A leaven agent is a material that induces the expansion of doughs and batters through the release of gasses within those mixtures, creating porous-structured baked goods. These agents include air, steam, yeast, baking powder, and baking soda. Leavening can be achieved by the fermentation process, which releases carbon dioxide. Baker’s yeast, consisting of living cells of the Saccharomyces cerevisiae strain of yeast, is available as a pressed cake and in powder form. When added to the dough, the yeast begins fermentation by acting on certain sugars that have been added to other dough ingredients, releasing carbon dioxide and substances that have an effect on the flavor and aroma of the baked product.
Yeast-leavened products include certain types of breads and rolls and sweet-dough products such as coffee cakes, doughnuts and Danish pastries. The sourdough process used for rye breads uses a small portion of the dough or sponge in which the sugar-fermenting bacteria have been able to mature. When added to a fresh mixture of dough, the sponge produces fermentation. Commercial sour crops are often used as replacements for naturally fermented sourdoughs.
How Leavening Agents Work
Dough is made from wheat flour, which comprises a pair of proteins called gliadin and glutenin. When water is added and mixed, gliadin and glutenin are fused to form a new protein called gluten. Gluten molecules are arranged into chains that can be very long and elastic. This elasticity explains why a piece of bread dough can be spread between fingers. The harder you knead the dough, the more it stretches.
The gas created by the leaven agent creates thousands of tiny bubbles in the dough, which causes it to grow. Imagine that thousands of small balloons are blown up in the air. The dough becoming stretchy, almost like balloons. If it weren’t, instead of popping a balloon, it would be like blowing a bottle of water with a straw: the bubbles would instantly explode and the gas would escape. Owing to the elasticity of the dough, the bubbles expand without exploding, leaving the gas contained in the bubbles long enough for the third phase of the reaction to take place.
The heat of the oven heats the dough, allowing it to set, while the tiny bubbles are in their inflated state. So, as soon as the gas eventually exits, the air pockets retain their form instead of deflating. The size of the air pockets defines the texture of your baked foods. Small air pockets create a smooth texture, like a cake. Larger ones produce a rougher texture, like crusty pizza.
Types of Leavening Agents
In baking, leavening is the air that allows bread, cakes, and other baked products to rise as they go into the oven. This gas is generated in various ways, depending on the form of leaven agent used. This, on the other hand, varies according to what is being baked. But the best way to think of it is that the leaven agent creates gas, and the gas allows the dough or the batter to expand.
There are four main types of leavening agents: biological, chemical, mechanical and steam.
Biological Leavening Agent
Yeast is the most commonly used biological leaven agent in baking. It is made up of single-celled organisms (a kind of fungus) that experience an existence far removed from what we would know as “life,” and play a vital role in cooking. Yeast is responsible for the fermentation process, without which there would be no such thing as beer, wine or bread.
In reality, in the fermentation process, the yeast consumes sugar and creates carbon dioxide (CO2) gas and alcohol. In most cases, the yeast doughs rise once, are beaten and then rise again. They end up in the oven, where the heat rouses the yeast to one more big CO2 expulsion before they hit 140 F and die.
Chemical Leavening Agents (Baking Soda and Baking Powder)
Chemical leaven agents are commonly referred to as baking soda (sodium bicarbonate or bicarbonate of soda) are white powders that come in a package and have a pH level of 8 or 9 which indicates that they are base in nature. When combined with an acidic component, it can produce a chemical reaction that causes CO2 to be released. Some of the acidic ingredients used to activate baking soda are buttermilk, lemon juice, yogurt, sour cream, molasses and honey. In its dry state, the baking soda is inert but reacts instantly after it has been triggered.
In comparison to the reaction of yeast, which happens slowly over a long period of time, baking soda works rapidly, which is why the bread and muffins it makes are called quick breads.
Baking powder is a substance made up of baking soda plus some other acidic portion, also in powder form. It’s inert as long as it remains dry. When moisturized, the chemical reaction starts. It’s less rapid, though, than a pure baking soda reaction, because it’s double-acting, which ensures that it continues to work when combined and then lets out another blast of gas when warm. That’s why some quick bread batters, including pancakes, can be kept for a while without losing their strength. It is however important to note that baking soda cannot be substituted with baking powder or vice-versa.
Mechanical Leavening Agent
Mechanical leavening process is carried out by creaming, which is a process of beating sugar crystals and solid fat (usually butter) together in a mixer. This creates tiny air bubbles into the mixture, as the sugar crystals are mechanically sliced into the structure of the fat. Creamed mixtures are typically further leavened by a chemical leaven such as baking soda. This is widely found in cookies.
Using a whisk on some liquids, in particular cream or egg whites, foams may also be created by means of mechanical activity. This is the process used to create sponge cakes, where the egg protein matrix formed by intense whipping provides almost all the structure of the finished product.
Steam (Vaporous) Leavening Agent
Unlike yeast, baking powder, and baking soda, all of which produce CO2 gas, steam is merely water vapor created when the water in the dough reaches 212 F and vaporizes.
Steam can sound dull as compared to the interesting processes mentioned above, but it is a strong force. When water is steamed, its volume raises by about 1,500 times. The force in which this expansion takes place has been enhanced by higher temperatures. Puff pastry and choux pastry are two examples of pastry that only uses steam as a leaven agent and, when correctly baked, is superbly airy and flaky.
The trick to this leaven agent is to ensure that the steam is absorbed by the dough. For puff pastry, which is achieved by applying butter to the dough and then folding it into book folds. This technique produces hundreds of layers which, as a result of the steam created by the liquid in the dough and the water in the butter, swell into separate flaky layers.
Choux pastry, which is used to produce cream puffs, eclairs and beignets, uses a particular technique. The glutens are partly denatured by boiling the dough once on the burner, which decreases the elasticity of the dough. In the meanwhile, the starch in the flour is gelatinized, which serves to structure the flour. As a result, when baked, the steam inflates the pastry, but instead of snapping around, it keeps its form and the air pockets in the middle of the pastry remain unchanged.