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.