What Is Yeast? A Single-Celled Fungus Doing Most of the Work in a Kitchen

Yeast consumes sugars and, in the absence of sufficient oxygen, converts them into ethanol and carbon dioxide while extracting energy for itself, which is fermentation. That reaction is the basis of the entire application, with the gas inflating dough and the alcohol being the product in brewing and winemaking, and the same organism doing both jobs. Given oxygen it instead respires, converting sugar fully to carbon dioxide and water and growing far faster, which is how commercial yeast is produced. Alongside the two main products it generates hundreds of minor compounds, including esters, higher alcohols and acids, and those account for most of the flavour and aroma that distinguish one fermented product from another, which is why strain selection matters enormously in brewing and why the same wort ferments into different beers with different yeasts.

Baking yeast comes in several preparations and they are not interchangeable without adjustment:

• •Fresh yeast, a moist compressed block, which is the traditional form and keeps for a couple of weeks refrigerated
• •Active dried yeast, which requires rehydrating in warm liquid before use
• •Instant dried yeast, milled finer and mixed directly into flour without rehydrating
• •Rapid or bread machine yeast, which is instant yeast with additives to accelerate the rise
• •Sourdough cultures, which are a mixed community of wild yeasts and bacteria rather than a single strain
• •Brewing and wine strains, selected for alcohol tolerance and flavour rather than for gas production

The organism was used for millennia and identified only in the nineteenth century, and the sequence is a standard case in the history of science. Fermentation was assumed to be a purely chemical process, with the leading chemists of the period holding that it was a decomposition requiring no living agent, and the observation of yeast cells under the microscope was dismissed as incidental. Pasteur established in the 1850s and 1860s that fermentation required living organisms, that different organisms produced different results, and that spoilage was caused by unwanted organisms rather than by spontaneous change, which founded microbiology and produced the heat treatment named after him. Buchner later showed that cell-free extract could ferment, which established that enzymes rather than the whole living cell did the chemical work and reconciled the two positions.

The domesticated strains used in baking and brewing are a small selection from a much larger natural population, and where the wild ones live turned out to be a real question. The species was assumed to live on fruit and in vineyards, which is where it is found in association with human activity, and genetic surveys have established that wild populations occur on oak bark and in forest soils across several continents, with the domesticated lineages tracing back to those populations. Separate domestication events produced the lineages used for beer, for wine, for bread and for sake, which diverged and were selected independently for different properties, and brewing strains in particular show clear signs of selection under cultivation including loss of the ability to reproduce sexually. That history parallels the domestication of crops and animals and was worked out only recently.

The same species became one of the most important organisms in biological research. It is a eukaryote, meaning its cells are organised like those of plants and animals rather than like bacteria, while being single-celled, fast-growing and easy to manipulate genetically, which makes it a model for processes that bacteria cannot illustrate. Its genome was the first eukaryotic genome sequenced, completed in 1996. Fundamental discoveries about the cell cycle, about protein trafficking and about autophagy were made in it, and several Nobel Prizes have recognised work using it. It is used industrially to produce insulin, vaccines and a range of chemicals through engineered strains. The combination of ancient culinary use and central scientific importance is unusual, and the practical familiarity is part of why it became a model organism.

The organism converts sugar to carbon dioxide and alcohol without oxygen, which raises dough and makes beer with the same reaction, and hundreds of minor compounds produced alongside account for the flavour differences between products. Fermentation was assumed to be purely chemical until Pasteur showed it required living organisms. The same species was the first eukaryote to have its genome sequenced.