My career as a baker has to a great extent been defined by yeast. Of course, the species of yeast that is dearest to me is the one that leavens bread dough, Saccharomyces cerevisiae. This affinity for fermenting grain is, for bakers, what makes it special among the 600 or so other yeast strains that ferment grapes, grain, molasses, and potatoes for wine, beer, and whiskey, or that cause all sorts of bodily afflictions like prickly heat, athlete’s foot, and candidiasis. Since my relationship with yeast, while deep and abiding, exists in the narrow, though wide-reaching context of bread, that’s what I’ll be focusing on in this primer.
Yeast is, both literally and metaphorically, a transformational agent, in that it has the ability to change things into something else, through the process of fermentation. In bread baking we refer to it as leaven, which means “to raise, to bring to life or vivify”; yeast turns a lifeless lump made from flour, salt, and water into a living, growing mass that we then call bread dough. For this reason, yeast has an almost magical or alchemical reputation. Before the late 19th century, when scientists were able to begin analyzing it properly, yeast was viewed with an even greater sense of almost mystical awe, but now, even though we know what it is and how it functions, it still retains an aura of mystery.
What Is Yeast?
Yeast, in its categorical sense, is a fungus, from the same general kingdom as other fungi such as mushrooms, truffles, and molds. There are more than 100,000 formal species of fungus but some scientists believe there are probably more like 1.5 to 5 million uncataloged types. Because the cell walls are made of a substance called chitin rather than cellulose (as found in plants), and do not produce chlorophyll, fungi are not considered part of the plant or animal world, but exist in their own distinct kingdom. Fungi can be visible to the naked eye, such as with mushrooms, or can be microscopic in size, as are the single-celled fungi that fall in the genus we know as yeast.
As with bacteria, yeast and other fungi can be beneficial—serving as the basis of many foods as well as medicines such as penicillin and other antibiotics—or harmful, toxic, or psychotropic. (Indeed, fungi can be transformational by either healing or killing us.) And, while yeast fungi have been acting upon other organisms in the world for millions of years, living in both combative and symbiotic relationships with bacteria, plants, and animals, it has been only about 6,000 years (as far as history can tell us) that they have come under directed application and cultivation to do our bidding.
How Is Commercial Yeast Manufactured?
Commercial yeast production, a late-19th-century innovation, is a fascinating process. I often show my baking students a video, made a number of years ago by the Fleischmann Yeast Company, of how baking yeast is manufactured. A small vial of active yeast cells is inoculated into a large vat of carbohydrate-rich water, spiked with molasses and other nutrients. It is then pumped with air, causing the yeast to rapidly reproduce through a process called budding, splitting into 2 identical cells, then 4, then 8, then 16, and on and on until, in just hours, the entire vat is swimming with trillions of living Saccharomyces cerevisiae cells. This yeasty liquid is then either shipped as fluid “cream yeast” to large-scale production bakeries, or dried on rotating drums into a paste-like substance that can be pressed into cakes and sold to smaller bakeries and even to home bakers through a few specialty markets as fresh, compressed yeast (aka “baker’s yeast”). Or, the liquid can be dried even further into a dry granulated powder called dry active yeast, to be used by both home and commercial bakers. More recently (for about 40 years or so now), a heartier, newer strain of Saccharomyces cerevisiae has also been cultivated and then dried into an even smaller sandlike grain and packaged under a variety of product names such as instant yeast, bread machine yeast, perfect rise yeast, or rapid rise yeast.
How Yeast Works: Feeding and Fermentation
What was so fascinating about the video was that it showed how quickly the yeast cells multiplied under those ideal growth conditions in the lab, but how much more slowly and differently they worked when deployed in bread dough. This is because yeast can grow both aerobically and anaerobically, but it multiplies much faster when in the presence of lots of dissolved oxygen in that vat pumped with bubbles of air. In bread dough, though, which has little air, it must rely on slow, anaerobic growth, fed by available sugar in the dough rather than by oxygen. Hardly any multiplication or budding of the yeast occurs in dough; instead, the yeast metabolizes the sugars and breaks them down—it ferments them, causing them to release carbon dioxide (like burping) and ethyl alcohol, or ethanol (like sweating). The rate of this fermentation is directly proportional to the temperature of the dough; the warmer it gets the faster the metabolic transformations take place—to a point (more on that later).
