Add acid to tomatoes before processing
Today, our current varieties of tomatoes are not as high in acid as they used to be. Today, researchers have found that new tomato varieties; stressful growing conditions; or over-mature fruit may affect acid levels. Other factors that affect the acidity level of tomatoes include adding low-acid ingredients to tomatoes such as onion and peppers; making juice versus tomato solids; and the canning process itself. Because many factors impact the acidity level of tomatoes and may support the growth of Clostridium botulinum, USDA recommends adding acid to all home-canned tomatoes and tomato products.
Today’s canning recommendations require that acid be added to all canned tomato products even if they are pressure canned. Horticulture researchers have concluded the acidity of heirloom tomato plants is no different from the non-heirloom varieties. In fact, there are some heirloom varieties that are more low-acid than hybrid varieties. As a result, the same acidification recommendations apply for canning heirloom tomatoes
Acid can easily be added directly to the jars before filling with tomatoes. Added acid can be either citric acid or bottled lemon juice. Citric acid is available where canning supplies are sold or ordered online.
citric acid or bottled lemon juice.
- Add 2 tablespoons of bottled lemon juice or 1/2 teaspoon of citric acid per quart of tomatoes.
- For pints, use 1 tablespoon of bottled lemon juice or 1/4 teaspoon of citric acid.
- Instead of the lemon juice or citric acid, vinegar can be used, but it will cause a more noticeable flavor change. Add 4 tablespoons of vinegar per quart or 2 tablespoons of vinegar per pint.
- Do not use freshly squeezed lemon juice because the acidity level varies and there is a chance of contaminating the juice from the rind.
- Bottled lemon and lime juice contain sulfites. If you or family members have a sulfite sensitivity or allergy, use citric acid or vinegar or substitute frozen lemon juice (not lemonade) that you find in the grocery store frozen section—use same amounts as bottled lemon juice.
- Tomato canning tablets that may be found on the market should not be used because they are ineffective.
|Citric acid||Little change in flavor||1/2 tsp/quart
|Bottled lemon juice||Easy to use||2 Tbsp/quart
|Vinegar (5% acidity)||Noticeable flavor change||4 Tbsp/quart
For more information, check out National Center for Food Preservation
Home Food Preservation Newsletter. July 2012. University of Minnesota Extension.
- Adding Acid to Home-Canned Tomatoes
- How to Freeze Homemade Stewed Tomatoes
- To Thicken or Not to Thicken Canned Tomato Products
Revised by Suzanne Driessen 2017