Double acting.

Baking powder is a dry chemical leaving agent which is a mixture of bicarbonate and a weak acid. The base and acid are prevented from reacting prematurely by the inclusion of a buffer such as corn starch. Baking powder is used to increase the volume and lighten the texture of baked goods. It works by releasing carbob dioxide gas into a batter or dough through an acid base reaction, causing bubbles in the wet mixture to expand and thus leavening the mixture. The first single-acting baking powder, which releases carbon dioxide at room temperature as soon as it is dampened, was developed by food manufacturer Alfred Bird in England in 1843. The first double-acting baking powder, which releases some carbon dioxide when dampened, and later releases more of the gas when heated by baking, was first developed by Eben Norton Horsford in the U.S. in the 1860s.

Baking powder is used instead of yeast for end-products where

  1. fermentation flavors would be undesirable,
  2.  the batter lacks the elastic structure to hold gas bubbles for more than a few minutes,
  3. and to speed the production of baked goods.

Because carbon dioxide is released at a faster rate through the acid-base reaction than through fermentation, breads made by chemical leavening are called quickbreads. The introduction of baking powder was revolutionary in minimizing the time and labor required to make breadstuffs. It led to the creation of new types of cakes, cookies, biscuits, and other baked goods.

Ingredients: Sodium acid pyrophosphate, sodium bicarbonate, corn starch, monocalcium phosphate

Baking Powder Biscuits Recipe:

2 cups all-purpose flour,1 tbsp baking powder, 1/2 tsp salt, 1/4 cup shortening, 1 cup milk.
Preheat oven to 450 degrees F (230 degrees C). Lightly grease a baking sheet. In a bowl mix the flour, baking powder, and salt. Cut in the shortening until the mixture has only pea sized lumps. Add milk and mix until dry mixture is absorbed. Drop dough by heaping spoonfuls onto the prepared baking sheet. Bake for 12 to 15 min in the preheated oven, until golden brown.