Bread rises because of yeast, a fungus made of a single cell. The yeast in bread dough is alive. Yeast eats sugar and gives off carbon dioxide gas. Scientists call the process where yeast cells use up sugar and release gas fermentation. Yeast also gives off a second product as it eats sugar, an alcohol called ethanol. Dry yeast needs warm water to wake up. Bread dough is set in a warm place to rise until it doubles in size. Rising dough is covered, often with a clean, damp towel. Punching down risen dough means pressing out some of the air with your fist.

Wheat flour has two proteins, glutenin and gliadin, that join to make gluten. Before kneading, they stay separate. Mixing and kneading lets them form chemical crosslinks into a protein network called gluten. Adding water hydrates the glutenin proteins in flour, which swell and become stretchy and flexible. Gluten works like a net that traps the gas in dough. Kneading means pushing, pulling and stretching the dough. It also breaks up the pockets of air in dough. The air pockets between gluten strands fill with gas from the yeast, and the smaller the pockets, the finer the bread's texture.

Baking soda and baking powder make carbon dioxide gas bubbles that help cakes and pancakes rise. Baking soda starts to make gas almost as soon as it touches an acid. Baking soda heated with enough acid gives off twice as much gas as baking soda heated without acid. Heat alone breaks down sodium bicarbonate into carbon dioxide at about 80 degrees Celsius (180 degrees Fahrenheit) or more. An acid-base reaction gives off one carbon dioxide for every sodium bicarbonate, so it needs half as much.

Baking powder has an acid already mixed in with the baking soda. Double-acting baking powder makes gas at two different times. In baking powder, about 15 percent of the gas is released while the batter is still cold and about 85 percent is released in the oven, starting at about 105 degrees Fahrenheit. Baking powder has a second acid that does not react until the batter is both wet and hot, so the batter keeps rising for longer in the oven.

In the 1800s sour milk or cream of tartar supplied the acid for baking soda, but the acidity of milk varied and the supply of cream of tartar was unreliable. There was also a search for a way to make bread without the hours that yeast takes to work. Harvard chemist Eben Horsford hit on a combination of acid phosphate and sodium bicarbonate, and he replaced cream of tartar in baking powder with an acid phosphate. He added a bit of corn starch to keep it dry until it was used. In 1843 British chemist Alfred Bird made a baking powder for his yeast-allergic wife.

In yeast dough, the gluten proteins in flour do a job like that of egg proteins in chemically leavened baked goods: they help trap air bubbles until the dough sets. This is why bread flour has more protein than cake flour. The proteins in eggs become permanently denatured by heat, which means they unfold and cannot refold. The altered proteins give the liquid foam of a batter a solid structure that holds its shape. Air beaten into a batter gets trapped between protein structures and adds to the final lift of the baked product.

Water in a flour mixture makes steam when heated. Steam expands to many times its original volume and is the main leavening agent in popovers, pie crusts and pastries. When dough first goes into a hot oven it rises fast, which bakers call oven spring, and the steam from the water in dough pushes the dough up. Gas bubbles in a batter keep expanding until the starch gelatinizes and the cells become rigid. This starts at about 140 degrees Fahrenheit and is mostly complete by about 167 degrees. Yeast dies when it gets too hot.

Wild yeast floats in the air and lives on flour and fruit. Sourdough bread rises with wild yeast and bacteria instead of yeast from a packet, and a sourdough starter is kept going by adding fresh flour and water to it. As of November 2026, a food historian quoted by Live Science says the first known leavened bread made with semi-domesticated yeast dates to around 1000 B.C. in Egypt. Scholars debate its origin because Mesopotamians also made yeast-risen bread.

Rich doughs, such as brioche and dinner rolls, contain a lot of fat, sugar and eggs, which give the bread a softer feel and richer taste. Children need a grown-up with them in the kitchen when they bake. Baked loaves are cooled on a wire rack so the bottom does not get soggy. A chemical reaction makes the crust of bread dark and toasty, and bread is done baking when it turns golden brown.

Words to know

• fermentation: yeast using sugar
• gluten: dough net
• denatured: unfolded for good
• gelatinizes: swells and sets
• crosslinks: chemical ties