Bread dough rises because of yeast. Yeast is a fungus made of a single cell, and the yeast in bread dough is alive. It eats sugar and gives off a gas called carbon dioxide. Scientists call the process where yeast cells use up sugar and release gas fermentation. It also gives off a second product as it eats sugar, an alcohol called ethanol. 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.

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. Kneading means pushing, pulling and stretching the dough, and it makes the gluten stretchy. Gluten works like a net that traps the gas in dough.

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. 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. A sourdough starter is kept going by adding fresh flour and water to it.

Baking soda and baking powder make gas bubbles that help cakes and pancakes rise. Baking soda starts to make gas almost as soon as it touches an acid. In the 1800s sour milk or cream of tartar supplied the acid. But the acidity of milk varied, and the supply of cream of tartar was unreliable. Bakers once made soda-and-sour-milk breads and then cream of tartar versions. Early baking powder was baking soda and cream of tartar sold together in separate packets, so they would not mix until the dough was wet.

In the 1800s there was 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. He replaced cream of tartar in baking powder with an acid phosphate. He also 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. Baking powder has an acid already mixed in with the baking soda. Double-acting baking powder makes gas at two different times.

Rich doughs, such as brioche and dinner rolls, contain a lot of fat, sugar and eggs. These give the bread a softer feel and richer taste. Salt slows down yeast, and yeast dies when it gets too hot. A dough left to rise too long is over-fermented, so the gas cells can burst and the loaf falls. When dough first goes into a hot oven it rises fast, which bakers call oven spring. Water in dough turns to steam in the oven, and the steam pushes the dough up. Heat sets the proteins in eggs, which turns a liquid foam of batter into a solid. 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. Bread is done baking when it turns golden brown.

Words to know

• fermentation: yeast using sugar
• gluten: dough net
• crosslinks: chemical ties
• ethanol: an alcohol
• sourdough: wild yeast bread