From Birth to Bake: How Bubbles Form in Batters and Doughs

This story is as old as the stalest bread, which is to say at least 14,000 years. The act of mixing flour with water, raising it with a leavener, and baking it goes back at least that far. The process is deeply familiar, yet most of us don't know much about what is happening down in that mass of dough. Let me tell you, it's complicated.

The science of doughs and batters can fill books upon books and still not cover all there is to know about how it works. So a relatively brief article like this one that's focused on just one facet of a much larger story is guaranteed to have holes in it, which is fitting, because the subject here is exactly that—the bubbly spaces that inflate breads, cakes, and countless other leavened baked goods. Without the bubbles that aerate them, breads would be rock-hard lumps that we'd have to suck on in hopes of dissolving a bit with our saliva, and cakes would be dense and rubbery discs that might work best as drain stoppers.

Ask people how air bubbles form in doughs and batters, and they'd likely say that yeast or a chemical leavener like baking soda produce gas bubbles that provide aeration. And they'd be partly correct, but the full story is more complicated—and more interesting. Why exactly do cakes have such fine, tiny bubbles while breads can have huge hollows in them? It's not an easy question to answer, as doughs and batters are incredibly complex systems.

The importance of bubbles, though, isn't just about the bubbles themselves, it's about what they make possible. Only through an elaborate dance of a multitude of chemical and physical processes can a loaf of bread or moist cake exist. Understanding this science will not automatically make you a better baker (though it certainly can!), but it will help you understand why so many baking recipes work the way they do.

I want to take you on a journey to witness firsthand the lifecycle of bubbles in doughs and batters. To do it, we're gonna fire up the miniaturization-laser of our imaginations and shrink ourselves down, Honey-I-Shrunk-the-Kids-style, to visualize the bizarro world of bubbles firsthand.

But First, What Are Batters and Doughs, Anyway?

Baked right into English is a distinction: There are doughs, and then there are batters. And indeed, there are critical differences between them. Doughs and batters also share a fundamental similarity: They're both aerated mixtures of flour and water. And while most of us who have spent our lives biting into sandwiches and birthday cakes are unlikely to have realized it, doughs and batters are foams, as are the solid breads and cakes they bake up into. The next time you use a froth of toothpaste to scrub compacted bits of morning toast out of your molars, take a moment to appreciate how weird it is that you are using foam to clean foam.

Doughs, as you've probably noticed, are drier than batters. Doughs may be sticky, and they're very much malleable, but batters are far more wet and flowing. This is because doughs have more flour than water, while batters have more water than flour. Doughs also tend to be simpler in terms of ingredients, often consisting of just flour, water, leavener, and salt. Many batters have several additional ingredients, including eggs, sugar, flavorings, and various sources of fat (butter or oil, milk, the egg yolks, etc.). Bread doughs, meanwhile, are more often leavened with yeast, while batters tend to be leavened by gas-producing chemicals like sodium bicarbonate (baking soda).

Yet all of this is a generalization. There are doughs with eggs and fat (hello brioche!), and batters that are little more than flour and water. Similarly, there are doughs that are raised with baking soda (they don't call it soda bread for nothing) and batters teeming with yeast.

To put it bluntly, there's more complexity and nuance to these categories of food than I can account for here, and many recipes exist on a spectrum somewhere between the doughiest dough and the batteriest batter. So, in the interest of getting the basic points across, we'll be looking at the most stereotypical versions of bread dough and cake batter.

Conception: The Beginning of Bubble Life

Close your eyes and picture this: You're inside a mass of bread dough that has just had its ingredients of flour, water, yeast, and salt combined. Stretching off in all directions are chains of gluten proteins that seem to go on for miles, so long they fade into the murky distance. You can see that the gluten proteins are just starting to bond with each other to form a network that will be critical to all that happens next. They're going to give this bread dough strength and elasticity—which will be essential for trapping the gas bubbles later on.

