In the last few posts, we’ve been talking about leavening gases, which give our baked goods volume and tenderness. Last time, we focused on baking soda and baking powder, which quickly produce carbon dioxide through chemical reactions. In this post, we’ll take a closer look at yeast. We’ll describe what yeast are, how they leaven, and what to consider when working with them.
Yeast are tiny organisms.
Yeast produce carbon dioxide, but unlike chemical leaveners, yeast are living organisms and they do their work over longer periods of time. As shown in the illustration at the top of the page, individual yeast cells look like tiny eggs, and they reproduce by budding. One gram of compressed yeast, which is only about 30% yeast, contains about 20 billion individual cells!
Just like any living organism, yeast need food for energy. They use this energy to survive, grow, and reproduce. And like all living organisms, as yeast eat, they produce waste. The byproducts we’re most interested in are carbon dioxide, ethanol, and acidic flavor compounds, all of which we trap in our bread dough for leavening and flavor.
Yeast metabolize sugars.
Yeast use sugars for energy. In the mixing bowl, enzymes in the flour or within the yeast themselves convert many types of sugar into glucose. Then, through a series of chemical reactions called glycolysis (literally “breaking sugar”), enzymes in the yeast break down the glucose to release its energy. The cell stores the energy in the form of a molecule called ATP, which it then uses to power its survival, growth, and reproduction.
Sugars contain energy in their molecular structure.
Why is there energy in the glucose to begin with? Molecules like glucose are made of atoms held together by chemical attractions or bonds. Think of the bonds like the attraction between two magnets. We can separate the magnets, but we have to exert energy in order to do so. The same is true of atoms within a molecule: we can separate them, but it requires energy. In glucose, these chemical bonds are as strong as industrial magnets. The amount of energy required to pull them apart is huge.
Luckily, our cells have proteins called enzymes, which are specifically designed to break these bonds. Enzymes are like robots that can easily separate the magnets for us. And because we don’t expend much energy to break the molecules down, we instead gain that potential energy and convert it to a form our cells can use directly. When our cells finally need that energy, it’s readily available as the molecule ATP.
Breaking down glucose
Most organisms have the enzymes to break down glucose through glycolysis. The process is a bit like an assembly line. Glucose moves down the line, and at each station, a different enzyme robot makes its modification. Some of these enzymes merely rearrange the atoms within the molecule. But other enzymes add or remove molecular parts, and these parts are shuttled to and from the assembly line by molecules that move among the many assembly lines within the cell factory.
ATP, which carries energy, is one such molecule. Some steps of glycolysis actually require energy, and the enzymes get their help from ATP. But at other steps on the assembly line, enzymes transfer the energy from the glucose to ATP, which then powers other enzymes throughout the cell factory.
Another shuttle for molecular parts is called NAD+. NAD+ clears trash from the assembly line. When enzymes remove electrons and hydrogen atoms from glucose, NAD+ carts them away. If the factory runs out of NAD+, the glycolysis assembly line backs up as the waste builds up. The cell can no longer perform glycolysis, and it cannot derive energy from glucose.
By the time a single molecule of glucose reaches the end of the assembly line, the enzymes have transformed it into two molecules of pyruvate. In the process, they produced two net molecules of ATP and used up two molecules of NAD+. The cell receives a net gain of ATP, so the process of glycolysis converted glucose into usable energy, but in order for glycolysis to be a sustainable process, the cell must regenerate the trash-clearing NAD+ by using the electrons and hydrogens it took.
Regenerating trash removers
Glycolysis is a nearly universal process among living organisms, but there are three potential assembly lines that come next, depending on the organism and its environment. In most of the cells in our bodies, for example, the pyruvate enters another set of chemical reactions called the citric acid cycle or the Krebs cycle. This process is ideal because it creates more ATP energy molecules, but it requires oxygen. If we don’t have enough oxygen, as is the case in our muscle cells when we exercise, the cells must use a second assembly line.
On this line, NAD+ is regenerated by converting the pyruvate into a compound called lactate, which forms lactic acid. Lactic acid buildup is actually what causes our muscles to feel sore after exercise. Some microorganisms, such as bacteria, also undergo lactic acid fermentation when they don’t have oxygen. Acids produced by fermenting bacteria actually create the characteristic tangy tastes of cultured foods such as yogurt and sourdough.
In yeast, however, pyruvate enters an assembly line called ethanol (or alcohol) fermentation. On this assembly line, pyruvate is converted into ethanol and carbon dioxide to regenerate NAD+. The NAD+ can return to the glycolysis assembly line to clear more trash, and the yeast exude the ethanol and carbon dioxide in a liquid as waste.
