The cutoff is a single number: pH 4.6. Foods below this value are considered high-acid, and those above it low-acid; they are processed by two different canning methods. The distinction is based on measurement, not taste: a food that tastes sour can be low-acid, and a sweet-tasting food can be high-acid. Some foods sit right at the cutoff and form their own group.
There is a single cutoff for this distinction: pH 4.6. Foods below this value are considered high-acid, and those above it low-acid. This number is not arbitrary: Clostridium botulinum cannot grow in environments below this cutoff, so acidity itself acts as a protective barrier. Above the cutoff, that barrier is absent, and heat is the only protection. All of the distinctions between canning methods follow from these two statements; the first question in determining how many minutes and in what equipment to process a jar is which group the food belongs to. An incorrect answer to this question invalidates every step that follows.
The high-acid group consists largely of fruits and foods prepared with vinegar. Apples, plums, apricots, sour cherries, grapes, strawberries, and all citrus fruits belong to this group, as do jam, marmalade, and fruit compote. Pickles prepared with added vinegar belong here too. What they have in common is that the acidic environment stops bacteria from the outset. These foods can be safely processed using a boiling-water bath and do not require a pressure canner; boiling in the bath kills molds and yeasts and creates the vacuum seal.
The low-acid group is much broader and includes most of the foods commonly used in the kitchen. It includes all vegetables, dried legumes, corn, peas, beans, chickpeas, spinach and other leafy greens, carrots, beets, potatoes, mushrooms, meat, poultry, fish, and milk. For these foods, acidity does not provide protection; heat is the only protection. There are no recommended methods for canning unpickled vegetables in a boiling-water bath; this group is processed only in a pressure canner. The list shows that most foods canned at home belong to this group.
There is a third group, where the most confusion arises: borderline foods. Tomatoes, figs, melons, and bananas have pH values close to the cutoff; depending on the variety, ripeness, and where they were grown, they may fall just below or just above it. They are not considered high-acid on their own. The safe approach is to add acid in the amount specified by the recipe to make an acidified product, or to process it like a low-acid food. Taste is not the criterion here.
Because sour taste and acidity level are not the same thing, taste cannot distinguish between them. Sugar masks acidity, so a very sweet jam can be more acidic than a compote that does not taste sour. Likewise, a vegetable dish with plenty of lemon may taste sour on the plate, but it does not create a safe acidic environment in a jar, because the relevant measure is the amount, not the taste. Do not guess which group a food belongs to; check a recognized source’s list, and treat a food as low-acid when in doubt. Also note the source name and publication year, because tables are updated over time.
A Note for Readers
This content is for informational purposes only and is not a substitute for medical advice. Consult your doctor before making any decision about your health.