Stepping back for a moment to apply just this rudimentary information, you can see how the action of this fungal cerevisiae yeast initiates a series of transformations in foods—and not just in bread dough—by causing fermentation. (There are other fermentation types besides yeast-generated, such as bacteria-generated lactic fermentation, which creates acids in foods—think pickles and cheeses.) What’s really going on during any kind of fermentation is that complex molecules are being broken down, metabolized, and converted into smaller units. This is first initiated by enzyme activity, a chemical rather than biological transformation in which starch and protein molecules are broken into smaller units, such as the simple sugar glucose, by natural enzymes. The released simple sugars then serve as yeast food; yeast is able to feed off of glucose, metabolize it, and expel it as ethyl alcohol and carbon dioxide. Other strains of yeast, different from Saccharomyces cerevisiae, can ferment available food, but the by-products could be toxins, fungal molds, or other undesirables. The benevolent carbon dioxide and ethyl alcohol outcomes are why Saccharomyces cerevisiae has emerged as the preferred fermenter for bread- and beer-making (winemakers prefer a related species called Saccharomyces ellipsoides), and why it is cultivated in huge quantities in scientifically controlled laboratories.
Bread Baking and the Yeast Sacrifice
In bread baking we can witness the results of fermentation within a few short hours, as the generated carbon dioxide causes dough to expand, trapping the carbonic gas in the intricate mass of gluten threads and starch mass. One of the fascinating and ironic aspects of yeast fermentation is that, in order for the yeast to complete its designated work—to raise the dough into an aerated, light product, imbued with the flavor compounds of the ethyl alcohol and acids produced by both yeast and bacteria—it must also die. Death arrives as soon as the internal temperature of the baking dough exceeds the “thermal death point” of all biological organisms (known as the TDP to bakers and cooks), which is commonly believed to be 139°F/59.5°C. Thus, even before the dough has completed its own transformational journey of changing into bread—which doesn’t occur until the internal temperature passes at least 180°F/82°C and the starches have fully gelatinized—the yeast has already fulfilled its leavening function and becomes denatured.
A Walk on the Wild Side of Yeast
To complete the picture, it’s important to touch upon the diverse community of naturally occurring wild yeast that is generically known as Saccharomyces exiguus. These yeasts find homes on the skins of fruit, grain, and vegetables, lying dormant until the skins crack and allow them to feed upon the sugars within. They occur as all sorts of subspecies, so it is safe to assume that what we call sourdough yeast, or wild yeast, is composed of many strains and in combinations that can vary from region to region and climate to climate. This is also true of various bacteria, which are, like yeast, dependent on climate conditions and the soil from which the ingredients are grown or where the wheat is milled into flour or the flour is transformed into dough. But, and this is what distinguishes wild yeast from commercially cultivated yeast, the various challenges of surviving in the wild cause these strains to be more resilient to otherwise growth-suppressing conditions, such as highly acidic environments. In doughs that we call sourdough starters, wild yeast and bacteria actually coexist, sharing the simple sugars created by enzyme activity while generating different by-products. Unlike commercially produced yeast, which can’t tolerate acidity very well, the exiguus wild yeast strains actually thrive, and thus live symbiotically with the acid-producing bacteria (at least until the simple sugars run out). The result, happily for those who like sourdough bread, is a complex, acidic flavor that is not achievable by commercial yeast fermentation alone.
