Yeast Fermentation: How Does Yeast Fermentation Work?

Yeast is the most commonly used leavener in bread baking and the secret to great bread making lies in its fermentation, or the metabolic action of yeast. It is the magical process that allows a dense mass of dough to become a well-risen and flavorful loaf of bread. In order for fermentation to take place, all yeast needs food, moisture and a controlled warm environment. Its byproducts from consuming food are the gas carbon dioxide, alcohol, and other organic compounds. The gas is the rising agent in bread, and the other "waste" products create the subtle flavors and texture that make a good loaf. It is introduced into the bread's ingredients using different bread mixing methods

In bread baking, we are trying to ferment grain in order to leaven it. We are also trying to release sugars trapped in the complex starch molecules to be used as yeast food, and much of it for flavor and crust color (caramelization). Yeast is a single-celled organism and only certain strains are used for fermenting grain.

Yeast activation and the initiation of fermentation are triggered by hydration, from either water or some other liquid, and the presence of a food source. Fermentation ends at 140 degree F during baking when heat kills the yeast. (Fermentation can end earlier, if the yeast is killed by other factors.)

Yeast feeds on sugar derived from the complex starch molecules from flour, a complex carbohydrate. The starch molecules are broken apart into simpler sugar molecules from enzymes in the flour when hydrated. Flour tastes like sawdust because its sugar components are too complex to differentiate on the tongue. The enzyme is a catalyst, which breaks apart the threads, freeing them so they become accessible to yeast and bacteria. Yeast lacks amylase and cannot break down starch into sugar. Since flour's endowment of sugars can only feed yeast cells for a short period of time, flour millers add malted wheat or barley, grains that have been allowed to sprout and develop enzymes that break down starches into sugars, or enzymes extracted or purified from microscopic molds ('fungal amylase'). See also "Diastatic malt". Did you know that amylase is present in human saliva, where it begins the chemical process of digestion?

The yeast breaks down these simple sugars, such as glucose and to a lesser extent, fructose, into smaller and simpler molecules with every step, for energy (food), from which it grows and multiplies (budding known as mitosis), and exudes a liquid that releases carbon dioxide and ethyl alcohol into existing air bubbles in the dough. Fermentation typically ends with the bread baking stage.

Yeast also feeds on added sugar. As little as 1 or 2 teaspoons of sugar / sweetener gives the yeast a boost and make the dough rise.

Bacterial fermentation from Lactobacilli is another type of fermentation that affects bread, especially with sourdough or wild-yeast pre-ferments. These are rod-shaped bacteria that assist the process of fermentation and produce flavoring acids, such as lactic and acetic acids, plus too many to name, along with CO2 as by-products of metabolism (fermentation). They are held in by an elastic gluten network developed in the bread dough, formed by mixing, kneading and/or rising moistened wheat flour, which leavens or causes the bread to rise. The alcohol expands as a gas during the early stages of baking, adding significantly to oven spring and also adds to the bread's flavor. Both the carbon dioxide and alcohol evaporate during baking, leaving behind a well risen loaf, with flavor from the alcohol.

When you stir together wheat flour and water, two proteins in the flour, glutenin and gliadin, grab water and each other to form a bubblegum-like elastic mass of molecules that we call "gluten". In bread making, we want to develop as much gluten as we can because it strengthens the dough and holds in gases that will make the bread rise.

Factors affecting fermentation - Slower fermentation is best for the development of flavor and gluten strength.

  1. Temperature of the dough; optimal fermentation temperature is 78 - 82 degrees F
  2. Temperature of the room: optimal temperature being 75 - 80 degrees F. (When the temperature exceeds 85 degrees F, off flavors result.) Dough can still rise in cooler environments, but much more slowly.
  3. Fermentation time; allows for the development of distinctive flavor and texture, depending on type of pre-ferment
  4. Amount of yeast; the more yeast the faster the fermentation. Too much can add an undesirable yeasty flavor.
  5. Type of yeast; instant active dry yeast contains fast acting yeast
  6. Amount of salt; typical Baker's Percent is 1.8 to 2.5
  7. Amount of sugar; small quantities (up to 5 Baker's Percent) increases yeast activity. Above 10 Baker's Percent, slows yeast activity
  8. Type of sugar; sucrose, glucose and fructose are fermented rapidly; maltose is fermented slowly; lactose is not fermented at all
  9. pH of dough; optimal pH is acidic 4 to 6. Above, fermentation slows. As yeast ferments, it produces acids to lower the pH to that range
  10. Presence of antimicrobial agents; Most spices, have antimicrobial activity, such as cinnamon and can slow fermentation. Be careful how much is added to the dough directly

Effect of Temperature
Temperature has an impact on fermentation rates and how fast the dough rises. Rising times in a recipe are general guidelines only. The ideal temperature to raise dough in is 75 to 80 degrees F, until the temperature reaches 140 degrees F, when the yeast dies. This typically happens during baking, but can happen when proofing or mixing the dough if any ingredient is too hot.

