The safety of canning tomatoes primarily rests on the pH, or acidity, of the tomatoes themselves. Some myths state that older tomato varieties, or heirloom varieties, are more acidic than todays modern tomatoes. But research has shown that this is not true.
Research conducted at three universities, North Dakota State University, University of Illinois, and Utah State University, show that many varieties of heirloom tomatoes are actually less acidic than todays tomatoes. To be considered a high acid food, the pH needs to be below 4.6. Many are above that pH level and into the 5.0 or above pH range.
This is why, no matter what variety of tomato you preserve, it is important to add acid in the form of citric acid, bottled lemon juice or vinegar to can them safely. See instructions at www.bookstore.ksre.ksu.edu/pubs/MF1185.PDF.
Sources: North Dakota State University Extension; Utah State University; and University of Illinois
