The Chemistry
Two acids, one transformation. Grape juice contains many acids, but two dominate: malic acid (the acid in green apples — sharp, biting) and tartaric acid (the acid in tamarinds — round, less aggressive). After yeast finishes its work, the malic acid remains in the wine, making it taste tart and unbalanced.
Then bacteria step in. They literally eat malic acid and excrete lactic acid — the same acid that gives yogurt its softness. Same number of acid molecules, but the new acid is gentler on the palate. The wine's pH rises slightly, the texture rounds out, and a subtle buttery note (diacetyl) develops.
This isn't optional in most wineries. The bacteria are already in the cellar, often in the wood itself. The winemaker just decides whether to let them work, suppress them with sulfur, or filter them out.
