what is acid

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