Yeast is the engine of bread making. It is important for leavening the bread and also affects the flavor and smell of bread. The longer the period of fermentation, the more pronounced the yeast flavors become because the microbes have more time to produce aromatic compounds. Yeast also contributes to the crust’s color by releasing compounds critical to the Maillard reaction during baking.
For thousands of years, bakers have been harnessing yeast effluent to inflate their doughs. Scientists now understand the biochemistry of yeast respiration and fermentation in great detail. We’ll cover a breakdown of that process here, as well as some other yeast tips and tricks.
The Science Behind Yeast
These single-celled fungi feed off the simple sugars in the dough, then produce carbon dioxide and ethanol as very welcome by-products, in a process known as fermentation. These gases are what make bread dough rise, inflating the bubbles in the dough, just as steam does in the oven during baking. Yeast also plays a role in gluten development.
- The process of fermentation begins soon after the ingredients are mixed together and continues during the bread-making stages of bulk fermentation, final proofing, and even baking. Simply put, fermentation is a metabolic process during which yeasts (sometimes in tandem with lactic acid bacteria [LAB]) break down nutrients (sugars, oxygen, amino acids, micronutrients, etc.) in order to extract the energy and raw materials they need to live and grow.
- Immediately after mixing, when oxygen is present and sugars are in short supply, yeasts expel carbon dioxide and water in a process known as respiration. When the oxygen is used up, fermentation begins, and yeasts produce carbon dioxide as well as ethanol (which is alcohol—it evaporates during baking).
- 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 folds, 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.
- High salt or sugar concentrations and low moisture slow yeast growth and fermentation. For instance, low-hydration, sweetened, or heavily salted doughs take longer to ferment.
- Bakers can also control microbial growth and fermentation rate by adjusting the proofing temperature.
- The dough is ready to be degassed when it is puffy—by this time carbon dioxide accounts for over half of its total volume. Degassing, which happens when dough is folded during bulk fermentation, reduces the volume of the dough by 20%–40% and breaks large carbon dioxide bubbles into smaller ones.
- Once degassing stops, the bubbles start filling again with carbon dioxide. The point of degassing is not just to add to the number of bubbles, although that is important. The release and growth of bubbles, along with the stretching of the dough, also strengthens its netlike gluten matrix.
- Working the dough shifts molecules of water and protein and stretches out proteins so that more gluten chains can form. To become bread, the dough must contain the gas the yeasts have created, and to do that, the dough needs something that serves as a membrane, like the stretchy surface of a balloon.
- In well-mixed dough, the lattice of gluten chains acts much like rubber: it provides structure, but it’s also extensible—as the bubbles produced by the yeasts expand, the membrane of gluten chains is flexible enough to expand as well.
- During shaping and proofing, yeasts continue to ferment.
- Yeast loves a warm environment, and when the dough first begins to heat up in the oven, the yeast experiences a burst of frenzied activity (contributing to what is known as oven spring—see page 124 of Modernist Bread at Home and Modernist Bread vol. 3:290) before the rising temperature finally kills it.
Helpful Tips for Working with Yeast
- There are three ways to incorporate yeast into a dough: adding commercial yeast directly to the dough (known as the direct dough method), using what is known as a preferment (see page 42 of Modernist Bread at Home and Modernist Bread vol. 3:14), or using a combination of the two.
- We prefer instant yeast for making direct breads because it has a longer shelf life than fresh yeast and more leavening power than active dry yeast.
- Dry yeasts, both instant and active, will benefit from blooming (although it’s not technically necessary for instant dry yeast), which means hydrating the yeast in water or other liquid, like milk. Blooming ensures that the yeast hydrates fully and disperses effectively throughout the dough. You can also just mix it into the flour if you are using instant dry yeast, but we get a slightly greater loaf volume when we bloom the yeast first. For tips on blooming yeast, see Strategies for Incorporating Yeast on page 40 of Modernist Bread at Home and Modernist Bread vol. 3:12.
- 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 is perfect for speeding up fermentation. You can find more information about how the environment influences fermentation in Course 3: Making Bread with Busy Schedules.
- Sometimes dough is accidentally overproofed, which is when the dough rises too much. Fortunately, dough CPR can help save your dough so that you don’t have to throw it out.
Further Reading
- Modernist Bread: Chapter 12—Fermentation
- Modernist Bread at Home: Chapter 3—Fermentation
- Planning to bake bread—Modernist Bread at Home, page 10
- Strategies for incorporating yeast—Modernist Bread at Home, page 40 / Modernist Bread vol. 3:12
- Is fresh yeast best?
- Why does baking bread smell so good?