Yeast is the name of a lifestyle, not a related group of organisms, and as of November 2026 about 1,500 species of fungi have taken it up, in whole or in part. Baker's yeast reproduces by budding, which looks like a cell blowing up a balloon, the daughter cell. Yeasts make bread, beer and wine by harvesting energy from sugar without oxygen, a process called fermentation. Yeast eats sugar and gives off a gas called carbon dioxide, which lifts bread, and it gives off a second product as well: an alcohol called ethanol, which evaporates in the bread oven.

Louis Pasteur's research on the diseases of beer and other fermented products established yeast's connection to alcoholic fermentation, which inspired his germ theory of disease. 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, but scholars debate its origin because Mesopotamians also made yeast-risen bread.

Standard baker's yeast is one strain of Saccharomyces cerevisiae with very high carbon dioxide output but few flavorful by-products, so it gives a rapid rise but a limited flavor. Both yeast and chemical leaveners make gas, but yeast is essential for the unique flavors of breads and some other baked goods. 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. In a sourdough starter, wild yeast and bacteria live together in a symbiotic relationship. The bacteria depend on the yeast to break down proteins into peptides, and in return release excess glucose that feeds the yeast. Lactobacillus bacteria in a sourdough starter turn sugars from flour into lactic acid and turn the yeast's alcohol into acetic acid. These acids give sourdough its sour flavor and help it keep longer.

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. Dry yeast needs warm water to wake up, and salt slows down yeast. Bread dough is set in a warm place to rise until it doubles in size, and rising dough is covered, often with a clean, damp towel. Proofing is a rest time when yeast makes gas in the dough, and 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, flour's gliadin and glutenin proteins stay separate. Mixing and kneading lets them form chemical crosslinks into a protein network called gluten, and adding water hydrates the glutenin proteins in flour, which swell and become stretchy and flexible. In bread dough the gluten network is tight enough to trap the yeast's carbon dioxide so the dough can rise. 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. In yeast dough, the gluten proteins 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.

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. Baking powder has an acid already mixed in with the baking soda, and 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. 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. 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, and 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, and 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.

Once dough or batter goes into the oven, heat starts to change what is inside it, one step after another. 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. When dough first goes into a hot oven it rises fast, which bakers call oven spring, and water in dough turns to steam in the oven, and the steam pushes the dough up. Yeast dies when it gets too hot.

A chemical reaction makes the crust of bread dark and toasty. Caramelization is the action of heat on sugars: at high temperatures, melting sugars change chemically into a deep brown color and new flavors. The Maillard reaction is a different reaction, between sugars and proteins at high heat. Children need a grown-up with them in the kitchen when they bake. Bread is done baking when it turns golden brown. Baked loaves are cooled on a wire rack so the bottom does not get soggy.

Words to know

• fermentation: yeast using sugar
• symbiotic: living together
• caramelization: heat browning sugars
• denatured: unfolded for good
• strain: yeast type