Baking is applied microbiology. That may seem like an odd way to look at it, but it is only a modest exaggeration. All yeast breads and sourdoughs owe their shapes and textures to the actions of microbes, living creatures that are microscopic in size and typically consist of just a single cell.
Despite its size, yeast is a powerhouse of an ingredient. Yeast cells are living, single-celled fungi that behave like minuscule factories that seem to specialize in the production of bubbles and booze, which is one of the main reasons bread is so complex and special. Working with a living ingredient can sometimes be difficult, but having an understanding of how yeast behaves will ultimately help you bake better bread.
1. Yeast are territorial — much to our benefit.
The carbon dioxide and ethanol that the organisms release are their waste — and also a defense against competitors that find ethanol so off-putting that the substance keeps other microbes from encroaching on the yeast's food supply.
Yeasts use these two different modes to break down and process nutrients in flour in order to extract the energy and raw materials they need to live and grow.
Yeasts begin to respire immediately after coming into contact with food. In bread baking, respiration primarily occurs right after mixing. As long as oxygen is in ample supply and sugar is not, yeasts crank out carbon dioxide and water as byproducts. But if oxygen is scarce or if fermentable sugars are available, the cells ramp down respiration and ramp up fermentation, which causes them to spit out ethanol and even more carbon dioxide.
During the stage that bakers refer to as fermentation — from the time the dough is mixed to just before baking — yeasts swing between their two metabolic modes each time their environment changes. As a baker kneads and degasses the dough, more oxygen flows into it, so the yeasts can respire for a while. They then switch back to fermenting as the oxygen gets used up and more fermentable sugars become available.
2. You can control fermentation by controlling yeast's environment.
Dough is a complex environment that keeps the eager microbes in check. High concentrations of salt or sugar in the dough create an osmotic pressure on yeast cells that slows their growth and fermentation. That's one of the reasons why accurately measuring these ingredients is so important. Low moisture content has the same effect. As a result, tight, low-hydration doughs take longer to ferment. (So do sweetened and heavily salted doughs.)
Bakers can also stimulate or restrain the growth and fermentation rate of the microbes by controlling the temperature of loaves as they proof. Refrigeration slows down fermentation. Conversely, a warm and humid proofer is like a yeast sauna that's perfect for speeding up fermentation.
3. It's why bread smells so good.
Many bakers think of a yeast as just a gas factory to inflate dough, however it's also a biochemical processing plant that is modifying thousands of chemicals in bread. A number of these compounds happen to be delicious-smelling aromatics that contribute to the flavor and smell of the bread.
The longer the fermentation, the more pronounced the yeast flavors become since the microbes have more time to produce aromatic compounds. Conditions including the amount of sugar in the dough or the environmental temperature during fermentation will change the flavors and make them more or less pronounced while also affecting fermentation time.
The full flavors of these compounds sometimes develop only during baking. The heat of the oven triggers browning reactions, including the Maillard reactions, that give rise to the familiar flavors, smells, and colors of a well-baked crust. Thanks to yeast, the dough is stocked with amino acids that are an integral component of the flavorful reactions.
4. Yeast impact the consistency of your dough.
Two of the chemicals produced by yeasts, the enzyme protease and the tripeptide glutathione, can essentially turn the microbe into a reducing agent, a type of additive that is used to soften dough. Both these substances weaken the gluten structure: protease chews up proteins into bits, and glutathione breaks down the bonds that connect adjacent gluten molecules. This is a double-edged sword because while thriving yeasts inflate a well-developed dough, damaged or dead yeast cells can slacken it by excreting these two chemicals.
Some bakers use a small amount of deactivated yeast to relax dough for that very reason; when too much inactive yeast is added, however, the overabundance of protease enzymes and glutathione makes the dough too weak to use. Dough that is weak can feel coarse and easily rips when handled.
5. One type of baker's yeast can be used for another.
There's an ongoing debate as to which type of yeast is best for baking bread: active dry, instant, or fresh. Some bakers extol the virtues of working with fresh yeast, which has given dry yeast a stigma. It's widely accepted that fresh yeast does have more leavening power compared with the same quantity of active dry yeast, which contains fewer live yeast cells per gram. But it has drawbacks, too: Fresh yeast is fragile, and it has a relatively short shelf life.
If a baker uses the right techniques, there is no reason to use fresh yeast over instant yeast — in a lineup of baked loaves, you'd be hard-pressed to distinguish one from the other in terms of the yeast used. We actually prefer to use instant yeast in recipes that call for commercial baker's yeast because we like its convenience and reliability.
The Soul & Science of Sourdough
Kitchn is partnering with Modernist Cuisine, the brilliant masterminds behind a new masterwork devoted to bread, Modernist Bread (October 24, The Cooking Lab), in our series The Soul & Science of Sourdough.
We're obsessed with sourdough bread and how it blends both soul and science, history and modernity, and we invite you to discover the magic of its fundamentals together. Bread is a treasured part of life — how can it fit in yours? Find out this month at Kitchn!