Acid

In cooking, acid means any sour ingredient: citrus juice, vinegar, wine, tomatoes, yogurt, buttermilk, tamarind and many fruits. Acid makes food taste brighter and fresher, cuts through richness, and balances salt and sugar. It also changes how food behaves, from firming fish to making cakes rise.

Why it works

Sour is one of the basic tastes. A little of it makes our mouths water and makes other flavors seem stronger. Chemically, acid changes proteins and pectin. It firms the surface of fish in ceviche, helps keep green vegetables firm but dulls their color, keeps cut apples from browning, and helps jam set.

How to use it

• To finish a dish. If a soup, stew or sauce tastes flat even with enough salt, add a squeeze of lemon or a teaspoon of vinegar. Start small, about 1/2 teaspoon per serving, and taste. This is part of how you adjust the seasoning.
• To balance fat. Rich foods such as pork belly, fried food and creamy pasta taste lighter with a sharp sauce or a pickled side.
• In dressings. A vinaigrette is roughly 1 part acid to 3 parts oil.
• To preserve. Pickles use vinegar or fermentation to make food acidic enough to keep.
• In baking. Acidic ingredients such as buttermilk, yogurt and brown sugar react with baking soda to make gas that helps cakes rise.

Common mistakes

• Adding acid to green vegetables early, which turns them olive drab. Add lemon just before serving.
• Marinating meat or fish too long in strong acid, which makes the surface mushy. See marinate.
• Adding acid to hot milk or cream sauces, which can make them curdle. Cream with a high fat content handles it better.
• Cooking acidic foods for long periods in bare cast iron or aluminum, which can give a metallic taste.

In recipes

"Brighten with lemon", "add a splash of vinegar" and "finish with a squeeze of lime" all mean add acid at the end. Some recipes say "balance with acid" when a dish is too sweet or too salty.