Acidifying Tomatoes When Canning

Why do I have to add acid when canning tomatoes in the pressure canner?

Tomatoes that are acidified for canning are done so to prevent botulism poisoning and other bacterial concerns by a combination of acid and heat; the control in vegetables, meat and other naturally low-acid foods is by heat alone.

The bacteria that cause botulism poisoning can grow and produce toxin in sealed jars of moist food at room temperature if the pH (measure of acidity) is above 4.6.  Vegetables, meat, fish, etc. are naturally fairly high above pH 4.6 (close to 6.0) and so pressure processes were developed for those to kill the heat-resistant spores of C. botulinum bacteria that are likely to be contaminating them.

Tomatoes also can have a natural pH above 4.6 (at least up to 4.8).  But rather than develop a pressure-only process as if they were all low-acid, since they are so close to 4.6, USDA decided instead to recommend a small amount of acid be added so they can be treated as a food with a pH less than 4.6 for home canning.  Therefore they are suitable for boiling water canning when the acid is added.  (The commercial industry often also adds citric acid to tomatoes to be able to give them a less severe heat treatment than would be needed for botulism and other bacterial controls.)

When you see the tomato product recommendations in USDA canning directions that offer both boiling water and pressure canning options, those pressure processes are still only the same amount of heat treatment as the boiling water option.  (Higher temperature=shorter process time.)  Those pressure processes are not the amount of heat and time that would be required for canning a low-acid food to control for botulism.  There has not been a properly researched process for pressure canning of low-acid tomatoes without added acid, so the available process times still require the addition of acid as if they are being processed in boiling water.

Another example of how an acid food has both a boiling water and pressure process available is canned peaches.  Peaches (in pint jars) can be canned for 20 minutes in boiling water or 10 minutes at 5 pounds pressure in weighted gauge canner.  That pressure process is not a botulism control either, just because it is pressure canning.  The two time-temperature combinations are the equivalent amount of heating with regard to killing bacteria.

There are some tomato products in the USDA canning procedures that only have a pressure process listed (for example, tomatoes with okra or zucchini, spaghetti sauces, Mexican tomato sauce, etc.).  If a pressure process is the only listed option, then it is the required processing method and we do not have a boiling water process option available. These products made according to the stated recipes and procedures are low-acid food mixtures.

September 4, 2013
National Center for Home Food Preservation


The University of Georgia Cooperative Extension has now published a 6th edition of its popular book, So Easy To Preserve. The book was reviewed and updated in 2020. Chapters in the 388-page book include Preserving Food, Canning, Pickled Products, Sweet Spreads and Syrups, Freezing and Drying.