Baking Soda: Chemically it is sodium bicarbonate. It liberates carbon dioxide gas when heated. It also liberates the same gas when mixed with an acid, either hot or cold. When used alone reacts with the shortening and a residual flavor is felt besides causing an unpleasant taste brown colour and alkaline odor. Therefore, it is not used alone and when used, a suitable quantity of acid is added so that neutral residue is formed.
Baking powder: Baking powder is combination of sodium bicarbonate and an acid salt. When heated in presence of moisture it evolves gas which leavens the product giving volume and light texture. A baking powder should release its gas in the batter to saturate it with carbon dioxide gas uniformly during baking to hold the raised batter until set. This tends to give a uniform crumb and prevent shrinkage and cakes from falling. Baking powders contain an inert filler most commonly corn starch. It act as a buffer between soda and the acid and prevents reaction when exposed to air by absorbing moisture. The starch also helps the powder to release standard amount of carbon dioxide, Baking powder is of three different kinds:
Fast acting: It releases most of its carbon dioxide gas during bench operation and very little gas is released during baking i.e. mixing operation.
Slow acting: It does not release much of gas during bench operation but is released when it comes in contact with heat.
Double Acting: They are most widely used in baking. They release part of gas during bench operation (while mixing), increasing fluidity of cake batter. This facilitates easy weighing operations and transferring to mould for baking. The remaining carbon dioxide gas is released during baking, which gives volume to the end product.
The time and rate of gas evolution from baking powder can be regulated by the selection of different baking acids that react faster or slower with sodium bicarbonate.
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