Acid, in cooking, refers to any of the compounds that give foods a sour, tart, or tangy taste, either occurring naturally or added during preparation. Common culinary acids include citric acid (in citrus fruits), acetic acid (in vinegar), lactic acid (in fermented dairy), and malic acid (in apples and other fruits). Acids brighten and balance rich or sweet flavors, tenderize proteins in marinades, trigger reactions such as milk curdling or the "cooking" of raw fish in ceviche, and help preserve foods through pickling. The word is often used interchangeably with "acidic," which more precisely describes the quality or degree of sourness a food has rather than naming a specific compound.