Baking sets the final structure of baked goods through heat and mass transfer: heat travels from the surrounding air to the interior while moisture escapes from the core to the outside by evaporation. Baking has three major stages: expansion of the dough, drying of the surface, and crust browning. Baking makes bread lose about 8 to 12 percent of its weight, mainly as moisture and volatile compounds, especially in pan breads and buns.
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. Yeasts make bread, beer and wine by harvesting energy from sugar without oxygen, a process called fermentation, and its by-products are carbon dioxide, which lifts bread, and ethanol, which evaporates in the bread oven. Yeast eats sugar and gives off a gas called carbon dioxide, and it gives off a second product as it eats sugar, an alcohol called ethanol. Baker's yeast reproduces by budding, which looks like a cell blowing up a balloon, the daughter cell. 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.
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. 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, but scholars debate its origin because Mesopotamians also made yeast-risen bread.
Sourdough bread rises with wild yeast and bacteria instead of yeast from a packet, and wild yeast floats in the air and lives on flour and fruit. 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 the starter turn sugars from flour into lactic acid and turn the yeast's alcohol into acetic acid, and these acids give sourdough its sour flavor and help it keep longer. As of October 2024, Syracuse University researchers had tested 10 strains of acetic acid bacteria in synthetic sourdough communities. Every strain lowered the pH, which goes with more sourness, but closely related strains released quite different compounds. Wild-yeast sourdough takes longer and its timing is less predictable, and a baker may slow it further by refrigerating the dough on purpose, while packaged commercial yeast gives a faster, more consistent result.
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, and mixing and kneading lets them form chemical crosslinks into a protein network called gluten. In glutenin, disulfide bonds cross-link the subunits, which are linear polymers with large and small branches. 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. Salt has a strengthening effect on gluten, and the firmer gluten holds water and gas better and lets the dough expand without tearing.
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, and 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, whereas an acid-base reaction gives off one carbon dioxide for every sodium bicarbonate, so it needs half as much. Too much baking soda in a cake batter without enough acid turns the normally white crumb yellowish-brown and leaves a strong smell of soda. 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 carbon dioxide 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, sodium acid pyrophosphate or sodium aluminum sulfate, 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 there was a search for a way to make bread without the hours that yeast takes to work. Bakers once made soda-and-sour-milk breads and then cream of tartar versions, and 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. 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. Horsford added a bit of corn starch to his baking powder to keep it dry until it was used. In 1843 British chemist Alfred Bird made a baking powder for his yeast-allergic wife.
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 a flour mixture makes steam when heated, and steam expands to many times its original volume and is the main leavening agent in popovers, pie crusts and pastries. Gas bubbles in a batter keep expanding until the starch gelatinizes and the cells become rigid, which starts at about 140 degrees Fahrenheit and is mostly complete by about 167 degrees. 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, and 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, and with protein denaturation this converts the dough into bread and sets its structure.
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 in flour do a job like that of egg proteins in chemically leavened baked goods: they help trap air bubbles until the dough sets, which is why bread flour has more protein than cake flour. Air beaten into a batter gets trapped between protein structures and adds to the final lift of the baked product. Kneading breaks up the pockets of air in dough, and 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 high-ratio cakes, which have more sugar than flour, the flour-to-sugar ratio strongly affects starch gelatinization, protein coagulation and water evaporation. With little flour, a cake needs more structure-building ingredients such as whole eggs.
Maillard browning in a crust takes place above 105 degrees Celsius and needs a reducing sugar together with an amino acid, and Maillard reactions are non-enzymatic browning reactions that make malty, toasted, bready and nutty flavors. 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, and it is a different reaction from the Maillard reaction, which takes place between sugars and proteins at high heat. As of April 2009, Penn State food scientists had studied whether phenols from grains alter the Maillard reaction: when phenol was added to a heated sugar and protein, the toast-like pyrazine aroma practically disappeared. A chemical reaction makes the crust of bread dark and toasty.
As elevation rises, atmospheric pressure and air density fall, and lower pressure lets liquids vaporize at a lower temperature. So water boils at a lower temperature at altitude and food takes longer to reach the desired internal temperature. At high elevation, the lower air pressure lets bubbles in batter rise faster. Yeast breads and cakes are more susceptible to failure without proper adjustments. A dough left to rise too long is over-fermented, and the gas cells can burst so the loaf falls, and the bread can turn out hard and tough. 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
- gelatinization: starch swelling
- caramelization: heat browning sugars
- denature: unfold for good