You can also see starch granules everywhere, suspended in the watery matrix of gluten proteins like boulders tangled in an underwater net. They will eventually swell with water and later, when heat is applied, they'll gel and set, turning a soft dough into something much more solid.

You can also see yeast cells beginning to feed. They're eating the starch granules, digesting the glucose inside to generate energy and producing alcohol and carbon dioxide (CO2) in the process. Despite imagining ourselves at such a tiny scale, the carbon dioxide is still much, much smaller, and therefore impossible to see. The CO2 molecules are diffusing across the yeast cell walls into the watery solution that surrounds them—not as bubbles of gas, but as molecules dissolved in the water. The yeast will continue to eat the glucose in the starch granules and produce more and more alcohol and CO2, but it will take a while; they work slowly and are just getting started. Good thing is, in a bread dough, time is on our side: The yeast have a massive supply of food and there's a lot left for the baker to do before the dough is ready for the oven.

Speaking of the baker, they're about to do something important: Knead the dough. If you get motion sick, you may not want to imagine you're inside the dough at this point, but a lot happens here. First, the baker is mixing the ingredients more thoroughly, distributing the starch, proteins, salt, and yeast more evenly throughout the mass, which will ensure a more even crumb later. All that mixing is also working the gluten proteins round and round, helping them to bond to each other and building an even stronger network to trap air. But there's a third thing that's often overlooked in the explanation of why we knead dough: Air pockets and bubbles are being worked into it.

Without mixing and kneading, it would take longer for a yeasted dough to aerate, and the size and distribution of the eventual air bubbles will be more uneven in both size and distribution (some breads are kneaded minimally precisely to encourage large, uneven bubbles). By working air into the dough mechanically, the baker gives a jump-start to the aeration process, offering sites for larger bubbles to form throughout the dough, while also evening out the bubbles in the crumb.

At the same time, the dissolved CO2 that's being excreted by the yeast is diffusing through the water in the dough. Anywhere these dissolved CO2 molecules encounter an irregularity in the mass of dough—and there are irregularities everywhere, from the varied shapes of starch granules to impurities in the dough and bits of salt—they will gather and cluster to form the teensiest, tiniest bubbles. This is the same thing that happens in a glass of beer or soda, where microscopic irregularities on the surface of the glass provide nucleation sites, as they're called, for bubbles to form, eventually break free, and float upwards.

The big thing to know here is that in a dough, there are two pathways for bubbles to form: Larger ones of atmospheric air (mostly nitrogen and oxygen) that are incorporated mechanically when the dough is mixed and kneaded, and miniscule ones of CO2 that are forming at nucleation sites throughout the dough. Additional dissolved CO2 will find its way as it travels through the watery phase of the dough to these bubbles, where it can then escape into them as a gas.

Let's now move by power of imagination into a batter. Sploop. We're now bobbing in a thick slurry that was stirred together just moments before. Here, you have a lot more water along with several other ingredients—dissolved sugar, eggs (including emulsifier-rich yolks), fats, flavorings (is this a chocolate cake? Let's pretend it's a chocolate cake—I can definitely see chocolate now!), and even more fats, sugar, and proteins from milk. Gluten proteins float through the swampy mix with us, but they seem to be struggling to form much of a network—the fats in the batter appear to be attracted to the gluten proteins, and they're getting in the way of the kind of gluten-to-gluten bonding we saw in the dough.

In this swampy batter, we see no yeast. Instead, there's a chemical fizzing away: sodium bicarbonate (baking soda), which has wasted no time reacting with acids in the batter. The byproduct, once again: CO2. The whole pace of activity is different here. Unlike the snail-like pace of the yeast in the dough we were just in, the baking soda in this batter is just going absolutely nuts. One thing is clear: batters like this one are developing on a much shorter timescale than doughs—there's no time to wait for yeast to slowly build up a supply of CO2, it's being created in much greater quantities by the baking soda as soon as the batter is mixed.

