Food Chemistry: How Acids and Bases Transform Flavour, Texture and Colour in Cooking

pH is measured on a scale from 0 to 14. Below 7 is acidic, above 7 is alkaline (basic), and 7 is neutral. In practice, most cooking happens between pH 4 and pH 8. Common kitchen acids include acetic acid in vinegar (pH 2–3), citric acid in lemon juice (pH 2–3), lactic acid in yoghurt and buttermilk (pH 4–4.5), and tartaric acid in cream of tartar. Common kitchen bases include baking soda (sodium bicarbonate, pH 8–9), baking powder (a buffered mixture of baking soda and acid), and wood ash lye historically used in nixtamalisation.

At the molecular level, acids donate hydrogen ions (H⁺) and bases accept them. This seemingly simple exchange has profound consequences. Proteins change shape (denature) at different rates depending on pH. The same is true of enzymatic reactions — the browning enzyme polyphenol oxidase, for instance, is inhibited below approximately pH 3, which is why lemon juice prevents cut fruit from turning brown. Anthocyanins — the pigments responsible for the red, purple and blue colours in foods like red cabbage, blueberries and red onion — shift colour dramatically with pH, turning bright red in acid and turning green or yellow in alkali. Chlorophyll in green vegetables loses its vivid green colour in prolonged acid conditions as hydrogen ions displace the magnesium ion at the centre of the pigment molecule.

pH interacts with the other fundamental cooking variables in ways that experienced cooks use instinctively but rarely articulate. Temperature accelerates acid-catalysed reactions — a citrus marinade at room temperature tenderises chicken more slowly than the same marinade applied to warm meat. Time matters because acid denaturation of proteins (as in ceviche) is gradual rather than instantaneous; leave seafood in lime juice for too long and the texture becomes mealy rather than just-cooked. Concentration determines the magnitude of the effect — a teaspoon of vinegar brightens a soup while half a cup would make it unpalatable.

In baking, the ratio of baking soda to acid is critical. Baking soda requires an acidic ingredient (buttermilk, yoghurt, honey, cocoa powder, brown sugar) to react and produce carbon dioxide. One teaspoon of baking soda (approximately 6 g) requires around 240 ml of buttermilk or 1.5 teaspoons of cream of tartar to fully neutralise. Excess baking soda leaves a soapy, metallic taste; too little and the leavening is insufficient. Baking powder, which contains its own acid, is more forgiving but produces a slightly different crumb texture.

In professional kitchens, pH management is rarely articulated but constantly practised. When a seasoned chef tastes a sauce and reaches for lemon juice, they are not simply adding flavour — they are suppressing bitterness through competitive inhibition at taste receptor sites, boosting the perception of other flavours by increasing salivation, and brightening colour by shifting the pH of any anthocyanin-containing ingredients. Thomas Keller's French Laundry cookbook details acidulating blanching water to maintain vegetable colour. Ferran Adrià and the molecular gastronomy movement made pH manipulation explicit, using sodium alginate spherification (which requires precise pH to form gels properly) and sodium citrate to create emulsified cheese sauces.

In Japanese cuisine, the addition of rice vinegar to sushi rice is not merely flavouring — the acidity inhibits microbial growth, extending safe holding time, while also affecting the gelatinisation of starch to produce the characteristic slightly sticky texture. Alkaline cooking has its own distinguished tradition: the nixtamalisation of corn (treatment with lime — calcium hydroxide — at pH 11–12) transforms the nutritional profile of maize by releasing bound niacin, a process that prevented pellagra in societies that discovered it.

