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How Your Grandmother’s Home Canning Recipe Could Mean Death

Timothy Boyer Ph.D.'s picture
Canning food warnings.

Thinking about canning produce this year? Here is one home canning mistake that could mean death for the first-time canner.


Why Canning

One of the joys of growing your own produce is not only watching the fruits of your labors develop over time, but also turning your produce into meals that beat restaurant-quality taste. This is not an exaggeration. While cooking shows may reveal culinary tricks of the trades used by chefs, taste relies primarily on the foundation of the plants in your garden.

As I’ve revealed in an earlier article, the secret to the best tasting vegetable soup lies in your tomatoes. While fresh, truly vine-ripened tomatoes are typically not available to everyone everywhere throughout the year, the home-canned ones are an excellent option that are reliable, available, and just as tasty.

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Outdated Canning Information Warning

That said, a recent article in Mother Earth News about home canning cardinal rules for first-time food preservers caught my attention because it made a good point about the danger of using outdated information when it comes to safe canning.

Canned Foods, BPA and Safety Tips

One of the resources many kitchens carry are old cookbooks and family recipes passed down from well-meaning family members. However, what you have to keep in mind is that the conditions under which safe-cooking occurs can and do change over time.

According to Mother Earth News writer Mary Moss-Sprague, one of the cardinal canning rules of safe canning is to be aware of the importance of acidity in canning:

“Many newer food preservation books are on the market now, but I will confidently recommend the always-reliable Ball Blue Book publications for sound recipes and instructions. If it's more than five years out of date, I'd strongly recommend replacing it with a current edition.

The chemistry within vegetables changes with hybridization and other genetic factors, which influence the natural acidity, so recipes and processing times may be different from before.”

That last statement about how “…hybridization and other genetic factors could influence the natural acidity…” caught my attention, making me wonder if this was really true. Are tomatoes today any different in their pH levels than they were during yesteryear?

My practice during canning was to always follow the guideline of adding 1 tablespoon of commercial lemon juice to every pint of tomatoes canned. The reason for this is that most produce is not acidic enough on its own when canned to prevent microorganism survival and/or growth that can lead to botulism poisoning. But some people believe that you don't need to add lemon juice, if you can with the more-acidic varieties of heirloom tomatoes.

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How Low Does a Tomato Go?

So, just how acidic are tomatoes without the added lemon juice?

A North Dakota State University Food & Nutrition publication about why you should add lemon juice to tomatoes before canning tells us that a pH of 4.6 or lower is required for safe canning without the use of pressure processing. While pressure cooking is surer way to kill off microorganisms during canning, many people rely on a hot water bath method in a large pot to can tomatoes, but need the lower acidity to can safely. With pressure cooking, the addition of lemon juice is also recommended as a safe practice.

In the publication, they show that out of 14 varieties of mixed modern and heirloom tomatoes tested, their individual pH values ranged from 4.93 to 5.20—too high above the required minimum safe pH value of 4.6. Their proviso for canning tomatoes in a hot water bath then was that the tomatoes regardless of their variety do need added lemon juice to lower the pH to safe levels for preserving.

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Another Gardening Myth Bites the Dust

Digging a little deeper into the pH question, it turns out that there is a gardening myth where some canners claim that the tomatoes your grandmother canned were more acidic than today’s varieties; And therefore, grandma didn’t add lemon juice to her tomatoes when canning.

An explanation about the “truthiness” of this myth touches on the North Dakota State University Food & Nutrition publication (as well as some other studies) and is well-described in the Healthy Canning website that addresses the question whether heirloom tomatoes are more acidic.

The key points they make is that aside from studies showing that the actual pH levels of heirloom tomatoes are all over the pH map, other factors influence the safety and acidity of canned tomato products, such as that:

• Tomato juices are less acidic than tomato solids.

• One or more over-ripe tomatoes in a jar will decrease the overall acidity. Choosing to can tomatoes off of a dead vine is a bad practice; Ripeness, decay and bruising result in elevated pH values above a safe level.

• Adding low-acid ingredients like onions, celery, garlic, peppers will decrease the acidity.

• Canning itself can decrease acidity.

• Older tomato varieties were if anything, prone to be less acidic than modern varieties. Tomatoes, old or new varieties, are just not reliably always on the low side of the 4.6 safety divide.

• People were canning tomatoes without any added acidification “back in the day”; however, it is likely many more unreported cases of food poisoning occurred due to unsafe practices than what we hear about today.

Fake Gardening News: Bee-lieve it or Not

Lastly, the Mother Earth News article warning that “…recipes and processing times may be different from before,” is true and applicable today. A comparison of recommendations for canning time processing under heat has increased remarkably longer today than it was back in our parents’ and grandparents’ generations.

So, the point here is that it may be a good idea to take a critical look at that old canning recipe whether it’s from a dated cookbook or a 3” x 5” passed from one generation to the next, and see how it compares to recommended cooking practices today.

If you have an old family recipe passed on from your relatives that you find questionable, share it with us in the comments section below.

Timothy Boyer has a Ph.D. in Molecular and Cellular Biology from the University of Arizona. For 20+ years he has been employed as a freelance health and science writer. Today, with a background in farming and an avid home gardener, Timothy continues writing about science with a focus on the connection between plant biology and gardening for healthy living. For continual updates about plants and health, you can also follow Timothy on Twitter at TimBoyerWrites.


"Home Canning Cardinal Rules for First-Time Food Preservers” by Mary Moss-Sprague; Mother Earth News 4/9/2020

Why Add Lemon Juice to Tomatoes and Salsa Before Canning?” North Dakota State University Food & Nutrition publication, reviewed Jan. 2018.

"Heirloom Tomatoes: Were They Really More Acidic?" Health Canning website.