Yeast

Yeasts are generally unicellular fungi reproducing asexually by the process of budding or fission and sexually by spore formation. Emil Christian Hansen’s studies, over 30 years, have provided an insight into the biological features of yeasts and facilitated their differentiation and their characterization as species. Currently, greater than 500 species of yeasts, belonging to around 50 genera, are known. The majority of yeast species belong to Ascomycotina, a few are basidiomycetes. Bakers’ yeast and the yeasts used in brewing, winemaking, and distilling are some of the strains of Saccharomyces cerevisiae, belonging to the family Saccharomycetaceae in Ascomycotina.

Phylogeny and taxonomy

The name yeast is a morphological term that defines unicellular fungi. Convergent evolution has mainly resulted in unicellular fungi in many different groups involving Zygomycetes (bread moulds), Basidiomycetes (club fungi) and Ascomycetes (cup fungi). The majority of the unicellular fungi (yeasts) are ascomycetes but even within this phylum, some yeasts are not closely related. Commercial yeast (such as Saccharomyces cerevisiae) is an ascomycete, as in fission yeast (Schizosaccharomyces pombe), another yeast used in brewing is also an important model organism with its entire genome sequenced. Although fission yeast and baker’s yeast possess a similar ecology and are in the same phylum they are not closely related, having diverged from each other over 300 – 1000 million years ago.

Structure

Yeasts, in general, are unicellular fungi and in form and size, they are very similar to bacteria. Like all other fungi, they also possess a cell wall made up of chitin and contain a nucleus and other organelles, specifically mitochondria. In many different ways, they represent fungi that have evolved to become somewhat ‘bacteria-like’ in their form and ecology. Baker’s yeast is roughly spherical and is around 5 um in diameter.

Sex and reproduction

Two haploid yeast cells of opposite mating groups; then the two cells grow towards one another stretching their sides; further, these two cells merge resulting in the formation of an odd ovular shape with a depression in the middle.

Brewer’s yeast mainly reproduces asexually, by the process of ‘budding’, which is mainly cell division but here the daughter cell starts as an outgrowth (i.e. bud) of the parent cell and gradually separates. Brewer’s (and fission) yeast are capable of copulation when the diploid cells undergo meiosis, resulting in the formation of cells that can fuse (i.e. act as gametes) to maintain the diploid condition. Both fission yeast and commercial yeast may occur as haploid or diploid cultures. Haploid cultures are generally maintained by not bringing together different mating strains, while diploid cultures can be maintained because specific culture conditions (i.e. nitrogen starvation) are required to bring about meiosis. When haploid cells of different mating strains meet each other, then chemical communication (i.e. pheromones) activates the production of extensions that allow cells to fuse (this is known as plasmogamy). 

Uses

In food manufacture, yeast is used to cause fermentation and leavening. The fungi feed on sugars, producing alcohol (ethanol) and carbon dioxide; in beer and wine manufacture the former, is the desired product, in baking it is the latter. In sparkling wines and beer, some of the carbon dioxides get retained in the finished beverage. The alcohol produced in bread making is taken off while the dough is baked. The fermentation of wine and sourdough bread is generally initiated by naturally occurring yeasts present in the air. One yeast cell can ferment approximately its weight of glucose per hour.

In commercial production, selected strains of yeast are usually fed a solution of molasses, mineral salts, and ammonia. When growth stops the yeast gets separated from the nutrient solution, washed, and packaged. Yeast for baking is mainly sold in compressed cakes comprising starch or in a dry granular form mixed along with cornmeal. Commercial yeast has about 50%protein and is generally a rich source of vitamins B1, B2, niacin, and folic acid. Brewer’s yeast and nutritional yeast, which gets deactivated (nonliving), can also be eaten as a vitamin supplement.

Conclusion

Yeast has interactions with humans in providing food products and beverages. The ‘native habitat’ of both brewer’s yeast and fission yeast is the coverings of sugar-containing fruits like grapes, apples and pears, making the ‘invention’ of wine-making comparatively easy. In addition to ethanol, genetically engineered yeast is also used to produce a variety of compounds including insulin. As brewer’s yeast is non-pathogenetic some yeast species can cause disease in humans and other organisms. Here we come to an end of this topic, we hope that you were able to grasp a clear concept of the topic of yeast, its structure, phylogeny and taxonomy, their sexes and mode of reproduction lastly the uses of yeasts.