A pastry instructor I once had used to tell us that bakers make their living selling air. It sounds deceptive, but that’s part of why baked goods are so delicious (and don’t break your teeth!). Air bubbles make cakes fluffy, make breads soft (not gummy or hard), meringues pillowy, and more. And part of the job of a baker is using techniques and ingredients that give the right amount of air to a baked good in order to produce a desired texture.
There are different types of leavening. Broadly, the ones of interest in a home kitchen are:
- Natural and chemical leavening (agents that produce gas)
- Mechanical leavening (incorporating air into a batter or dough)
- Steam (present in all baked goods, especially important in some)
Natural leavening agents
Natural leavening agents include yeast- both commercially available strains and naturally occurring yeasts in sourdough cultures. Different types of yeasts are suited for different tasks, but they all use the same process: fermentation. Yeasts take sugars available in a dough or batter (either added in, or produced by enzymes in the flour or yeast itself) and metabolize them into carbon dioxide and ethanol. These gases need to be trapped until the dough or batter sets, which happens in the oven as the items bake. Because they’re living organisms, yeasts act at different rates depending on the temperature of the dough: they’re inactive below 0°C, very sluggish at fridge temperatures, most active from 20-30°C, and permanently inactive past 60°C (RIP). They’re the slowest of all of the leaveners, but they do produce flavour compounds like “yeasty” smelling esters and aldehydes. By lowering the temperature of the dough, you can allow leavening to happen slowly enough that the yeasts can also produce these flavours, usually by doing one of the rises in the fridge.
Chemical leavening agents
Chemical leavening agents produce gas through chemical reactions that occur in the presence of water. Baking soda (sodium bicarbonate) is an alkaline (basic, the opposite of acidic) ingredient that releases carbon dioxide when it reacts with an acid in water- this reaction happens as soon as the ingredients come in contact and doesn’t require heat. Baking soda should be balanced with the acid in a recipe- that usually comes from things like brown sugar, buttermilk, yogurt, fruit juice, and natural cocoa- otherwise they can taste bitter and soapy from an overly alkaline mix.
Baking powder is another chemical leavener that has both the acid and the base included. Baking powders act at two stages- once at mixing and again in the presence of heat. These are called double-acting baking powders, but single-acting baking powders are pretty uncommon. Because baking powders are already balanced independently of the other ingredients, they can be added in greater quantities than baking soda. However, using too much baking powder (like way more than a recipe calls for) can give you “off” flavours and excessive rise that can’t be supported. That produces things like a cake that rises beautifully at first but ends up sinking in the middle. Baking powders are also good for things that don’t see heat right away, like a pancake batter that rests.
Another commonly used strategy is mechanical leavening: introducing air through the process of mixing. Cookies and cakes are often made using the creaming method- butter and sugar are beaten together until light and fluffy, then the other ingredients are added in stages. The act of creaming together the butter and sugar causes the sugar to punch tiny holes in the butter- these little bubbles are how air is incorporated, and the volume of the entire mix increases. Meringues (and any foamed eggs like in sponge cakes) hold air in the eggs as well, which is introduced by lots of whisking and is held inside the mixture with egg proteins. In most of these cooked applications, the increase in volume due to incorporated air is noticeable before baking, and then air held inside these mixes expands when heated, creating rise in the oven.
Steam can produce some of the most dramatic leavening in baked goods. This is particularly noticeable in something like puff pastry or choux paste (the dough used for eclairs, cream puffs, etc. pictured above). Essentially, when the dough/batter/etc. is heated up and baked, the water inside the mixture turns to water vapour, expanding in bubbles to many times its own size. This water can come from the dough/batter itself, or from components like butter that’s been rolled in (as in laminated doughs), or a combination of both. That expansion forces the mixture to expand as a whole and makes the baked item less dense than when it went into the oven (with some of the water vapour escaping once it’s done its job expanding the dough/batter). As baking goes on, the structure of the expanded bubbles sets (due to some combination of starches, proteins, etc. depending on the baked item). Some items are leavened entirely by steam like in the case of puff pastry and choux, but pretty much all baked goods are leavened by steam to some extent.
Note that baked goods often use more than one type of leavening, and that they often overlap. For example, a cake made using the creaming method would utilize mechanical leavening to add air bubbles into the butter before the other ingredients are added, but usually involves a leavening agent like baking powder, and may be leavened to a certain extent by the steam produced as water turns to vapour in the batter as it bakes.