Leavening in the Kitchen: Incorporating Air

In the introduction to leavening, we discussed how gases in our batters and doughs expand to add volume and tenderness to our baked goods. Over the next few posts, we’ll explore how those gases get there in the first place. Today, we’re talking about air, which is the foundation of all leavening. It’s easy to see air’s role in meringues and sponge cakes, where it’s beaten in for volume and fluffiness. But air is crucial to the success of other leaveners like baking soda, steam, and yeast. In this post, we’ll explore what air is, how we use it, and how it behaves in the kitchen.

Air is a mixture of gases.

As we mentioned in the introduction, the air around us is rich with invisible molecules zooming by. They’re predominantly nitrogen, but there’s also oxygen and trace amounts of carbon dioxide, hydrogen, and other gases. Whenever we add air to our batters and doughs, we add these gases. And when they heat up in the oven, the gases expand to raise our baked goods. Since air is the sole leavener in products like meringues, soufflés, and some sponge cakes, it’s easy to see its critical role. We need enough gas molecules to push against the batter and give it a good lift. But air is actually crucial to all baked goods, even those that are leavened predominantly by baking powder or yeast.

Air is a mixture of gases including nitrogen, oxygen, and carbon dioxide.

Air bubbles are the foundation of leavening.

Air is the foundation for all leavening. No other leavener—baking powder, baking soda, steam, or yeast—creates new air pockets. Instead, any gases that they create expand existing air bubbles. If we put a cake batter with no air bubbles into the oven, no amount of baking powder will save it. Any gas that’s created will simply leave the baked good.

Why is this? Remember the concept of diffusion? We talked about sugar and water diffusing into and out of fruits in a sugar solution, but diffusion’s also at work here. Essentially, molecules try to spread as far apart as possible, like commuters on a subway during a pandemic. And because the air contains a lot of space for the gas molecules to spread out, like an empty subway car, gases diffuse there. To the gas molecules, it makes no difference whether that air is a tiny pocket in the batter or the large open space of the oven. The concentration of gas molecules is the same. To a commuter, it’s like walking down a subway and seeing one person seated every other row the whole length of the train. It doesn’t matter if some of the subway cars are longer; any car would offer the same amount of space. Similarly, for a gas molecule, the only goal is to find the space with the least concentration of gases.

Before baking (left), gas molecules created by leaveners (green) are concentrated in the batter. Other gas molecules (blue) come from the air. As the batter bakes (right), gas molecules diffuse from the batter into the air, where there is more space.

Thus, for proper leavening, we add air bubbles within the batter. Only then can we effectively trap the additional gas molecules from other leavening agents to create a product with high volume and desirable texture.

We can add different numbers and sizes of air bubbles.

So how do we add air to our batters? It’s actually impossible to avoid. Sugar, for example, has rough edges that carry air into the batter or dough with it. Many cake and cookie recipes take advantage of this by creaming sugar with butter. The sugar carries air into the butter, creating the air pockets that other leaveners ultimately expand. In other recipes, air is beaten into eggs until they increase several times in volume. Even actions such as stirring, mixing, or sifting add air.

We can also redistribute air bubbles more evenly throughout an uncooked batter or dough. As we’ll learn in the next post, baking soda typically creates carbon dioxide gas as soon as it’s combined with wet ingredients. The carbon dioxide expands air bubbles that we already mixed into the batter, which lightens the batter and makes it easier to handle. If we continue to work this batter, we break the enlarged air pockets into several smaller ones. Similarly, the process of punching down risen dough also breaks apart large bubbles created by clumps of yeast. Air bubbles that start off the same size typically expand at a similar rate in the oven, so the baked product ends up with a finer, more uniform crumb. If the distribution of air is uneven to start, as shown in the first column in the figure below, the differences are exacerbated during baking to form large holes and a coarser, less even crumb in the final product.

The distribution of air bubbles within a batter influences the crumb, texture, and volume of the final baked good.

