Low-Acid Foods and Canning: The pH 4.6 Line in Plain Words

If you're new to canning, this is the one idea that makes everything else fall into place. Once you know which side of the line a food is on, you know which kind of canner it needs.

Which foods are low-acid?

• Red meat, chicken, seafood: low-acid. Pressure canner (USDA, CDC).
• Milk: low-acid, says the USDA. But being on the list doesn't mean a home process exists. See what you can pressure can.
• All fresh vegetables except most tomatoes: low-acid. Pressure canner (USDA, CDC).
• Figs: slightly above 4.6. Add lemon juice or citric acid if you can them as a high-acid food (USDA).
• Tomatoes: usually high-acid, but some are slightly above 4.6. Same acid step (USDA, National Center).
• Mixes of low-acid and high-acid foods: usually low-acid, unless the recipe has enough lemon juice, citric acid or vinegar (USDA).
• Fruit: high-acid. The USDA recommends a boiling water canner because it's faster.
• Pickles, sauerkraut: high-acid. Boiling water canner, by tested recipe (USDA).
• Jam, jelly, marmalade, fruit butter: high-acid. Boiling water canner, by tested recipe (USDA).

The CDC's list says the same thing with slightly different examples. It adds figs and "some tomatoes" to the low-acid group. The two agree on what matters.

Why is 4.6 the line?

The USDA explains it with one germ. Low-acid foods are not acidic enough to stop Clostridium botulinum from growing. It can cause botulism, a deadly kind of food poisoning. High-acid foods have enough acid to block its growth, or to kill it faster when heated.

The spores are harmless on fresh food, because they only grow without air. A sealed jar of moist, low-acid food at room temperature gives them what they need. Our botulism page reports what the CDC and the National Center for Home Food Preservation (the US canning experts at the University of Georgia, "the National Center" from here on) say about the illness.

Temperature then decides the canner. The USDA says the spores are very hard to kill at boiling-water temperatures. Low-acid foods should be heated to 240° to 250°F, which pressure canners can reach. It adds that pressure itself doesn't kill germs. High heat held long enough does.

Which foods catch people out?

Tomatoes

Tomatoes feel acidic, and most are. But the USDA says some are slightly above 4.6. So tested tomato recipes add lemon juice or citric acid. The National Center says the acid is required even when a recipe offers a pressure option, because those options were not made for tomatoes without added acid. The amounts belong to the recipe, so take them from the National Center's tomato section.

Figs

They look like fruit, and fruit is usually high-acid. But the USDA says figs are also slightly above 4.6, so they need the same acid step.

Salsa, relish and other mixes

The USDA says most mixes of low-acid and high-acid foods stay above 4.6 unless the recipe has enough acid. That's why the exact recipe matters so much. The National Center says you can make your own salsa only with a recipe scientifically tested for that exact mix. The tested recipes page shows where those live.

Pickled vegetables

Green beans in plain water are low-acid. Dilled beans from a tested recipe are not, because of the vinegar. Penn State explains the difference. The National Center says adding vinegar to fresh vegetables on your own is no shortcut. If this is you, read canning without a pressure canner.

What doesn't "low-acid" tell you?

• It says nothing about the time. That depends on the food, how it's packed and the jar size, says the USDA. We publish no times. Your tested recipe and your canner's manual do.
• It doesn't mean any pressure machine will do. The National Center's instructions were written for stovetop canners holding four or more quart jars upright. The CDC says not to use electric multi-cookers. See Instant Pot canning.
• It doesn't mean a process exists. Pumpkin puree is low-acid, and the National Center says the research to set a safe home method was never done. See what you can pressure can.
• You can't judge it by eye or taste. The National Center says spoiled low-acid food may show little or no sign. Never taste a suspect jar. A seal does not show that the food got the heat it needed.

What does this mean when you choose a canner?

If you only make jam, fruit and pickles from tested recipes, a boiling water canner may be all you need. If beans, carrots, corn, meat or stock are in your plans, you need a pressure canner. Start with our stovetop canner guide. It's a simple line once you've seen it, and now you have.