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, acidssuch as wine and lemon juiceare natural tenderizers because they break down connective tissue and cell walls.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.