Compounds that release hydrogen ions in solution, giving foods their characteristic sour taste. Organic acids — citric in citrus, malic in apples, lactic in yogurt, acetic in vinegar — shape the flavour of fruits, fermented foods and pickles while preserving them by lowering pH. Food chemists distinguish pH, which measures acid strength, from titratable acidity, which measures total acid present, since both affect taste, safety and processing behaviour. Acids are also functional tools: they set pectins in jam, firm proteins in cheese, leaven with baking soda, and keep canned foods safe.