Yeast - we sort of know what it does, but why? Here's some easy science for you!
What do most (if not all) breads have in common? On the most basic level, they all involve cooking a mixture of milled grains and water.
Some are amazingly simple -nothing more than flour and water, baked until crispy. Others involve complex interactions between flour and the leaveners that give them their porous, tender quality.
Leaveners can be either powder or soda and yeast.
Baking powder or baking soda work quickly, relying on chemical reactions to inflate dough or batter. Baking powder and baking soda are used to leaven baked goods that have a delicate structure, ones that rise quickly such as quick breads like cornbread and biscuits.
Yeast, is a live, single-celled fungus. Most people are familiar with yeast in its mass-produced form: the beige granules that come in little paper packets. This organism lies dormant until it comes into contact with warm water. Once reactivated, yeast begins feeding on the sugars in flour, and releases the carbon dioxide that makes bread rise (although at a much slower rate than baking powder or soda). Yeast also adds many of the distinctive flavors and aromas we associate with bread.
But leavening agents would just be bubbling brews without something to contain them. Here’s where flour comes in. There are lots of different types of flour used in bread, but the most commonly used in raised bread is wheat flour. This is because wheat flour contains two proteins, glutenin and gliadin, which, when combined with water, form gluten. As you knead the dough, the gluten becomes more and more stretchy. This gum-like substance fills with thousands of gas bubbles as the yeast goes to work during rising.
Starch, a carbohydrate that makes up about 70% of flour by weight, also gets in on the act. When starch granules are attacked by enzymes present in flour, they release the sugars that yeast feeds on. Starch also reinforces gluten and absorbs water during baking, helping the gluten to contain the pockets of gas produced by the yeast.
Sometimes, a baker will let the dough rise several times, allowing the gluten to develop more completely and the yeast to add more of its flavors. When the dough is finally cooked—either in an oven, over a fire, or in a steamer, depending on what kind of bread you’re baking—the yeast inside it continues feeding, and the pockets of gas in the dough continue to expand. As the temperature of the cooking dough rises, the yeast eventually dies, the gluten hardens, and the dough solidifies. Et voilà! Bread!