Guide
Water-Bath vs Pressure Canning: High-Acid, Low-Acid, and the pH 4.6 Line
The short answer
There is a single dividing line in home canning: pH 4.6. Clostridium botulinum spores will not grow if a food's pH is 4.6 or less, so high-acid foods (most fruits, pickles, jams, properly acidified tomatoes) can be safely processed in a boiling water canner at 212 F. Low-acid foods above pH 4.6, such as green beans, other plain vegetables, meats, poultry, and seafood, cannot.
Low-acid foods must be pressure canned. The reason is temperature: C. botulinum spores can survive boiling water (212 F) and then grow in a sealed jar of low-acid food, producing a deadly toxin. Only a pressure canner raises the food to the 240 to 250 F required to destroy those spores. A boiling water bath simply cannot get hot enough, so you must never can a low-acid food in one.
Tomatoes are the famous borderline case. Their pH sits slightly above 4.6, so tested recipes require adding acid, typically bottled lemon juice or citric acid, to bring them safely into water-bath range. Because processing times depend on the food, jar size, and your altitude, treat this as background and follow tested NCHFP recipes for any specific product.
Why pH 4.6 is the whole game
Home-canning safety is built around one organism, Clostridium botulinum, and one number, pH 4.6. The spores of C. botulinum will not grow if the pH of a food is 4.6 or less. That is why acidic foods are relatively forgiving: the acid itself blocks the bacterium, so gentle boiling-water heat is enough to seal the jar and kill the molds, yeasts, and other microbes that would spoil it.
Above pH 4.6, the acid no longer holds the bacterium in check, and the danger becomes the spore. C. botulinum spores can survive boiling water at 212 F and then, in the oxygen-free, low-acid environment of a sealed jar, germinate and produce botulinum toxin. Boiling water cannot destroy those spores, which is the entire reason low-acid foods need a different tool.
Match the food to the canner
- High-acid (pH 4.6 or below) - fruits, pickles, relishes, jams, jellies, acidified tomatoes: boiling water canner at 212 F is safe with a tested recipe.
- Low-acid (pH above 4.6) - plain vegetables, green beans, corn, meats, poultry, seafood: must be pressure canned to reach 240 to 250 F and destroy botulinum spores.
- Tomatoes: borderline, pH slightly above 4.6; add bottled lemon juice or citric acid per a tested recipe before water-bath canning.
- Never process a low-acid food in a boiling water bath; 212 F cannot kill the spores and the sealed jar becomes an ideal botulism environment.
⚠ Only a pressure canner can heat low-acid foods to the 240 to 250 F needed to destroy Clostridium botulinum spores. Canning plain vegetables or meats in a boiling water bath is unsafe and can cause fatal botulism.
The tomato exception, and where to get real numbers
Tomatoes straddle the line, so tested recipes require added acid. NCHFP directs adding, per jar: 1 tablespoon bottled lemon juice or 1/4 teaspoon citric acid to pints, and 2 tablespoons bottled lemon juice or 1/2 teaspoon citric acid to quarts, and warns not to leave the acid out. (Vinegar can be used but may cause undesirable flavor changes.) That acidification is what lets tomatoes be safely water-bath canned.
This page is a map, not a recipe. Actual processing times and pressures change with the specific food, pack style, jar size, and your altitude, and getting them wrong is dangerous. For every real canning project, use a current tested recipe from the National Center for Home Food Preservation or your state extension service, and follow it exactly.
Sources
Reviewed 2026-07-18. We cite university Extension, government, and established horticultural sources — and never invent pesticide rates or dangerous treatments.
- Clemson HGIC - Canning Foods, the pH Factor — Foods with a pH of 4.6 or lower are acid foods and may use a water bath canner; foods above 4.6 are low-acid and require pressure canning; Clostridium botulinum spores can survive boiling water (212 F) and grow in a sealed jar of low-acid food; low-acid foods (vegetables, meats, poultry, seafood) require a pressure canner reaching 240 to 250 F to destroy the spores; tomatoes are borderline with pH values slightly above 4.6 and must have acid added; use only tested recipes.
- NCHFP - Tomato Acidification Directions — To ensure safe acidity, add per pint 1 Tablespoon bottled lemon juice or 1/4 teaspoon citric acid, and per quart 2 Tablespoons bottled lemon juice or 1/2 teaspoon citric acid; do not leave the acid out; using vinegar may cause undesirable flavor changes.