Quick pickling illustrates acid chemistry with satisfying precision. Combine 240 ml white wine vinegar (pH 2.4, approximately 5% acetic acid), 240 ml water, 1 tablespoon sugar and 1.5 teaspoons salt. Bring to a simmer until solids dissolve, then pour over thinly sliced vegetables — red onion, cucumber, radish or carrot work well. The science operates on several levels simultaneously. Acetic acid diffuses across cell walls through osmosis, gradually dropping the internal pH of the vegetable cells to around pH 3.5–4, a level at which most spoilage bacteria cannot survive. The low pH denatures surface proteins on the vegetable, shifting the texture from raw-crisp to lightly yielding. Anthocyanins in red onion turn a striking bright pink as the pH drops. The sugar buffers some of the sharpness by providing a contrasting taste stimulus, and the salt both seasons and accelerates osmotic water loss from the vegetable, creating the characteristic slight translucency. For maximum colour vibrancy in red onion pickles, avoid adding any alkaline ingredients — even a small amount of baking soda added by mistake would shift the anthocyanins toward an unappetising blue-grey.

Buttermilk (pH 4.4–4.6) is the canonical tenderising marinade for fried chicken, and the chemistry justifies its pre-eminence. The lactic acid in buttermilk partially denatures the surface proteins of the chicken, breaking some peptide bonds and loosening the protein matrix. This does not fully cook the meat but makes it more tender by disrupting the tight protein fibres that produce toughness. The low pH also helps break down collagen in the connective tissue, particularly effective in chicken thighs. Simultaneously, the calcium ions in buttermilk activate the proteolytic enzyme calpain, which further tenderises muscle fibres from within. For the best result, marinate chicken pieces in buttermilk for at least 4 hours and ideally 24 hours refrigerated. After marinating, allow the chicken to approach room temperature for 20 minutes before dredging in seasoned flour — cold meat straight from the fridge will cause the coating to slide and the oil temperature to drop dramatically. Dredge in flour containing 1 teaspoon of baking powder per 200 g flour: the alkaline baking powder reacts with the residual acid on the chicken surface, creating CO₂ bubbles that make the crust light and craggy rather than dense.

The most frequent acid-related error in baking is adding too much baking soda. Baking soda (pH 8.3 in solution) not only provides leavening but also promotes browning by raising the pH of the batter, which accelerates Maillard reactions. This is why Dutch-process cocoa (alkalised, pH 7–8) produces darker chocolate cakes than natural cocoa (pH 5–6). Too much baking soda results in a cake that browns on the outside before the interior sets, and which carries a distinctly soapy aftertaste from unreacted sodium carbonate. The fix is to balance soda with sufficient acidic ingredients, or switch to baking powder.

Over-marinating proteins in acid is another common error. After approximately 2 hours, citric or acetic acid continues denaturing proteins past the point of tenderisation into the territory of mushiness, creating a texture chefs describe as 'citrus-cooked'. For delicate fish in ceviche, 15–30 minutes is sufficient; for chicken, 12–24 hours; for tougher beef cuts, up to 24 hours.

Adding lemon juice to a braise too early is a subtler mistake. Acids inhibit the softening of pectin in vegetables and collagen in meat when added at the start of a long cook. Adding acid in the final 10 minutes of cooking brightens flavour without preventing the structural changes that make braised food tender.

Three simple experiments make the science of pH visible without specialist equipment. First, the anthocyanin indicator test: shred a quarter of a red cabbage and simmer in water for 10 minutes. Strain out the cabbage and divide the purple liquid into three glasses. Add a teaspoon of white vinegar to the first (it turns bright red-pink), leave the second plain (purple), and add a pinch of baking soda to the third (it turns blue-green). This illustrates how anthocyanins shift across the pH spectrum. Second, the green vegetable colour test: blanch two batches of broccoli florets in boiling water for 2 minutes. To one batch, add a teaspoon of white vinegar to the blanching water; to the other, add a pinch of baking soda. The acid batch will lose its bright green colour faster; the alkaline batch will retain intense green for longer (though with some textural softening). Third, the buttermilk versus milk biscuit test: make two batches of biscuits using identical recipes, substituting plain milk for buttermilk in one. The buttermilk batch will rise higher, have a more tender crumb, and brown more evenly because the baking soda reacts fully with the available acid. The results are striking and immediately edible.