Bread baking is really about the balancing act of time, temperature, and ingredients, so there are many ways in which to make dough. But the single most important ingredient is the leaven, the yeast, because it is the most responsive of all the ingredients to the manipulation of time and temperature in terms of creating flavor. It takes only a little yeast to raise an entire loaf. But yeast exists within a complex biosphere and, as a result, creates wonderfully complex possibilities for those who know how to channel its potential. In the world of pharmacology, for instance, scientists are only scratching the surface of its healing properties, and in the culinary world, even after 6,000 years, we are still finding new ways to use yeast to make bread, beer, wine, and all sorts of other delicious fermentable foods.
Practical Advice on Working with Yeast:
Answers to Five Common Questions
What are the types of yeast available to home bakers and how should I choose one?
The three types of yeast available to home cooks are fresh/compressed, dry active, and instant (aka rapid rise, perfect rise, fast rising, and bread machine yeast). Most people choose their yeast based on availability, familiarity, and shelf life (if you bake infrequently it would be better to use dry yeast, which can be stored for up to a year in an airtight container, whereas fresh yeast lasts only about three weeks). The only type that needs to be dissolved first in warm water is dry active, because the yeast grains are too big to dissolve directly in the dough. The term instant yeast refers to the fact that it dissolves instantly in the dough, as soon as liquid is added for mixing, though it can also first be dissolved in warm water if desired. Instant yeast is 25 percent more concentrated than dry active yeast due to the processing method (25 percent of the dry active yeast cells—which are more fragile than the newer strain of cerevisiae used in instant yeast—die during packaging, whereas practically no yeast cells die during packaging of instant yeast).
How do I substitute one type of yeast for another?
The substitution formula for replacing one type of yeast with another type, by weight, is 100 percent fresh compressed yeast = 40 percent dry active yeast = 33 percent instant yeast. In other words, instant yeast is three times more concentrated than fresh compressed yeast. Unless otherwise stipulated, in most types of bread baking all three types of yeast can be substituted for each other as long as the weights are adjusted accordingly. Regardless of which type of yeast you use, as long as you make the proper replacement adjustment, the flavor of the finished bread will be the same.
Is there a difference among brands of yeast?
Almost all brands of commercial yeast are comparable to each other in strength and reliability. The only difference is in type, so be sure to ascertain whether you are using instant (which is confusing since it goes by so many names), dry active, or fresh compressed yeast, and follow the instructions on the package regarding storage and life expectancy of the yeast.
What kind of yeast works best in sweet or acidic doughs?
There is a specially cultivated strain of cerevisiae yeast that is more resistant to acidity or excessive sweetness in dough called osmotolerant instant yeast, but it’s usually available only to professional bakers (the name refers to its ability to resist the penetration of the cell walls of the yeast by sugars or acids). Some specialty catalogs and Web sites, such as King Arthur Flour, also carry it for serious home bakers. This type of yeast is especially useful for sweet, rich doughs and also for slow-fermenting doughs that develop a lot of acidity. (Yeast can process only simple, single-chain sugars, like glucose, so doughs that are high in granulated or brown sugar—sucrose types—can actually overwhelm yeast and put it to sleep while the yeast waits for the dough enzymes to break down the sucrose into digestible glucose and fructose.)
I have found that regular instant yeast will do nearly as well as osmotolerant yeast if you first hydrate it in lukewarm water, as you would for dry active yeast, which seems to give it a kind of head start in the rich or sweet doughs.
What are preferments?
Artisan bakers have developed methods for adding to their doughs small quantities of old dough. The purpose of this so-called preferment is to instantly add complex fermentation flavors to the new batch of dough. Some of these prefermented doughs are called by names such as biga, poolish, sponge, pâte fermentée, or simply, old dough.
Bread-baking authority Peter Reinhart is the author of eight books, including Artisan Breads Every Day and the James Beard Award–winning Whole Grain Breads. He’s also a baking instructor and faculty member at Johnson & Wales University and the host of the Web site Pizzaquest.com, where you can learn more about him.