Acid Levels in Canning

Why does acid matter in canning?

Dangerous bacteria, including the one that causes botulism, do not grow well in a high-acid environment. In a sealed jar without oxygen, low-acid food gives them a chance. Acidity is therefore the main factor in deciding whether a food can be safely canned in a boiling water bath or needs a pressure canner.

Acidity is measured on the pH scale, where lower numbers mean more acid. You cannot judge acidity by taste. A food that tastes sour can still sit near the safe boundary, and a food that tastes mild may be low in acid.

Because the science depends on acidity, tested recipes are developed in laboratories that measure each product in the form the recipe describes. That is why even a small change cannot be assumed safe. Fruit juice, vinegar and other acid ingredients vary in strength, so the tested recipe usually names the kind to use. Keep the label so you can show your extension office exactly what you used if you have questions.

What are high-acid and low-acid foods?

Many fruits are naturally high in acid, which is why most jams, jellies and many fruit products can be water bath canned. Most vegetables, meats, poultry, fish and soups are low in acid and need pressure canning. Some foods, including tomatoes, fall near the line, and tested recipes handle them in specific ways.

Look up your specific food in a tested source. Varieties, ripeness and growing conditions affect acidity, and this is why recipe instructions are written so carefully.

If you grow unusual varieties, such as heirloom or low-acid tomatoes, do not assume they behave like the ones in the recipe. Follow the tested source for how to treat the variety you have.

• Many fruits: usually high in acid
• Most vegetables and all meats: low in acid
• Borderline foods: follow a tested recipe exactly
• Taste is not a reliable guide

How are pickles and tomatoes handled?

Pickling safely depends on getting the right acidity in the finished product. Tested recipes set the proportions of vinegar, water and other ingredients, and those proportions matter. Do not dilute vinegar, swap vinegars without a tested recipe, or reduce acid to suit your taste.

Tomatoes are borderline and tested recipes call for added acid in specific ways. Do not guess how much or which kind, and do not use untested recipes. The tested source provides the exact method.

Keep your acid ingredients as the recipe specifies and store them properly. Ask your extension office or the tested source what to do if your ingredient is not the exact kind listed.

Can you change a recipe's ingredients?

Do not change a tested canning recipe. Adding low-acid ingredients such as onions, peppers, garlic, herbs or extra vegetables to a recipe designed for a particular acidity can make the final product unsafe. Reducing vinegar or lemon juice, adding thickeners, or changing the proportions can have the same effect.

If you want a new flavor, look for a different tested recipe, or add flavors after opening the jar. Ask your extension office for tested options.

If a tested recipe does not exist for the food you want to can, the safe choice is to freeze it, refrigerate it or use another tested method, not to improvise a canning method.

What about acid testing and ingredients?

Home pH testing is not a reliable substitute for a tested recipe, since proper testing requires calibrated equipment and the right method, and results can mislead. Use tested recipes that have already been evaluated by laboratories.

Use bottled or tested ingredients as the recipe states, since acidity of fresh juice can vary. Check the recipe for what it says about acid ingredients, and follow it exactly. This article does not give amounts or recipes. Ask your extension office to explain any term in a recipe that you do not understand before you start, not partway through.

Where do you find safe answers?

The National Center for Home Food Preservation at nchfp.uga.edu, the USDA Complete Guide to Home Canning and your county extension office are the sources to rely on. Ask them about any food you are unsure about, and whether a recipe has been updated.

Costs fall into categories: tested recipe resources, which are often free, quality jars and lids, and the right equipment for the method. The cheapest protection is following the recipe exactly.

When you teach others, pass on the habit of checking the source, not just the steps. That habit is what keeps new canners safe.

Last reviewed: November 12, 2026. Written by the Homestead & Small Farm Editorial Team. See our editorial policy.