The importance of proper mixing

Another cause of coarser crumb is undermixing. If we don’t mix the batter enough, there are few air bubbles in the baked good. As shown in the second column above, as the batter bakes, these air bubbles expand a lot, creating the large holes that define a coarse crumb. Furthermore, because there aren’t enough air pockets to hold all the gases created by other leavening agents, a lot of gas molecules will diffuse into the oven instead. As a result, the baked good contains little air and is low in volume.

An overmixed batter, which has too many air bubbles, also has low volume. As the air heats up in the oven, each one of the bubbles expands, stretching the batter between them thinner and thinner. The batter becomes so thin that it is too weak to hold up the structure of the baked good, and the structure collapses as if there was never any leavener in the first place. The result is dense texture and low volume. (Overmixing can also cause other problems, like in these muffins.)

A properly mixed batter, then, has the correct number of air bubbles to take full advantage of leavening gases, whether they come from the air, from baking soda, or from steam. As the batter or dough bakes, the air bubbles all expand and contribute volume. They stretch the batter around them to tenderize the baked good. When the batter solidifies, the pressure of the gases breaks the walls surrounding them to produce a porous network with a fine, even crumb. And when the baked product is removed from the oven, the structure maintains its rigidity and shape even as the gases contract. (For a more detailed discussion of this process, see the introduction.)

Leaveners expand air bubbles.

Once we have a good foundation of air bubbles, we can add more gas molecules to really help the batter expand in the oven. Here, I provide a brief introduction to the main sources of leavening in the home kitchen, but we will discuss them in more detail in future posts.

Chemical leaveners produce carbon dioxide.

Chemical leaveners such as baking soda and baking powder undergo chemical reactions to produce carbon dioxide. They work quickly (relative to yeast) to produce a lot of gas to leaven cookies, cakes, muffins, biscuits, and more. If we add too much chemical leavener, it is possible to produce too much carbon dioxide and overleaven. In an overleavened batter, the air bubbles expand quickly, rise, and burst at the surface before the batter sets, leaving the baked good dense and flat, as if it had been overmixed or underleavened. In the next post, we’ll delve into much more detail about chemical leaveners and how they work.

Yeast produce carbon dioxide and alcohol.

Yeast are another source of carbon dioxide, but because they are living organisms, they’re considered biological leaveners. When yeast eat sugar, they produce carbon dioxide as a byproduct. Yeast work on a much longer time scale than chemical leaveners, so they’re used for breads, which ferment for hours. Yeast also produce alcohol as a byproduct, which, like water, vaporizes to contribute to leavening.

Liquids vaporize into gas.

In the oven, liquids reach temperatures so hot they vaporize into gas. In addition to water from ingredients like egg whites, butter, and milk; alcohols, such as those made by yeast and the ones in flavorings like vanilla extract, vaporize to give baked goods additional volume. Because these ingredients enter the oven in their liquid state, their molecules are packed much more tightly than the gas molecules from the air. As a result, when liquids do vaporize, they actually contribute a lot more gas molecules and leavening power than air. Baked goods such as cream puffs and puff pastry that rely primarily on steam for leavening use a high oven temperature to quickly vaporize a lot of water and produce maximum lift.


All leavening begins with air. And although many baked goods rely on chemical leaveners, yeast, or steam to carry most of the leavening power, none of these leavening agents would be effective without a foundation of air bubbles. Thus, it’s crucial to correctly mix batters for the best volume and texture. In the next post, we’ll talk in more detail about the chemical leaveners that expand these air bubbles to produce light and airy baked goods.


Corriher, S. O. Bakewise; Scribner: New York, 2008.

Figoni, P. How Baking Works, 3rd ed.; John Wiley & Sons, Inc.: Hoboken, 2011.

Rathnayake, H. A.; Navaratne, S. B.; Navaratne, C.M. Porous Crumb Structure of Leavened Baked Products. International Journal of Food Science, 2018.

Stauffer, C. E. Functional Additives for Bakery Foods; Van Nostrand Reinhold: New York, 1990.

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