Lemon Juice Citric Acid for Canning Tomatoes
Answer - Tomatoes were once considered an acid food that could be safely canned in a boiling-water canner. However, because of the potential for botulism when some newer, less acidic tomato varieties are canned, certain precautions must now be taken.
Add 1 tablespoon bottled lemon juice to pints and 2 tablespoons bottled lemon juice to quarts of tomatoes. Or add one-fourth teaspoon crystalline citric acid to pints and one-half teaspoon crystalline citric acid to quarts of tomatoes. Acid can be added directly to jars before filling. Four tablespoons of 5 percent acidity vinegar per quart may be used instead of lemon juice or citric acid, however, it may cause undesirable flavor changes. Add sugar to offset acid taste if desired. Note: Don't use fresh lemon juice as its acidity varies. Tomato canning tablets should not be used as they are ineffective.
Be sure to read up on home preservation of tomatoes in the Let's Preserve series of fact sheets https://extension.psu.edu/food/preservation/safe-methods/tomatoes