Life cycle of yeast in the kitchen
Now that we have a better understanding of how yeast use sugar to produce carbon dioxide and ethanol, let’s examine their life cycle in the kitchen. Most recipes for the home kitchen (except sourdoughs) use dried yeast, either active dry yeast or instant yeast. In their dry state, the yeast are in a state of hibernation because their enzymes are immobile. When we add water, the yeast rehydrate and wake up, ready to ferment. As we’ll discuss in the next section, the temperature of the water and the dough is important for yeast activity.
Once they’re awake, the yeast start to ferment the sugars around them. These sugars can come from ingredients we add, like table sugar (sucrose). Enzymes called amylases also break down the starches in flour into sugars that the yeast can metabolize. As the dough ferments, yeast break down sugar and produce ethanol, carbon dioxide, and other acidic flavor molecules. They exude these compounds as a liquid into the dough around them. When this liquid reaches an air bubble we mixed into the dough, it releases carbon dioxide and ethanol. The carbon dioxide adds volume and raises the dough.
The yeast use the energy from the sugar to survive and grow. And as long as they have access to the oxygen in air, they divide into more yeast cells that repeat this process. As bread dough ferments, individual yeast cells duplicate time and time again to produce clumps of cells, all fighting for sugars and oxygen and producing carbon dioxide. When we punch down and shape the dough, we redistribute the yeast throughout the dough so that they have a fresh supply of air and food during the second proof.
When the dough finally contains enough gas, we bake it. The heat initially helps the enzymes work faster, and the yeast give one final burst of carbon dioxide and ethanol. This last push, along with the vaporizing ethanol, contributes to oven spring, which is the large increase in bread’s volume as it bakes in the oven. Then, the yeast die as temperatures enter inhospitable ranges. The carbon dioxide they secreted into the dough continues to leaven the bread until the crust hardens, and the acidic flavor molecules create the scent of freshly baked bread.
Yeast need the right environment to ferment.
Just as our bodies work best within a certain temperature range and level of humidity, yeast are happiest under a certain set of conditions, some of which we will explore here. As you will see, yeast fermentation is a balance among many variables such as time, flavor, and sugar availability.
It’s also helpful to note that, for good oven spring, the yeast must be alive with plenty more food to eat when the bread dough enters the oven. This way, they have the resources for their last burst of fermentation. Furthermore, the dough shouldn’t already be holding its maximum volume of gas. In other words, it is possible to overproof the dough, and the results are similar to those of overmixing. If the bread dough goes into the oven with too much air, the gluten will not be able to accommodate additional volume, and the bread will collapse.
Amount of yeast
Since yeast are the source of the carbon dioxide that leavens bread dough, it makes sense that bread rises more quickly the more yeast there are. But large amounts of yeast can lead to an unpleasant taste, and they can also exhaust their food before the bread hits the oven. Instead, it is preferable to start with a less yeast and ferment for longer periods of time.
Water is essential to life, and yeast are no exception. For one, water hydrates the enzymes that perform glycolysis. Active dry yeast and instant yeast are in a state of hibernation because they are dried, but all yeast require a sufficient amount of water to thrive. In bread dough, not only do liquids hydrate the flour, but they also hydrate the yeast. This is also why it’s important to keep dough covered. We want the water to stay in the dough instead of evaporating into the air.
In large quantities, other ingredients that grab water, such as salt and sugar, slow fermentation by withholding water from the yeast. In low quantities, both of these ingredients are important for bread—salt enhances flavors and tightens gluten while sugar feeds the yeast—but at high concentrations, they essentially tie up water and dry out the yeast through diffusion. Some yeast, such as SAF Gold Label and Fermipan Brown, are more tolerant of high-sugar environments, but they are hard to find. Regular yeast may also be used for doughs high in sugar, but they need extra time to ferment. You can also supplement high-sugar doughs with baking powder for additional leavening. But remember that bread doughs are typically low in sugar compared to other baked goods because sugar prevents gluten from forming.
Yeast have a happy medium for temperature, just like us. At temperatures below freezing, yeast are dormant. The enzymes that perform glycolysis and other life functions are essentially frozen and immobile. As the environment warms, the enzymes work faster, and the rate of fermentation increases until about 140°F (60°C), when the enzymes denature and the yeast die. This is why the temperature of the dough, especially the liquid, is so important. Too cold and the yeast will ferment slowly. Too hot and you could inadvertently kill the yeast.
Temperature control provides many ways for us to manipulate bread doughs. Ideal fermentation temperature is generally given at 78–82°F (25–28°C). But if we refrigerate the dough, we can develop more complex flavors. At colder temperatures, the yeast ferment more slowly, so fermentation time extends to build up enough carbon dioxide. At the same time, the bacteria that undergo lactic acid fermentation are more active. Thus, if we refrigerate bread dough, we add more flavorful acids to it.
