Adding acid to home-canned tomatoes
Home canning is the most popular preservation method for tomatoes but it must be done safely to avoid the risk of a foodborne illness and spoilage. It is very important to follow reliable recommendations exactly when canning tomatoes. Any changes such as adding extra ingredients or overcooking could result in an unsafe product.
Safe home canning of tomatoes involves a certain level of acid where botulism microorganisms cannot grow. 1994 research determined that the safe acidity level is a pH of 4.6 or lower.
Tomatoes used to be considered high-acid but today's newer varieties, over-mature tomatoes, tomatoes from dead or frost-killed vines and tomatoes harvested late in the season may put them at acid level greater than 4.6. To make sure home canned tomato products are at a safe acid level, it is important to add acid to every jar before canning.
Added acid can be in the form of citric acid (available where canning supplies are sold or ordered online) or bottled lemon juice. Fresh-squeezed lemon juice is not recommended because the level of acid is not standardized and there is a chance you will contaminate the juice from the rind or seeds.
Vinegar is not as effective as bottled lemon juice in increasing the acidity. The amount of vinegar needed to increase the acidity of canned tomatoes changes the flavor in plain tomato products and may not be pleasing. However vinegar is acceptable to use in products such as catsup and salsa.
Measure the correct amount of acid into canning jars before adding the tomatoes and processing in the water bath canner or pressure canner:
- To each pint: Add 1 tablespoon of bottled lemon juice or 1/4 teaspoon of citric acid or 2 tablespoons of vinegar
- To each quart: Add 2 tablespoons of bottled lemon juice or 1/2 teaspoon of citric acid or 4 tablespoons of vinegar
To offset the taste of added acid, add a small amount of sugar to each jar. Do not use tomato-canning tables because they are ineffective in creating a safe product.
Revised 2011 by Carol Ann Burtness. Peer reviewed 2011 by Suzanne Driessen.