Yeast is a single-celled fungus, and what “makes” it yeast comes down to a few defining traits: it’s alive, it feeds on sugar, and it produces carbon dioxide gas and alcohol as byproducts. That simple metabolic trick is why yeast makes bread rise, beer ferment, and wine develop alcohol. But yeast is also a remarkably complex organism with a rigid cell wall, a full set of 16 chromosomes, and over 6,000 genes packed into a cell roughly twice the size of a typical bacterium.
Yeast Is a Living Fungus, Not a Chemical
Yeast belongs to the fungal kingdom, the same broad group that includes mushrooms and molds. The difference is that yeast is unicellular. It evolved this single-celled lifestyle multiple times independently across different branches of the fungal family tree, which means not all yeasts are closely related. The most well-known group, the subphylum Saccharomycotina, contains roughly 1,200 known species. The one you’re most likely to encounter in your kitchen is Saccharomyces cerevisiae, commonly called baker’s yeast or brewer’s yeast.
Yeast cells are typically round or oval. Each cell is surrounded by a sturdy two-layered wall. The inner layer is built from beta-glucans and chitin, two tough polysaccharides that give the wall its mechanical strength. The outer layer is made mostly of mannoproteins. By dry weight, the cell wall is about 25% to 30% polysaccharides, with beta-glucans making up the largest share (29% to 64%), followed by mannans (around 31%), lipids (9%), and chitin (about 2%). That rigid wall is what lets yeast survive in harsh environments, from acidic dough to alcohol-rich fermentation tanks.
How Yeast Turns Sugar Into Gas and Alcohol
The process that makes yeast useful is fermentation. When yeast cells encounter sugar, they break it down through a series of internal chemical reactions called glycolysis. In the presence of oxygen, yeast can fully process sugar into energy, carbon dioxide, and water, much like your own cells do. But in low-oxygen or oxygen-free conditions, yeast switches to alcoholic fermentation. It still breaks down sugar, but instead of completing the process, it converts the intermediate products into ethanol (alcohol) and carbon dioxide gas.
This switch matters because it’s the entire basis of baking and brewing. In bread dough, the carbon dioxide gets trapped in the gluten network, creating air pockets that make the dough rise. The small amount of alcohol evaporates during baking. In beer and wine, the carbon dioxide either escapes or is captured for carbonation, while the ethanol remains. Yeast generates enough energy through glycolysis alone to keep growing and dividing even without oxygen, which is why sealed fermentation vessels work.
How Yeast Reproduces
Most yeast reproduces asexually through a process called budding. A small bump forms on the surface of a parent cell, gradually enlarging until it pinches off as a new, slightly smaller daughter cell. Under ideal laboratory conditions, a single yeast cell can complete this cycle in about 90 minutes. That rapid division rate is one reason yeast populations can explode quickly when they have enough food and warmth. Each budding event leaves a small scar on the mother cell, and over time these scars accumulate, effectively limiting how many times a single cell can divide.
Some yeast species reproduce by fission instead, splitting roughly in half like bacteria. Schizosaccharomyces pombe, the fission yeast, is the best-known example. Both budding and fission yeasts can also reproduce sexually under certain stressful conditions, exchanging genetic material to produce spores.
What Commercial Yeast Is Made From
The packets of yeast you buy at the store start from a pure laboratory culture of Saccharomyces cerevisiae. That culture is fed and scaled up through a series of progressively larger fermentation vessels. The primary food source is molasses, a byproduct of sugar refining that contains 45% to 55% fermentable sugars in the form of sucrose, glucose, and fructose. Before use, the molasses is pH-adjusted to between 4.5 and 5.0 (slightly acidic, which discourages bacterial contamination), clarified, and sterilized with high-pressure steam.
Yeast also needs more than just sugar to grow. Manufacturers add nitrogen (typically as ammonia or ammonium salts), phosphorus, magnesium, potassium, and calcium, along with trace minerals like iron, zinc, and copper. B vitamins, particularly biotin, thiamine, and pantothenic acid, are also essential. Once the yeast has multiplied to the target density, it’s separated from the liquid using centrifuges, then filtered into a thick paste called a filter cake.
From there, the process splits depending on the final product. For compressed (fresh) yeast, the filter cake is blended with small amounts of water and emulsifiers, extruded, cut into blocks, and wrapped. For active dry yeast or instant yeast, the cake is extruded into thin ribbons, dried in either a batch or continuous drying system, and then vacuum-packed or sealed under nitrogen gas to keep the cells viable during storage.
Not All Yeasts Are Helpful
While Saccharomyces cerevisiae is harmless and useful, other yeast species can cause infections. Candida albicans is the most common pathogenic yeast in humans, responsible for oral thrush, vaginal yeast infections, and in severe cases, bloodstream infections. Candida species are structurally different from baker’s yeast in important ways. C. albicans can shift between a round yeast form and elongated filamentous forms, which helps it invade tissue. It also handles oxygen and membrane chemistry differently: it can grow without certain membrane components that baker’s yeast requires.
On the beneficial side, Saccharomyces boulardii is a yeast strain used as a probiotic. It has been prescribed for over 30 years to help prevent and treat various forms of diarrhea, including antibiotic-associated diarrhea, traveler’s diarrhea, and diarrhea linked to Clostridium difficile infection. It works by mimicking some of the protective effects of healthy gut bacteria and appears to interfere with inflammatory signaling pathways in the gut. It’s typically sold as capsules containing either freeze-dried or heat-dried yeast cells.
Humans Have Used Yeast for Thousands of Years
Archaeological evidence of fermented beverages stretches back to the Neolithic period, around 6000 to 5000 BCE, in the region of modern-day Georgia. Extensive evidence of organized wine and beer production in Egypt, Mesopotamia, and the Near East dates to the mid-fourth millennium BCE. Ancient records include not just residue on pottery but administrative lists, recipes for different beer varieties, and paintings depicting the brewing process. Researchers have even isolated live yeast cells from ancient vessels, using them to study the lineages of domesticated yeast strains.
For most of that history, people didn’t know yeast existed as an organism. They simply knew that dough rose and grain water became alcoholic if treated a certain way. The identification of yeast as a living microbe came in the 1800s, and its genome was fully sequenced in 1996, making Saccharomyces cerevisiae the first eukaryotic organism (any organism with complex cells) to have its complete genetic code mapped. Its 16 chromosomes and roughly 6,275 genes continue to make it one of the most studied organisms in biology.