How to Test Acidity for Canning

Test the acidity of your batch for water bath canning to ensure that you are meeting the requirements for high acid foods.

Reader Contribution by Ilene White Freedman
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by AdobeStock/Yaruniv-Studio

Test the acidity of your batch for water bath canning to ensure that you are meeting the requirements for high acid foods. This is an educational tool and not a license to change lab-tested recipes. 

Canning guide books, such as the Ball Blue Book Guide to Canning, warn canners that they must carefully follow lab-tested recipes in order to ensure safety against botulism. The recipes are tested for proper acidity levels and stability during shelf life. It has always bugged me that the books say, “Trust us, follow our recipe, and you’ll be fine.” Oh, and otherwise the results could be fatal.

So I did something bold, something that Ball would surely not recommend. I bought a litmus paper pH testing kit. I use Whatman litmus paper test strips.

Acidity Basics

Water bath canning sterilizes jars of high acid food for a stable shelf life. The water bath boiling kills off the organisms that create mold and fungus, but it is not enough to kill off botulism. That is alright, because botulism cannot survive in the high acid environment anyway. So it is important to water bath process only foods with a high acidity (a pH under 4.7) that are not at risk for botulism. Foods that could harbor botulism — low acid foods with a pH of over 4.7 — must be pressure-canned to kill off any botulism-creating spores. Tomatoes teeter near the unsafe zone, up to 4.5, so lemon juice is added to bring the numbers down. Adding spices and additional ingredients to a jar of tomato puree can increase the pH to those unsafe numbers. Following tested recipes is important to help you steer clear of unsafe acidity levels.

Since the acidity is so important, and the recipe is full of garden products that could vary widely, wouldn’t it be wise for me to check the acidity level of my batch before I preserve it? My garden fresh ingredients are hardly controllable. My six jalapeños for salsa seem bigger than average. My yellow and orange heirloom tomatoes might be lower in acidity than typical reds. Let’s not fool anyone — garden-fresh ingredients vary widely, making it difficult to control for acidic safety. It is the added lemon juice that brings down the acidity to the safe level under 4.6.

Peace of Mind

My litmus test may not enough to put me in charge of changing a recipe, but it seems like a good idea to make sure that my carefully followed, carefully tested recipe has the acidity we say it should have. It provides peace of mind for me to give it a little check.

When I teach canning to others, I like to test the pH of my tomato puree with a litmus paper test. All the canning books tell us that tomatoes can go up to about 4.6 in acidity, right on the edge of safe acidity for water bath canning. It is interesting to test the pH of tomato juice and illustrate pH level. We are instructed to add bottled lemon juice to lower the pH into the safe canning range below 4.6. My litmus test confirms the drop in acidity into the safe zone and well-illustrates these points to my students.

Can I change recipes safely by using the litmus paper to confirm acidity?

No, sorry. It is not that simple. I am glad I researched it a bit further, as I now respect the process even more. Safe recipe testing is a lot more complex than a simple litmus test. First, there are more factors to recipe safety than acidity. Density is also considered, and I don’t even know what that’s all about. Second, acidity can change with shelf life, so multiple tests are done at different times after canning. Third, there are carefully calibrated meters that test the acidity more accurately than paper test strips.

In my research, I discovered that as of April 2015, the FDA requires those who sell canned products to batch test them with pH paper or a meter. The pH paper is only appropriate for products under 4.0 in acidity; other products closer to the edge of acidity must use the more accurate calibrated meters and undergo more frequent tests. My salsa is 3.5-4.0 in acidity. I appreciate knowing that, thanks to my litmus testing. It’s nice to know where you stand.

The litmus tests are an educational tool, not a lab safety gauge. They are great tools for teaching about canning and acidity levels. They are not a ticket to changing lab-tested recipes. Let Ball and Rodale continue that good work. I now know that there is a lot more to the testing process than a litmus test, especially for products with a borderline acidity (4.1-4.6). But still, I like knowing the initial pH of my salsa and my low-acid heirloom tomatoes, to take away some uncertainty and to understand where these products sit on the spectrum, even if I am not using this information to change or create recipes.

Ilene White Freedman operates House in the Woods organic CSA farm with her husband, Phil, in Frederick, Maryland. The Freedmans are one of six 2013 MOTHER EARTH NEWS Homesteaders of the Year. Ilene blogs about making things from scratch, putting up the harvest, gardening and farm life at her website House in the Woods.

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