Originally Posted by SRL
Acidic foods, like pure tomatoes, may be processed at a lower temperature in a boiling water bath. Less acidic foods require higher processing temperatures, hence a pressure canner.
Another slight correction. Not all today's tomatoes are NOT acidic and all published recipes for canning tomatoes require the addition of bottled lemon juice in order to bring the pH up as a safety measure because we have no way of testing which are which. Many recipes now actually will Pressure Can tomatoes, especially since they found out they actually taste better from that method.
Check out this link to find the pH for all kinds of foods, and of course those tomatoes. Acidic is 4.6 pH or lower. Higher numbers means it is low-acid.
US FDA/CFSAN - Approximate pH of Foods and Food Products