acetic acid
Pronunciation: [a-SEE-tihk]
Acetic acid is formed when common airborne bacteria interact with the alcohol present in fermented solutions such as wine, beer or cider. Acetic acid is the constituent that makes vinegar sour.
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.