Acidity

Acidity is sourness you can taste and pH you can measure. They are related, not identical. Organic acids (lactic, acetic, tartaric, citric, malic) are different molecules with different smells and different jobs.

In fermentation #

Lactic acid bacteria make lactic acid, and often acetic acid, in dough and in milk. That is why sourdough can taste yogurt-like or vinegar-like depending on temperature and hydration — a baking observation, not a city name. Cheese cultures acidify milk so casein will set; FAO’s acid coagulation is a pH-and-temperature problem ¹. Wine’s tartaric acid is largely from the Grape; fermentation also produces acids. The shared idea is Fermentation; the crafts apply it.

In jam #

Pectin gels need acid in a workable window. Extension: when a recipe calls for lemon juice, it is for gel formation ². Too little acid, weak gel. Too much, a gel that weeps. See Jam and Pectin.

In wine #

Acid keeps wine from tasting flat and increases how astringent Tannin feels ³. Taste it as watering of the mouth and a sharp edge, using Tasting wine.

In canning #

Here acidity is a safety line, not a flavor note. NCHFP/USDA: foods with pH above 4.6 are low-acid; botulinum spores are not reliably killed in a boiling-water canner; pressure canning is required. Acid foods at pH 4.6 or below may use boiling-water processes ⁴. Do not “fix” a low-acid canned vegetable by adding a splash of vinegar unless you are on a tested pickled recipe. See Canning, Pickling, and Food safety.

Taste sourness. Measure pH when the decision is safety. Do not confuse those jobs.