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. 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. The yeast used for baking is Saccharomyces cerevisiae, called baker's yeast. Standard baker's yeast is one strain of it, with very high gas 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 sourdough takes longer and its timing is less predictable. A baker may slow it further by refrigerating the dough on purpose, while packaged commercial yeast gives a faster, more consistent result; 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 the 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. A sourdough starter is kept going by adding fresh flour and water to it.
Yeast eats sugar, 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. 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 gas so the dough can rise; salt has a strengthening effect on gluten. The firmer gluten holds water and gas better and lets the dough expand without tearing.
Kneading means pushing, pulling and stretching the dough, and it makes the gluten in dough stretchy; gluten works like a net that traps the gas in dough. Dry yeast needs warm water to wake up. Kneading 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. Dough rises best in a warm place, above 75 degrees Fahrenheit, until it doubles in size; rising dough is covered, often with a clean, damp towel; proofing is a rest time when yeast makes gas in the dough. Punching down risen dough means pressing out some of the air with your fist. Salt slows down yeast, and a dough left to rise too long is over-fermented, so the gas cells can burst and the loaf falls, and the bread can turn out hard and tough.
Everything that happens inside the oven can be read as a series of steps, and each step is tied to a temperature. In oven spring, the dough gains volume fast in the first 5 to 8 minutes of baking, when its core is between 35 and 70 degrees Celsius. Water in dough turns to steam in the oven, and the steam pushes the dough up. Steam expands to many times its original volume and is the main leavening agent in popovers, pie crusts and pastries. Yeast and other microorganisms are killed in the oven at an internal temperature of about 60 to 70 degrees Celsius, when the cells can no longer add gas. From about 60 to 70 degrees Celsius the proteins in dough begin to denature, so the gluten becomes tougher and stiffer as it irreversibly forms a gel; moisture loss makes it more rigid too.
In bread dough, starch begins to gelatinize at about 76 degrees Celsius as the granules swell with free water; with protein denaturation this converts the dough into bread and sets its structure. In a cake, the proteins in eggs become permanently denatured by heat, which means they unfold and cannot refold, and the altered proteins give the liquid foam of a batter a solid structure that holds its shape. 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. 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. 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. Air beaten into a batter gets trapped between protein structures and adds to the final lift of the baked product.
In the 1800s there was a search for a way to make bread without the hours that yeast takes to work. 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. 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.
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.
Maillard browning in a crust takes place above 105 degrees Celsius and needs a reducing sugar together with an amino acid; sugars caramelize at about 160 degrees Celsius. 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, and 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
- gelatinize: swell and set
- caramelization: heat browning sugars
- amylases: starch-splitting enzymes
