Canning Tomatoes: Acidity and Other Concerns

Reader Contribution by Mary Moss-Sprague
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All around the country, this summer’s tomato crop is outstanding. This will likely lead to more canning of these garden yummies. While canned tomatoes on the shelf is a comforting sight, a few words of canning caution are still worthwhile.

A recent article on this site discussed the safety of old canning recipes and techniques. One of the main reasons why “Grandma’s Favorite” or recipes older than about five years shouldn’t be used is that chemical changes have occurred due to newer vegetable varieties being developed, particularly in tomatoes. Many varieties have lost their acidity through modern genetic tweaking and hybridizing. So, it’s necessary to add acid to tomatoes — including heirloom varieties — when canning them. Lemon juice is considered adequate and effective for this purpose and must always be used for water-bath and steam-pressure canning.

If you forget to add lemon juice, you’ll be in trouble. This was a hard lesson learned by one of my master gardener colleagues. During the prior day, she had harvested her crop and processed some dozen or more jars of tomatoes to be used for making sauce. Then she telephoned me in a panic, saying that she just realized she’d forgotten to add lemon juice due to a distraction. 

The jars had all gone through the prescribed canning cycle, had sealed correctly, etc. “Are these going to be okay?” she asked.  

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