Dough set out in a 75 degree F kitchen will rise in half the time than dough set in a 65 degree F kitchen, but it always depends on the type of ingredients included in the recipe. For example, bread dough laden with grains and seeds or whole wheat flour, rises much more slowly and will not double in bulk. If you happen to live at a high altitude, bread dough rises 25 to 50 percent faster than normal due to the low atmospheric pressure.

Warmer temperatures encourage the development of milder lactic flavors (milder natural sourdough acids), while cooler temperatures promote the growth of more acetic flavors (vinegar-like, sour and tart) and character (larger, irregular air holes, crustier crust) to the dough and finished baked loaves.

When the temperature exceeds 85 degrees F off flavors result. The pilot light of an oven usually results in temperatures of about a 115 degrees F which can actually kill the yeast. If you leave the oven light on however, it should be just the right temperature.

Dough will rise in the cold, too, often called the cool rise or refrigerator rise, where it can be left, overnight and up to two days or whatever is specified in the recipe. This causes yeast fermentation and dough rising to slow dramatically and take longer. Because of this, bread flavors are more complex with "sourness" or "wheatiness", and can have more interesting textures. Typically the amount of yeast needed in the recipe is reduced. However, dough held longer than the recipe specifies can debilitate the yeast.

SARAH SAYS: Dough stays warm for three hours to chill before the yeast fermentation slows. Before refrigerating, place the degassed or flattened dough in a large oiled resealable plastic bag. Deflate the dough every hour for the first 3 hours to speed up the chilling process.

Fermentation dramatically increases during oven spring (oven kick or ovenspring), until 140 degree F heat kills the yeast. It takes place during the first third of the bread baking cycle.

Humidity - Sarah's Microwave Bread Proofer
The more humidity, the faster the dough will rise. Professional bakers use atmosphere-controlled "proof boxes" to keep the dough at the right temperature and humidity for the best possible rise and a moister bread texture. I devised a way to have a proof box in my own home, without the great expense called, Sarah's Microwave Bread Proofer.

Fermentation and texture (gluten development)
Fermentation also helps with texture or gluten development in the bread. With every burst of carbon dioxide that the yeast releases into an air bubble, protein and water molecules move about and have another chance to connect and form more gluten. Proper fermentation provides a resilient crumb, which is soft and smooth to the touch. In this way, a dough's rising is an almost molecule-by-molecule kneading. Next time you punch down bread dough after its first rise, notice how smooth and strong the gluten has become, in part from the rise.

At this stage, most bakers stretch and tuck the dough into a round to give it a smooth, tight top that will trap the gases produced by fermentation. Then they let this very springy dough stand for 10 to 15 minutes. This lets the gluten bonds relax a little and makes the final shaping of the dough easier. This rounding and resting step isn't included in many home baking recipes, but it's a good thing to do.

Fermentation and flavor
Fermentation, whether it's acting on fruit juices to make wine or on flour to make bread, does exactly that, it breaks down large molecules into smaller, flavorful ones. Flavor has to do with how the baker manipulates fermentation through a delicate balance of the type of yeast and ingredients used, along with temperature, to control outcomes in the finished loaf. The environment in which it is fermented in also has a significant impact. This generates many volatile and non-volatile flavor precursors that create the unique fermentation flavor.

Bread flavor also comes from the ingredients themselves, especially the flour and Maillard browning that occurs during browning. As a general rule, the less ingredient enrichments a dough has, such as added sugar, dairy and fat, typically, the longer the fermentation necessary because most of the flavor comes from the wheat starches; they need time to release their natural sugars. Where enrichments are present, the flavor is derived from the enrichments rather than the flour, so a shorter fermentation time is preferable.

Flavor also comes from the action of the bacteria present in the environment, which compete for yeast and the sugar in the flour. These beneficial bacteria produce flavorful acids such as acetic and lactic acids, as in pre-ferments (sponges and particularly in sourdoughs) or, in slow, cool rises.

SARAH SAYS: There is bacteria in the dough from the beginning, but as long as the yeast is very active, it consumes sugars as quickly as they're produced, leaving no food for the bacteria, which also like sugar. But when bakers chill dough and slow down its rise, the cold dramatically reduces yeast activity. The bacteria, on the other hand, function well even in cold temperatures, so they now have an opportunity to thrive, producing many more marvelously flavorful acids.

Fermentation improves dough handling characteristics
The various complex reactions during fermentation produce a range of intermediate compounds. These fermentation by-products soften the dough protein structure which is "gluten". Long fermentation times allow for complete hydration of the gluten proteins, which also aids in its softening, allowing for improved dough machinability and handling.

Fermentation enhances gas retention in dough
As a direct consequence of gluten softening, the dough protein matrix is conditioned to hold more of the carbon dioxide produced by the yeast during fermentation and proofing.

Fermentation extends shelf-life
Breads that have gone through a proper fermentation process have a better shelf life than those that have not. While gluten modification definitely aids in this respect, it is possible that the action of amylases on broken starch during the long fermentation process causes the shelf-life extension.