This article is from the Food Preserving FAQ, by Eric Decker firstname.lastname@example.org with numerous contributions by others.
Tomatoes and tomato products are very hard to categorize. Tomatoes are
borderline acidic. Lemon juice or other acid (vinegar, citric or ascorbic
acid) must be added to all tomato products to insure adequate acidity. Added
acid is necessary whether the product will be pressure canned or boiling water
bath processed. Bacteria and spores die faster in an acidic environment, and
the recommended process times for pressure canning assume that the tomatoes
are acidic. The times would not be reliably adequate to insure safety if the
tomatoes were low-acid.
Green tomatoes are more acidic, and may be used safely in any recipe calling
for red tomatoes. Overripe and frosted tomatoes are less acidic and can not
be safely home canned. They can be frozen.
Addition of salt, while optional, does give a miniscule margin of safety. For
dietary information, one teaspoon of salt added to 1 quart of juice or sauce
adds about 526 mg sodium per cup.
Addition of low-acid vegetables to tomatoes decreases the acidity. The
amount by which the acidity is lowered depends on which vegetables, how much,
how finely they are chopped, if they are boiled in the tomatoes or not, if
seeds and skins remain in or are removed, and if the chunks of vegetable and
tomato remain, if they are ground together or sieved out. The initial
acidity of the vegetables and tomatoes depends on maturity, growing conditions,
post-harvest holding conditions, and soil/location of growth.
It is impossible to test every recipe. It has so far been impossible to
develop a set of proportions or an equation that would take into account all
the variables and give a reliable assessment of the acidity or the necessary
process times. The only safe recommendations can be made by comparing the
recipe in question with the USDA guide recipes. If more vegetable or less
acid (vinegar or lemon juice) is added than the USDA recipe, the recipe in
question can be changed or the product should be processed according to the
process times for the vegetables. Alternatively the product may be frozen
These proportions of vegetables have processing times in the USDA Home
Tomato-vegetable juice 22 lb tomato : 3 C chopped vegetable Spaghetti sauce 30 lb tomato : 8 C vegetable : no acid Ketchup #1 24 lb tomato : 3 C onion : 3 C vinegar Ketchup #2 24 lb tomato : 1 C peppers : 2.6 C vinegar Ketchup #3 24 lb tomato : 9 C vegetables : 9 C vinegar