Malolactic fermentation

Malolactic fermentation is bacteria on wine, not yeast on grape sugar. Lactic acid bacteria convert malic acid (the apple-like sharpness in many grapes) into lactic acid (the yogurt-like softness). The alcoholic ferment remains yeast’s job; this is a later, or overlapping, bacterial story.

Conversion percentages, pH cutoffs, and sulfur-dioxide ppm that inhibit MLF are not universal; each depends on the wine, organism, temperature, and cellar protocol.

Why the acid change is tasteable #

Acidity keeps wine from tasting flat and increases how astringent Tannin feels ¹. Softening malic acid into lactic acid changes that edge. You notice less bite, sometimes a buttery aroma when diacetyl is present — an observation, not a law that every malolactic wine tastes like popcorn. Some whites are blocked from this conversion on purpose to keep a green-apple line. Some reds go through it almost as default house practice. Neither is a geography rule; an AVA does not mandate it ².

What to notice #

Taste the same wine before and after only if you are in a cellar. In the glass, look for a rounder mid-palate and less malic snap, using Tasting wine. A yogurt or butter note can be MLF; a vinegar note is volatile acidity — a different failure. Cloudiness and gas during the conversion are cellar signs, not bottle faults if the wine was finished before bottling.

Wine fermentation is yeast. Lacto-fermentation of cabbage is the same kingdom of metabolism in brine. Do not copy pickle salt rules onto a barrel.