Baker’s Yeast

Saccharomyces cerevisiae, also called brewer’s yeast or baker’s yeast, is a species of yeast (single-celled fungal microorganism). The species has been instrumental in winemaking, baking, and brewing since ancient times. The yeast distributes ubiquitously in nature with clearly structured populations, with global genetic diversity mainly contributed by strains from Far East Asia, and ancient basal lineages found only in China. It is the microorganism which causes many common types of fermentation.

This ascomycete fungus has profoundly shaped human civilization for millennia, yet its scientific importance extends far beyond food production. It is one of the most intensively studied eukaryotic model organisms in molecular and cell biology, much like Escherichia coli as the model bacterium. Saccharomyces cerevisiae was the first eukaryote to have its genome completely sequenced, a collaborative effort involving hundreds of researchers worldwide, with results published in 1996.

Identification and Appearance

S. cerevisiae cells are round to ovoid, 5–10 μm in diameter. Its cell wall is a dynamic structure relatively rigid that provides cell protection, and osmotic support and determines cell shape. It produces from one to four ellipsoidal, smooth-walled ascospores. S. cerevisiae can be differentiated from other yeasts based on growth characteristics and physiological traits: principally the ability to ferment individual sugars.

Under the microscope, individual cells appear as small, rounded structures. There are roughly 15 billion individual yeast organisms in every gram of fresh, compressed baker’s yeast. After budding, yeasts have “outies”— circular scars where their offspring detached.

Life Cycle and Growth

It reproduces by budding. Growth in yeast is synchronized with the growth of the bud, which reaches the size of the mature cell by the time it separates from the parent cell. In well nourished, rapidly growing yeast cultures, all the cells have buds, since bud formation occupies the whole cell cycle. Under optimal conditions, yeast cells can double their population every 100 minutes.

Yeast has two mating types, a and α (alpha), which show primitive aspects of sex differentiation. Two haploid yeast cells of opposite mating type can mate to form diploid cells that can either sporulate to form another generation of haploid cells or continue to exist as diploid cells. Under conditions of stress, diploid cells can undergo sporulation, entering meiosis and producing four haploid spores, which can subsequently mate. Mean replicative lifespan is about 26 cell divisions.

Distribution and Habitat

Saccharomyces cerevisiae and its variant Saccharomyces boulardii, otherwise known as “baker’s yeast” (or “brewer’s yeast”), are ubiquitous in nature, cultivated from soil, plants, and fruit, and are occasionally isolated from the human genitourinary and gastrointestinal tracts. It is believed to have been originally isolated from the skin of grapes. iNaturalist data indicates the species has been recorded across multiple continents, with 8,551 total occurrences documented globally.

Wild S. cerevisiae distributes ubiquitously in nature, but the success rate of S. cerevisiae isolation from fruit samples (6.5%) was lower than those from rotten wood (9.2%), soil (10.8%) and tree bark (16.5%) samples. The fine white powder that often covers grapes is a yeast—often S. cerevisiae, the same species (but a different strain) that is usually used in brewing and baking.

Ecological Role

S. cerevisiae’s biotechnological usefulness resides in its unique biological characteristics, i.e., its fermentation capacity, accompanied by the production of alcohol and CO2 and its resilience to adverse conditions of osmolarity and low pH. Fermentation allows the microbe to survive in anaerobic (oxygen-free) and sugar-rich conditions as well as in its more usual state in the wild, in which oxygen is plentiful but sugars are not. By spewing out alcohol and acids that are toxic to most bacteria (and many other fungi), S. cerevisiae fends off other microbes as it ferments.

All strains of S. cerevisiae can grow aerobically on glucose, maltose, and trehalose and fail to grow on lactose and cellobiose. Galactose and fructose are shown to be two of the best fermenting sugars. Glucose is the primary substrate and in anaerobic environments the end products are carbon dioxide, ethanol and heat.

Edibility and Uses

The species has been instrumental in winemaking, baking, and brewing since ancient times. In baking, S. cerevisiae is used primarily for bread making, in which carbon dioxide released during fermentation causes dough to rise; the ethanol produced evaporates when the dough is baked. Saccharomyces cerevisiae is used to produce alcoholic beverages such as beer, wine, and distilled spirits. Beer is made through the fermentation of sugars derived from malted grains, while wine results from the fermentation of grapes or other fruits.

Saccharomyces cerevisiae is the main source of nutritional yeast, which is sold commercially as a food product. It is popular with vegans and vegetarians as an ingredient in cheese substitutes, or as a general food additive as a source of vitamins and minerals, especially amino acids and B-complex vitamins. Yeast is also good food. It is rich in protein and is an uncommonly good source of the B vitamins. Saccharomyces is frequently utilized in the preparation of probiotics to treat and prevent a variety of gastrointestinal illnesses, including Clostridioides difficile infection.

Conservation

No formal conservation status has been assigned to Saccharomyces cerevisiae, as it is not a wild organism requiring protection. S. cerevisiae is a organism which has an extensive history of safe use. Despite considerable use of the organism in research and the presence of S. cerevisiae in food, there are limited reports in the literature of its pathogenicity to humans or animals, and only in those cases where the human had a debilitating condition. The species is not threatened and remains one of the most abundant microorganisms on Earth due to its widespread cultivation and natural distribution.

Fun Facts

• Archaeological evidence indicates that humans began using this yeast more than 5,000 years ago, making it one of the earliest domesticated fungi.
• The S. cerevisiae genome is composed of about 12,156,677 base pairs and 6,275 genes, compactly organized on 16 chromosomes. Only about 5,800 of these genes are believed to be functional.
• It is estimated at least 31% of yeast genes have homologs in the human genome.
• S. cerevisiae is used in brewing beer, when it is sometimes called a top-fermenting or top-cropping yeast. It is so called because during the fermentation process its hydrophobic surface causes the flocs to adhere to CO2 and rise to the top of the fermentation vessel.
• Brewers, vintners, and distillers have diligently bred strains of S. cerevisiae to excrete more ethanol when fed particular blends of nutrients. This produces more of the interesting flavor compounds and fewer of the off-putting ones, and keeps the yeast fermenting for longer as the liquid becomes increasingly alcoholic.
• A prime example of key discoveries in the field of biology is the discovery of key regulators of the cell cycle, for which the Nobel Prize was awarded in 2001. Hartwell used S. cerevisiae to identify more than one hundred genes involved in cell cycle control, the cell division cycle or CDC genes.
• It has contributed to major scientific discoveries, including insights into cell cycle regulation and gene expression, and it continues to be applied in modern biotechnology for producing pharmaceuticals, biofuels, and recombinant proteins.