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.  

I told her it was too bad that she hadn’t realized the lemon juice omission immediately after the jars had come out of the canner. If she had, she could have opened all the jars, dumped the contents into a pot, reheated the tomatoes, and added lemon juice to each jar as she filled it. Then, she could have put the tomatoes through the canning cycle again, using fresh lids.  

Unfortunately, too much time had elapsed; I told her the heartbreaking news: “You’ll have to toss all of it.” There was too much risk that botulism could be lurking in her beautifully canned tomatoes. So, all of that hard work, and the tomatoes, had been wasted.  

This leads me to an important point: Canning, ideally should be avoided when very young children, pets, cellular telephones, television, or other distracting factors are part of the scene. If friends or family members can or want to help, more’s the better, so long as everyone understands that staying focused on the task at hand is an absolute must. I strongly recommend that “team canning” be organized, so that each participant is assigned to perform a couple of very specific tasks in the canning sequence. That helps avoid missed steps in recipes, confusion, and traffic jams in the kitchen! 

Next time, I will share some more insights and helpful tips about the art and science of food preservation. And, I will certainly welcome your comments and questions about this topic.

Mary Moss-Spragueis a certified Master Gardener and Master Food Preserver in Corvallis, Ore., and author of Stand Up and Garden: The No-digging, No-tilling, No-stooping Approach to Growing Vegetables and Herbs. Read all of Mary’s MOTHER EARTH NEWS posts here.

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