Temperature can also help us fit bread to our schedules. If we want to stop yeast activity completely to bake the loaf another week, we can freeze the bread dough. If we want to slow the yeast down to give ourselves a few extra hours, we can refrigerate the dough. If we want to speed the yeast up, we can put it in a warmer environment.
Most yeast thrive in a slightly acidic environment. As we discussed in the context of proteins, enzymes work less efficiently outside their optimal pH range. Although our doughs typically start at a neutral pH, the yeast increase the acidity of their environment with their acidic byproducts as they ferment.
Different ingredients can affect the rate of yeast fermentation. Ginger and nutmeg, for example, speed up yeast fermentation, while dry mustard inhibits it. Cinnamon encourages yeast fermentation at low concentrations but inhibits it at higher concentrations. Chemicals can also affect yeast activity. Chlorine, for example, can be inadvertently added to dough with tap water. At high enough concentrations, it inhibits yeast activity. Potassium, on the other hand, encourages yeast activity. Potato breads rise faster than other breads because both the potatoes and the potato water in the dough contain potassium.
Type of yeast
“Yeast” is a broad term that encompasses many species. They differ in characteristics such as their ideal temperature, preferred acidity, and fermentation rate, which suggests different considerations when working with each type in the kitchen.
Sourdough is made from wild yeast, that is, a combination of several yeast species from flour and from the air around us. When we combine flour and water, yeast and bacteria metabolize sugars from the flour’s starches. Over time and repeated feedings of fresh flour, these microorganisms replicate until they can provide enough leavening power for a loaf of bread. The exact combination of microorganisms depends on the life in the air around you, which is why sourdough bread from different parts of the world tastes different.
Because they’re not bred and cultivated the way commercial yeast are, wild yeast don’t ferment as quickly. This gives the bacteria in sourdoughs plenty of time to develop complex flavors. They metabolize glucose into lactic acid and acetic acid (the acid in vinegar) to give sourdough its signature tang. Wild yeast are more tolerant of acidic environments than their commercial counterparts, so they continue to ferment along with the bacteria. However, the acidity prevents other microorganisms such as mold from growing on the bread even after baking, increasing its shelf life.
All commercial yeast is baker’s yeast of the species Saccharaomyces cerevisiae, but there are different strains of baker’s yeast and there are a few ways to process them. Commercial yeast has been bred and packaged for baking, so they all ferment more quickly and more consistently than wild yeast. They also tend to prefer higher temperatures than wild yeast. Thus, breads made with commercial yeast are typically less time-intensive but also less flavorful. To get a greater depth of flavor, some recipes combine a portion of the yeast with a portion of the flour and water for at least half an hour to create a pre-ferment, during which some flavor molecules are made. The pre-ferment is then added to the rest of the ingredients, providing additional flavor without exhausting the food supply of the yeast.
Although all forms of commercial yeast are pretty much interchangeable with some slight modifications in amount and method, they do behave slightly differently. It’s helpful to understand these differences to predict yeast activity in bread doughs and to make sure the yeast are as happy as possible.
Compressed yeast comes in cakes, and it’s typically first dissolved into water (100°F or 38°C) to ensure even distribution throughout the dough. This type of yeast is less commonly used at home because it has a very short shelf life. If you find it at the supermarket, it may already be dead, so it’s a good idea to proof it before you use it. Just add some sugar to the warm water along with the yeast and check for foam after ten minutes or so.
Active dry yeast
Active dry yeast is often sold in strips of three foil packets. It’s dried to about 10% moisture, but the drying process is harsh, so every granule of yeast actually has a layer of dead cells on the outside. Dead yeast leak glutathione, especially in cold water, which weakens gluten and loosens doughs. Active dry yeast is typically first dissolved in warm water (about 105–115°F or 41–46°C) to ensure the yeast are hydrated, then added to the rest of the ingredients.
Instant yeast (also called rapid-rise yeast) is also sold in strips of foil packets. The grains are small and porous, so they hydrate easily. Instant yeast is added directly to the dry ingredients, and the dough should be about 70–95°F (21–35°C) when all the ingredients are combined. Instant yeast especially is formulated to ferment quickly, which means it produces a lot of gas in a very short period of time, making it ideal for breads with short fermentation periods. The drying process for instant yeast is also gentler than that for active dry yeast, so each packet contains less dead yeast and glutathione. Some instant yeast packets also contain ascorbic acid, which helps to strengthen gluten.
Yeast are fascinating little creatures that convert sugars into carbon dioxide, ethanol, and acidic flavor compounds for our breads. Because they are living organisms, they can be finicky. But if we give them the right environment, they can contribute substantial leavening power to give our breads tender texture and high volume.
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