Yeast is a single-celled fungus used primarily to make bread rise, produce alcohol, and add nutrition to food. Its ability to convert sugar into carbon dioxide and ethanol makes it one of the most versatile organisms in human history, powering a global industry worth an estimated $9 billion in 2025. Beyond the kitchen and the brewery, yeast plays roles in digestive health supplements and has been central to some of the most important discoveries in modern biology.
How Yeast Actually Works
Every use of yeast traces back to one basic process: fermentation. When yeast cells consume sugar, they break it down into two byproducts: carbon dioxide gas and ethanol (alcohol). The sugar first gets converted into a compound called pyruvate, which is then transformed into acetaldehyde and finally into ethanol. This entire process happens without oxygen, and it’s relatively inefficient. Yeast only extracts two units of cellular energy per molecule of sugar, compared to the 36 or so units a cell can get when oxygen is available. But that inefficiency is precisely what makes yeast so useful: those “waste” products, carbon dioxide and alcohol, are exactly what bakers and brewers want.
Baking: Making Bread Rise
In bread making, yeast’s job is to produce gas. When you mix yeast into dough and leave it in a warm place, the yeast cells begin feeding on the sugars in the flour. The carbon dioxide they release gets trapped as tiny pockets of air within the gluten network of the dough, causing it to expand and rise. The ethanol produced during this process evaporates during baking, so the finished loaf contains no alcohol.
Warmth is critical. Yeast activity speeds up in warm environments, which is why bread recipes call for letting dough rise in a warm spot. Too much heat kills the yeast cells, while cold temperatures slow them down dramatically. This is also why you can refrigerate dough overnight for a slow rise that develops more complex flavors: the yeast is still working, just at a crawl.
Brewing, Winemaking, and Distilling
In alcohol production, the goal flips. Brewers and winemakers care about the ethanol, not the gas. The species most commonly used, Saccharomyces cerevisiae (baker’s yeast and brewer’s yeast are closely related strains), ferments sugars from grains, grapes, or other fruit into alcohol. In beer, the sugars come from malted barley. In wine, they come from grape juice. In spirits, fermentation happens first, and then the liquid is distilled to concentrate the alcohol.
Yeast has natural limits on how much alcohol it can produce. As ethanol builds up in the liquid, it becomes toxic to the yeast cells themselves, eventually killing them off. Most wine and beer yeast strains top out somewhere between 12% and 18% alcohol by volume, depending on the strain and conditions. Grapes harvested very ripe contain more sugar, which can push the resulting wine to uncomfortably high alcohol levels. Winemakers sometimes manage this by selecting specific yeast strains or adjusting the sugar content of the juice before fermentation begins.
Nutritional Yeast as a Food
Nutritional yeast is a deactivated form of Saccharomyces cerevisiae, sold as yellow flakes or powder. It has a savory, slightly cheesy flavor that makes it popular as a seasoning, especially among vegans and vegetarians. A single serving of a popular brand provides about 5 grams of protein and 2 grams of fiber.
Many brands are fortified with B vitamins, including B12, which is otherwise difficult to get from plant-based diets. However, the B12 content varies widely between products, so checking the label matters. Unfortified nutritional yeast still contains several B vitamins naturally, but not reliable amounts of B12. It’s worth noting that nutritional yeast is not the same as brewer’s yeast (which has a bitter taste) or active baking yeast. The cells have been heat-killed, so they won’t cause any fermentation in your gut.
Digestive Health and Probiotics
A related yeast species, Saccharomyces boulardii, is used as a probiotic supplement for gut health. Unlike most probiotics, which are bacteria, S. boulardii is a yeast, which gives it a practical advantage: antibiotics don’t kill it. This makes it particularly useful for preventing diarrhea caused by antibiotic use, one of its best-supported applications.
S. boulardii has also shown effectiveness for preventing traveler’s diarrhea, reducing diarrhea in children and people on tube feedings, and lowering the recurrence of infections caused by the bacterium Clostridium difficile (a common and sometimes dangerous hospital-acquired infection). There’s more preliminary evidence for its use in irritable bowel syndrome, Crohn’s disease, and even acne, though the research on those conditions is less conclusive. It’s available over the counter in capsule form at most pharmacies and supplement stores.
Yeast in Scientific Research
Saccharomyces cerevisiae holds a unique place in science. It was the first complex organism (eukaryote) to have its entire genome sequenced, back in 1996. Because yeast cells share a surprising number of genes and biological processes with human cells, they’ve become one of the most widely used model organisms in genetics, cell biology, and disease research.
The most celebrated example: yeast was central to discovering the genes that control cell division, the process that goes wrong in cancer. Researchers used yeast to identify the molecular switches that tell a cell when to grow and when to divide. That work earned Leland Hartwell, Paul Nurse, and Tim Hunt the 2001 Nobel Prize in Physiology or Medicine. Yeast has also been instrumental in developing tools to study how proteins interact with each other inside cells, a technique called the yeast two-hybrid system that became a standard method across biology labs worldwide.
Today, researchers use yeast to model human diseases ranging from neurodegeneration to metabolic disorders. Its fast growth rate, cheap maintenance, and genetic similarity to human cells make it an ideal starting point for understanding disease mechanisms before moving to more complex animal models.
Industrial and Biofuel Applications
Beyond food and medicine, yeast is increasingly used in industrial biotechnology. Engineers have modified yeast strains to produce biofuels, converting plant sugars from agricultural waste into ethanol for fuel. The same fermentation process that makes beer also powers a growing segment of the renewable energy sector.
Yeast can also be engineered to produce specific compounds, essentially turning the cells into tiny factories. Pharmaceutical companies use modified yeast to manufacture insulin, vaccines (including the hepatitis B vaccine), and other biological drugs. Flavor and fragrance companies use engineered yeast to produce compounds like vanillin. The global yeast market is projected to grow at roughly 6.8% annually over the next decade, reaching an estimated $16.3 billion by 2035, driven largely by these expanding industrial applications alongside traditional food and beverage demand.