30.5 Chemical leaveners: baking powder, soda, ammonium

To leaven a dough or batter is to give it a source of gas that makes it swell when it meets moisture and heat. Yeast is the living leavener. Others are chemical, mechanical or physical, and a baker picks one to suit the product. Yeast belongs with fermentation see chapter 26: Alcohol and gas, and eggs as a leavener with eggs. see chapter 15: What eggs do in a recipe

Baking soda

Baking soda is sodium bicarbonate. Wet and heated, it gives off carbon dioxide gas. Wet and heated alongside enough acid, it gives off twice as much.

Some ingredients are slightly acidic and supply part of that acid.

• honey
• molasses
• ginger
• cocoa
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• bran

That is why some recipes use baking powder alone and others use baking powder and soda together. Buttermilk works the same way. see chapter 14: Cultured milk and cream

• Work quickly. Most of the gas comes off early, while the batter is still acidic, at a pH of about 5 to 6. Once the soda has used up the acid, the mix turns alkaline and the gas slows. Get soda batters and doughs into the oven without delay, or much of the gas is gone before baking starts.
• Do not overdo it. Too much soda with too little acid turns a white cake crumb yellowish brown and leaves a strong, unpleasant smell of soda.
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• Soda alone goes into gingerbread and spiced honey cookies, to give them color quickly in the oven while keeping them soft.
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• Soda in chocolate cakes. It is paired with baking powder to keep the pH where it should be, and it gives a better grain and color. When there is enough acid to release all the gas, the chocolate color comes out darker and richer. Devil's food cake owes its reddish, coarse, open crumb to soda being its main leavener.
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• Know your cocoa. Dutch-process cocoa has had some of its acid neutralized already, so less acid is left to react. With it, use more baking powder and less soda.
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How much soda depends on the product and on the rest of the formula. Cookies rich in fat and sugar need little leavening, or none. Typical amounts, as a percentage of the flour weight, are:

These figures look small beside baking powder's, because baking powder is only partly soda; the rest is its acid.

Baking powder

Baking powder is soda with one or more acids already mixed in. Moisture and heat set the acid and soda reacting, and the carbon dioxide they make forms bubbles that raise the product, just as gas from yeast does. It is reliable and of high quality.

The two main kinds differ only in how fast they react:

• Continuous-action (single-action) powder has one acid. It reacts steadily all through baking until its gas is spent.
• Double-action (multiple-action) powder has two acids. One gives off a little gas while the batter is still cool; the other, the main acid, gives off most of the gas at baking heat.
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What happens in the oven. The gas comes off in stages.

• About 15% of the gas comes off before baking, in the cold mix, and some of that is lost, though enough power usually remains.
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• The other 85% comes off in the oven, starting at around 40°C (105°F). The gas moves into the tiny cells that mixing or creaming made in the batter and swells them.
• At about 60°C (140°F) the starch begins to gelatinize and the cell walls start to set.
• By about 75°C (167°F) they are more or less firm. Any gas made after that just escapes through the open crumb. How starch sets with heat is explained with heat. see chapter 7: Starch gelatinization

Using it. Sift baking powder with the flour or other dry ingredients so it spreads evenly. For most cakes, about 5% of the flour weight works best. Weigh it carefully: a little too much can make the product collapse. Yeast is more forgiving, since too much of it usually just makes the dough rise faster. Weighing is covered with measuring. see chapter 5: Measuring by weight and by volume

Ammonium bicarbonate

Ammonium bicarbonate is a white powder made of crystals. Bakers use it in flat, spiced cookies such as gingerbread, and in éclair paste.

• Dissolve it in the cold liquid of the batter.
• At room temperature it barely breaks down. Near 60°C (140°F) it breaks down noticeably, and at oven heat it breaks down very fast.
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• Use it only in products that are low in moisture and not dense. Then no taste or smell of ammonia stays behind.
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