Added acid is always required when canning tomatoes
Once upon a time, tomatoes were considered a high-acid fruit. But research has since shown that they are, in fact, variable in acidity, with pH levels sometimes going higher than 4.6, the dividing line between high-acid and low-acid foods – and the line that determines which foods can be processed in boiling water/steam canners (high-acid foods such as pickles and most fruits) vs. those that must be processed in a pressure canner (low-acid foods such as meat and vegetables).
To account for the variable acidity level of tomatoes and ensure a safe product when canning them, recommendations now call for acidifying all tomatoes before processing. This is accomplished by simply adding a small amount of acid – either citric acid powder, bottled lemon juice, or vinegar of 5% acidity – to each jar of tomatoes. This goes for tomatoes processed in a boiling water/steam canner as well as those processed in pressure canners. (Pressure canning processing recipes were developed for high-acid tomatoes, and at this time there are no researched processes for canning tomatoes without added acid, even when pressure canning.)
Note that there are some tomato-based recipes, such as Spaghetti Sauce with Meat, that were developed specifically as low-acid products and thus they do not have added acid. These recipes will have a pressure canning option only, with longer processing times.
In summary: To be safe, always acidify tomatoes, for both boiling water/steam canning and pressure canning processes. Use a scientifically validated recipe from a reputable source, and follow the instructions exactly, which will include information on how much acid to add to each jar being processed.
For further information on canning visit the National Center for Home Food Preservation (NCHFP) at nchfp.uga.edu or contact your local Cooperative Extension office.
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Last updated: July 2025