Yeast is the name of a lifestyle, not a related group of organisms. As of November 2026, about 1,500 species of fungi have taken that lifestyle up, in whole or in part. The yeast used for baking is Saccharomyces cerevisiae, called baker's yeast, a fungus made of a single cell that is alive in bread dough. Baker's yeast reproduces by budding, which looks like a cell blowing up a balloon, the daughter cell. Wild yeast also floats in the air and lives on flour and fruit.
Yeasts make bread, beer and wine by harvesting energy from sugar without oxygen, a process called fermentation. Its by-products are carbon dioxide, which lifts bread, and ethanol, which evaporates in the bread oven. In bread dough, yeast eats sugar and gives off a gas called carbon dioxide, and it also gives off a second product as it eats sugar, an alcohol called ethanol. This is why yeast helps bread dough rise.
Scientists analyzing residue from wine jars in the tomb of an early Egyptian ruler named Scorpion, who lived around 3100 B.C., found fragments of Saccharomyces cerevisiae DNA. As of November 2026, evidence from about 6,000 years ago shows that people had learned to make leavened bread and beer around the time of the earliest known winery, in a cave in Armenia. Also 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.
Sugar is food for yeast, and flour supplies sugar for yeast, but the starch in flour has to be broken down first. Enzymes called amylases break the starch in wheat flour into shorter pieces such as maltose, and an enzyme called maltase splits maltose into two simple sugars. Proteins often make up 10 to 15 percent of flour, and they include the classes called glutenins and gliadins.
Wheat flour has two proteins, glutenin and gliadin, that join to make gluten. Before kneading, flour's gliadin and glutenin proteins 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 gluten works like a net that traps the gas in dough. Kneading also breaks up the pockets of air in dough. The air pockets between gluten strands fill with carbon dioxide from the yeast, and the smaller the pockets, the finer the bread's texture.
Yeast is not the only way to make gas. 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. 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 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. Heat sets the proteins in eggs, which turns a liquid foam of batter into a solid, and the altered proteins give the batter a 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.
Bakers once made soda-and-sour-milk breads and then cream of tartar versions. 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. 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.
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.
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. A dough left to rise too long is over-fermented, so the gas cells can burst and the loaf falls. Children need a grown-up with them in the kitchen when they bake. 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
- budding: how yeast reproduces
- amylases: starch-splitting enzymes
- crosslinks: chemical ties
- denatured: unfolded for good
