The word "acid" comes from the Latin acidus, meaning "sour. " All acids are sour to some degree. Sourness (acidity) is found in many natural ingredients such as vinegar (acetic acid), wine (tartaric acid), lemon juice (citric acid), sour-milk products (lactic acid), apples (malic acid) and rhubarb leaves (toxic oxalic acid). When used in a marinade, acids — such as wine and lemon juice — are natural tenderizers because they break down connective tissue and cell walls.