Baking powder is a mixture of bicarbonate of soda, which is an alkali, and cream of tartar, which is an acid. When liquid is added, the two react to produce carbon dioxide, and the heat of the oven expands these gas bubbles to raise the mixture. Baking powder also contains a filler in the form of a food starch, usually rice flour, which prevents the premature release of carbon dioxide during storage. You can make your own baking powder from one part bicarbonate of soda to two parts cream of tartar. Bicarbonate of soda can be used as a raising agent on its own, but it has an unpleasant aftertaste, so it is best used for strongly flavored, spicy cakes.
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