in a boiling water bath for over 30 years without issue, but a pressure canner is safer when large quantities of low acid ingredients are added to tomato products. The addition of vinegar, lemon or lime juice, or citric acid also improves safety by lowering the pH. After significant research I now can my salsa in a pressure cooker and add 1/4 teaspoon of citric acid to each pint (citric acid has less effect on flavor than vinegar or lemon juice). I did not notice any change in the flavor or texture of my salsa.
There are detailed instructions on canning in the Ball Blue Book Guide to Preserving, or online at
http://nchfp.uga.edu/publications/pu...ions_usda.html. Some studing on this subject is prudent.
TomNJ