Adding acid to canned tomatoes

August 30, 2005 (Updated December, 2007)
Carol Ann Burtness
Regional Extension Educator, Food Science
888-241-0720
Email: burtn002@umn.edu

You can preserve tomatoes by canning, drying, freezing, pickling or creating fruit spreads such as jams and marmalades. Whatever preservation method you use with tomatoes, it must be done properly to avoid spoilage and the risk of food poisoning. Creating your own method and recipes could result in an unsafe product.

Tomatoes require a certain level of acid for safe home canning. Some 1994 research determined that the safe level is a pH of 4.6 or lower. If the product has a higher pH level, the production of botulism toxins becomes an issue. Tomatoes used to be considered high-acid but new varieties, over-mature tomatoes, tomatoes from dead or frost-killed vines and tomatoes harvested late in the season may put them at a pH greater than 4.6.

To make sure whole, crushed, or juiced tomatoes are at a safe acid level, it is very important to add acid. Added acid can be citric acid (available where canning supplies are sold or ordered online) or bottled lemon juice. Do not use freshly squeezed lemon juice because the level of acid is not standardized; there's also a chance you will contaminate the juice from the rind. To offset the taste of added acid, add a small amount of sugar to each jar.

Small amounts of vinegar are not as effective as bottled lemon juice in increasing the acidity. Enough vinegar to increase the acidity of canned tomatoes will change the flavor in plain canned tomatoes or juice. However vinegar is acceptable to use in products such as catsup and salsa.

Measure the correct amount of acid into canning jars before sealing:

• To each pint: Add 1 tablespoon of bottled lemon juice or 1/4 teaspoon of citric acid or 2 tablespoons vinegar.


• To each quart: Add 2 tablespoons of bottled lemon juice or 1/2 teaspoon of citric acid or 4 tablespoons vinegar.

Low-acid ingredients such as carrots, celery, corn, green peppers, mushrooms or onions will decrease the acidity level of your tomatoes. Don't add more of these ingredients than called for in the tomato mixture, salsa or vegetable soup recipe.

Never add a thickening product such as flour, cornstarch, rice or pasta to tomatoes before canning. These products will also change the acidity level of your tomatoes. You may thicken tomato products before serving. If you do thicken your tomato soup mixture, it can be frozen successfully but never canned.

Open-kettle canning of tomatoes is very unsafe. This method involves packing hot tomatoes in jars and sealing them without any further heat processing. As a result, microorganisms may survive and the product can spoil or become harmful to eat.

For more information on preserving tomatoes, see the University of Minnesota Extension Food Safety website at http://www.extension.umn.edu/foodsafety and click on "Food Preservation."

(Carol Ann Burtness is a food science educator with the University of Minnesota Extension Regional Center, Brainerd.)