Also known as bicarbonate of soda, baking soda is an alkali used as a leavener in baked goods. When combined with an acid ingredient such as buttermilk, yogurt or molasses, baking soda produces carbon dioxide gas bubbles, thereby causing a dough or batter to rise. It also neutralizes acidity and produces tender baked goods. Because it reacts immediately when moistened, it should always be mixed with the other dry ingredients before adding any liquid; the resulting batter should be placed in the oven immediately. At one time, baking soda was used in the cooking water of green vegetables to preserve their color. That practice was discontinued, however, when it was discovered that baking soda destroys the vitamin C content of vegetables.
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.