Canning tomato products - safety guidelines

Add acid to all tomatoes before processing

A high acid level (pH of 4.6 or less) prevents the growth of Clostridium botulinum bacteria which causes botulism. Because many factors impact the acidity level of tomatoes, USDA recommends adding acid to all home-canned tomatoes and tomato products.

Treat heirloom tomato varieties the same as any other tomato. The acidity of heirloom tomatoes is no different from the non-heirloom varieties. Some heirloom varieties are more low-acid than hybrid varieties. Therefore, the same acidification recommendations apply for canning heirloom tomatoes.

Factors that affect acidity level

The acidity level of tomatoes can be affected by many factors, including:

• The variety of tomato.
• Stressful growing conditions.
• Over-mature fruit.

Also, the acidity level of canned tomato products can be affected by:

• Adding low-acid ingredients to tomatoes (such as onion and peppers).
• Making juice versus tomato solids.
• The canning process itself.

Citric acid, lemon juice or vinegar are good choices for added acid

Citric acid

Citric acid is available wherever canning supplies are sold. Canning supply companies like Mrs. Wages® and Ball® have a powder form of citric acid. Some health food stores carry citric acid and you can order it online. Be sure the product you buy is food grade. 

Lemon juice

Use commercially bottled juice. Don't use freshly squeezed lemon juice because the acidity level varies and there is a chance of contaminating the juice from the rind. You can safely use bottled lime juice instead of bottled lemon juice. 

Bottled lemon and lime juice contain sulfites. If you or family members have a sulfite sensitivity or allergy, use citric acid, vinegar or frozen lemon juice (not lemonade) from the grocery store (not homemade). Use the same amounts of frozen juice as bottled lemon juice.

Vinegar

All vinegars will noticeably change the flavor of your end product. You can add a small amount of sugar to help offset the flavor.

Do not use tomato canning tablets; they are ineffective.

Never add thickener before canning tomato products

Every year, home food preservers want to know how to thicken salsa or tomato soup with flour, cornstarch, rice, pasta or cream before canning. Never, under any circumstances, add a thickening product before canning.

Thickening products will change the acidity level of your tomatoes. They can create a thick product that does not allow good heat penetration. As a result, adding thickeners before canning may result in an unsafe product.

To produce a thicker tomato product:

• Try using Italian plum-style or paste tomatoes vs. large slicing tomatoes.
• Thicken salsas by adding tomato paste or by draining off some of the liquid after you chop the tomatoes.

Thicken just before serving

Thicken tomato products with flour, cream, cornstarch, etc. just before serving. If you do thicken your tomato soup mixture, it can be frozen successfully but never canned.