Swimming through this slightly viscous batter, we can see the lingering effects of the mixing that the baker did right before we dropped in: the mixture is homogenous, thanks to all that mixing, and little bubbles of atmospheric air are suspended in the floury soup. In this way, the batter is very much like a dough. But, unlike a dough, gluten didn't really have much of a chance to form here—there's no elastic network of wheat proteins to trap air nearly as effectively as in a dough.

From the beginning, these batter bubbles tend to be smaller than what we saw in the dough. That's because batters are rich with emulsifiers from egg yolks and other ingredients that work as surfactants to help form a stable shell around the bubbles—the more emulsifiers there are, the smaller the bubbles can be. If our baker were to reduce the amount of emulsifiers in the batter, we'd see larger bubbles because there simply wouldn't be enough surfactants available to cover the increased surface area of smaller bubbles.

Aside from these differences, the processes we're seeing in a batter are largely similar to a dough: the dissolved CO2 created by the baking soda diffuses through the water phase of the batter, nucleating on physical imperfections to form the tiniest bubbles, while mechanically-incorporated air provides a bubble-boost to further aerate the batter.

With the bubbles formed, it's off to the races. For bread, this will be a long-distance run. For the batter, it's more like the 100-meter dash. But there's no need for a winner here, we'll get our delicious and airy baked goods eventually.

Bubble Development: The Growing Years (or Days, or Hours, or Minutes…)

Let's stay in the batter for a moment, because some interesting things are happening here in the short time we have before baking. As I described above, we're floating in a more liquidy medium, rich with egg and wheat proteins, emulsifiers, starches, sugars, baking soda, flavorings, and, at this point, lots and lots of tiny bubbles.

Because of the relatively lower viscosity of the batter compared to the dough, the bubbles are moving around much more easily, and they're moving up due to their buoyancy. At the surface, bubbles pop and release their gas into the air, like sulphur burbling up through a muddy geothermal spring (thankfully, minus the sulphur part). The batter is degassing much faster than the dough—it has no good way to truly trap the air bubbles the way a stretchy dough can. It can slow them down, but they'll eventually find their way to the surface and out into the air. Hence why we need to bake the batter sooner rather than later.

The bubbles are also engaged in a fascinating series of interactions. Some bubbles are coalescing—literally fusing together into larger bubbles. Others are doing a bizarre trick called Ostwald ripening, in which a smaller bubble gives its air to an adjacent larger one, not by coalescing but simply by having its gas molecules escape across the liquid divide into the larger bubble, like people hopping across a stream, until the smaller bubble shrinks to nonexistence. It's always the smaller bubble that gives its gas to the larger one, a consequence of larger bubbles having a lower pressure, which is…well, the easy way to say it is that it's how nature wants it.*

* Okay, okay, that's a cop-out. Want to know why smaller bubbles have higher pressure than larger ones? Mostly, the answer has to do with the surface tension of the bubble's shell: Smaller bubbles have a more extreme curvature than larger ones, and more curvature puts the bubble under more pressure, similar to how a tiny balloon is so much harder to blow up than a large one.

All of this coalescing and ripening drives the bubble structure towards an equilibrium in bubble size. At the same time, there's an upper limit on bubble size in a batter. The reasons are many. Part of it is just time—a batter, being a shorter-lived foam compared to a dough, has less time to amass larger bubbles. Part of it is buoyancy: As bubbles become larger, they float to the surface more quickly, exiting at the surface of the batter. Bigger bubbles go bye-bye more quickly.

But another big part of it is the nature of the surrounding batter slurry, and once again we get to a fundamental difference between doughs and batters here. In a dough, the air is trapped in a strong but elastic gluten network that can swell and swell as the bubbles collect more gas. In a batter, though, there's no significant gluten network to trap the air. Instead, the bubbles are held stable by the emulsifiers in the wetter batter, and there's a threshold at which a bubble in a batter just can't get any larger or it'll pop due to instability. Plompfffffffff-blub, I think, would be the appropriate sound to imagine here.

