To ensure safe acidity levels in your whole, crushed or juiced tomatoes, you must add bottled lemon juice or food grade citric acid to each jar before processing. For quarts, use 2 tablespoons bottled lemon juice or 1/2 teaspoon citric acid per quart. For pints use 1 tablespoon bottled lemon juice or 1/4 teaspoon citric acid per pint. Food grade citric acid is available at some food stores or drug stores. Do not substitute ascorbic acid for citric acid. Four tablespoons vinegar per quart can be substituted for the lemon juice or citric acid, but it may cause undesirable changes in flavor.
Reference: Canning and Freezing Tomatoes, Pm 638, Iowa State University Extension
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