Acid — the category containing citric acid (lemon, lime), acetic acid (vinegar), lactic acid (yogurt, buttermilk), tartaric acid (wine), and malic acid (apples, grapes) — performs multiple simultaneous functions in cooking: seasoning, protein denaturation, colour stabilisation, enzyme activation/inhibition, and flavour perception modification. A cook who understands acid's specific functions in each context uses it as a precision tool rather than as a flavour additive.
**Acid as seasoning:** Acid activates salivation and freshens flavour perception, functioning as a brightness modifier rather than just a flavour. The correct use of acid is often to make food taste more of itself — a squeeze of lemon over a fried fish doesn't make it taste of lemon; it makes it taste more clearly of fish. Threshold effect: below 0.1% acidity, most acids are imperceptible. Above 0.5%, they begin to dominate. **Acid and protein:** - Acid partially denatures surface proteins — the principle of ceviche (fish "cooked" in lime juice) - At sufficiently low pH (below 4.5), all microorganism growth is inhibited — the principle of acidulated preservation - Acid breaks protein-protein bonds, tenderising meat when used in marinades — but excessive acid produces a mushy exterior without penetrating interior **Acid and colour:** - Green vegetables: acid accelerates the breakdown of chlorophyll to olive-drab pheophytin. Do NOT add acid to the blanching water for green vegetables. - Red/purple vegetables: anthocyanin pigments in red cabbage, beetroot, and red onion are stabilised by acid. They turn blue-green in alkaline conditions; bright red in acidic conditions. - Browning prevention: citric acid chelates the metal ions (primarily iron) that catalyse enzymatic browning — the principle behind acidulated water for cut vegetables. **Acid and fat:** Acid cuts the perception of fat richness by stimulating salivation and occupying the taste receptors that fat otherwise dominates. This is why lemon juice on fried food, vinegar in a rich braise, and pickles alongside fatty meat all "cut through" — not metaphorically but physiologically.
Modernist Cuisine Vol. 2