What’s the difference between a light, fluffy cake and a short, dense one? A flaky pie crust and a tough cracker? The answer lies in leavening. Leavening agents lift and expand batters and doughs to make fluffy cakes, flaky pastry, and light bread. There are several leaveners: air, baking soda, yeast, and steam are a few of the most common in the home kitchen. In this post, we’ll explore how all leaveners work. Then, we’ll dive deeper into specific leaveners in the next few posts.
Leavening is the work of gases, including those from air. Air is not devoid of matter. Instead, it’s rich with tiny molecules of nitrogen, oxygen, water vapor, and carbon dioxide. In the oven, these gases expand to give our baked goods volume and tenderness. To understand how this happens, let’s first review what gases are at a molecular level.
Gases are unbonded molecules.
Gases are one of the three states of matter we encounter in the kitchen. The other two are solids and liquids. The same substance can exist in any state of matter depending on the temperature. Water, for example, is solid ice below 32°F (0°C), liquid water from 32 to 212°F (0–100°C), and gaseous steam above 212°F (100°C). In all three states, individual water molecules do not change—they remain H2O—but their relationship to each other changes.
In “Proteins in the Kitchen,” we discussed the chemistry behind how water molecules form hydrogen bonds. Essentially, water molecules like to stick together because their hydrogens and oxygens attract each other. In ice, water molecules arrange themselves in a highly organized lattice pattern that contains as many hydrogen bonds as possible. They’re like tennis balls packed so tightly into a crate that they can’t shift, even if we stick our hand in. In ice, the molecules are fixed in place because the hydrogen bonds hold them there. As a result, ice is a hard solid that does not change in shape.
If we heat the ice a little, though, we add more energy into the water molecules. They wriggle more and more, and eventually, at water’s melting point around 32°F (0°C), they start breaking free of the lattice. It’s as if we smashed the crate holding the tennis balls. The balls collapse, spread out from the center, and roll along the ground. We can easily arrange them in different patterns. Similarly, as ice melts, the lattice of water molecules collapses. Some hydrogen bonds still hold the molecules together, but they break and reform as the liquid moves. Thus, water can change its shape as it flows.
If we continue to heat liquid water, the molecules move faster and faster. Eventually, at water’s boiling point around 212°F (100°C), water molecules completely sever their hydrogen bonds and escape the liquid as steam. It’s like our tennis balls have started zooming around the court, powered by invisible tennis rackets. Each ball’s movement is independent, and they’re difficult to catch and contain. In this state, the molecules are gas.
Temperature determines the state of matter.
I used water as an illustration here because it’s one of the few substances we see in all three states of matter. But any molecule can exist in multiple states. Our temperature range is just too narrow to see them. Dry ice, for example, is solid carbon dioxide, but it only solidifies at temperatures less than -109°F (-78°C). Carbon dioxide’s freezing point is so low compared to water because, due to its structure, its molecules barely attract one another. At temperatures warmer than -109°F (-78°C), they’re released from the solid lattice crate to zoom around the tennis court as gas molecules.
The specific melting and boiling points of a substance depend on its molecular structure and the consequent strength of its molecular attractions. Water molecules attract strongly, like industrial magnets, so it’s difficult to pull them apart, while carbon dioxide molecules barely attract, like cheap fridge magnets. Because water molecules need more energy to be separated, it takes more heat to vaporize them.
Gases expand to add volume and tenderness.
Now that we’ve reviewed what a gas is, let’s explore how it behaves in the oven. If we continue to heat gases, the molecules continue to gain speed. Imagine the free tennis balls flying faster and faster and moving further apart from each other. Eventually, they will reach the nets surrounding the court, which they will hit with more and more force. The nets bulge further and further out to contain them. Now, imagine that the tennis balls are trapped in individual air bubbles in our batter or dough. The same thing happens. As the gas molecules heat up in the oven, they zoom faster and faster, taking up more space. Eventually, they press against the batter and force it to expand like a balloon to contain them. When this happens in hundreds of individual air bubbles, the batter or dough rises.
Like a balloon, the batter also stretches thin as it accommodates the expanding gases. In this way, leaveners actually help to tenderize the baked good. As we discussed in the context of gluten, we can think about the relationship between structure and texture with spaghetti. Thin layers, like single spaghetti strands, are easy to bite through, while thick walls, like clumps of spaghetti, present more resistance.
When the batter or dough reaches a high enough temperature, gluten, starch, and proteins from eggs set and solidify the baked good. If we think back to the tennis court, it’s like the nets turn into wooden walls as the tennis ball gas molecules fly faster still. Eventually, the balls gain so much energy they smash through the wall, and some escape out of the tennis court. In the oven, something similar happens. The gas molecules continue to move faster and further as they get hotter, but because the batter no longer stretches to accommodate them, they build up pressure until they burst through the walls surrounding them and into the neighboring air pocket (see “Overmixing Muffins” for a dramatic example of this). Ultimately, the gases create a maze of air cells leading out of the baked good and into the oven. This is the crumb structure we see when we slice open a muffin or a cake. The baked good still contains gas, but as the gas molecules heat up and spread apart, any gas molecules that do not fit within the baked good escape into the oven.
When we take the baked good out of the oven, the gases within it cool down and lose energy. The gas molecules condense and contract, no longer pushing against the cooked batter or dough. If the structure molecules from eggs and flour have set, this will not be an issue. The rigid structure no longer needs the pressure of the gases to keep its volume. However, if the structure has not set, the baked good sinks, loses volume, and becomes dense. This is also why it’s so important to keep the oven door closed. As soon as the door opens, the oven temperature drops, the leavening gases contract, and the uncooked batter or dough collapses. This could lead to a loss of volume in the final baked good or an uneven bake.
As we’re starting to see, leavening is a delicate balance between gases and the rest of the batter or dough. If the conditions are right, we have plenty of gas to raise the batter or dough, and we trap enough of that gas to gain volume and tenderness before the batter or dough solidifies. In the next few posts, we’ll explore how we add gas into our baked goods. We’ll also discuss different leaveners and the specific characteristics that lend them to different recipes.
Corriher, S. O. Bakewise; Scribner: New York, 2008.
Figoni, P. How Baking Works, 3rd ed.; John Wiley & Sons, Inc.: Hoboken, 2011.
Wilbraham, A.; Staley, D. D.; Matta, M. S.; Waterman, E. L. Chemistry, Prentice Hall: Upper Saddle River, 2008.