The dough, meanwhile, is growing much more slowly, the gluten network expanding like a bunch of rubber balloons to contain a greater quantity of air as the yeast produces more and more of it. While coalescing and Ostwald ripening can happen in a dough, it's much less frequent due to the lack of mobility of the bubbles in a lower-hydration mass of dough; on top of that, the gluten network acts like barriers for bubble crowd-control, making it more difficult for those bubbles to interact freely. So much of bread-making at this point involves the baker, who can influence bubble size and distribution with a variety of techniques, from a very hands-off no-knead approach for bigger, more uneven bubbles to methods that involve active folding, punching down, slapping, and more, to divide bigger air bubbles into smaller ones while improving the evenness of their distribution.

Life Stage: Immortality (-ish)

It's time to bake. Thankfully, one of the benefits of imagining we're inside bread dough or cake batter is we can stay "inside" during baking without actually being roasted to death. Our batter has now been transferred to cake pans, our bread loaf is formed, fully proofed, and ready to go into the oven.

For cake batter, the oven temperature is generally a cooler 325°F or so. Bread goes into hotter ovens of at least 400°F, sometimes much hotter (think: Neapolitan pizza in an 800°F oven). Why the difference? With bread, we tend to want a dramatic and rapid oven spring, the dramatic increase in volume mostly caused by steam as the water in the bread vaporizes in the heat—once the exterior dehydrates enough to begin forming a crust, it won't allow much further expansion. This is also why breads are often baked with steam or spritzed with water in the beginning stages, to stave off crust development and allow more oven spring.

Cakes, on the other hand, do not require as dramatic of a rise, nor do we want them to form tough crusts, hence the lower oven temperature. Plus, cakes, with their finer bubble size, tend to be more dense than breads, so it takes heat longer to penetrate to the center; if the oven were too hot, the cake would harden on the exterior and still be raw in the middle. More moderate heat helps the cake cook through without over-baking on the outside.

Here, once again, we need to stop and appreciate one of the most important transformations that happens in the whole life story of the bubbles in these baked goods. Up until this point, the bubbles in both cases were forming what is called a closed foam, meaning each pocket of air is discrete and cut off from the rest. Think of the unbaked bread or cake like a huge house with tons of rooms (the bubbles) that each have an explosive device in them. Before baking, all the rooms have all their doors closed. If we were to light a fire in one room, it would eventually cause the explosive device to combust, blowing the door right off its hinges and opening the room up to the next. The heat would then rush into the next room, build, and blow the next device, and on and on until almost no rooms have doors anymore.

The same thing happens in a mass of batter or dough. As heat enters, starting from the outside and working its way to the center, water begins to vaporize and form steam, expanding a whopping 2000 times in volume compared to its liquid state. The bubbles expand, and then they burst into their neighbors' spaces, in a chain reaction of ruptures and explosions that drive heat much more rapidly towards the center, speeding up cooking, and turning the baked goods into open foams.

Which is to say, by the time they're baked, breads and cakes are no longer filled with thousands or millions of little bubbles, but rather one huge bubble that snakes and weaves its way through the crumb.

Up until baking, the bubbles in our batter and our dough had acted as a kind of internal support structure—in a sense, the bubbles were a solid framework holding up the liquid substance filled with starches and proteins. But as the bread and cakes cook, the starches gel and, in the case of egg-enriched batters, the egg proteins denature, firming up the crumb and setting the baked goods in their final solid form. 

Once they're cooled, as the accumulated gas and steam that provided so much expansion just minutes before drift off into the atmosphere, the loaves and cakes will not collapse. The bubbles that formed them have been immortalized in a final casting. Well, as final as any cake or bread will ever be, anyway, because I'm hungry. Can I interest you in a slice?