Belonging to the phylum of the Ascomycota, in the classical taxonomy the yeasts are identified by physiological characteristics that make it possible to differentiate the species.

In the presence of oxygen some yeasts use aerobic respiration, in the absence, instead, others can use a different metabolic process known as fermentation, in fact one of the most known features of these microorganisms is precisely the ability to ferment sugars and produce ethanol and carbon dioxide.

The reproduction of yeasts can be sexual or asexual. The asexual reproduction can occur by budding (more predominantly) or by fission (typical of the genus Schizosaccharomyces). In budding the mother cell of yeast after mitosis forms a lateral protuberance in which one of the nuclei migrates. At this point the protuberance grows and separates from the mother cell. Fission is the most common asexual reproduction mode, especially for simple organisms, such as bacteria, protists and plants, but also for some animals. It is characterized by the division of the organism into two or more parts, from which a complete individual develops. Sexual reproduction, on the other hand, occurs through the production of spores, generally very resistant reproductive forms. Sexual reproduction in ascosporigens (eg Saccharomyces cerevisiae) occurs in hostile situations, for example when the growth medium is free of fermentable sugars, or with little nitrogen.

“The double face of the coin”

The role of microorganisms in the transformation of organic material has been known since the 14th century. Yeasts have had and still have an important role from the microbiological, technological and sensorial point of view in the production of food and beverages.

The most notable examples of traditional microbiological processes concern the production of wine and beer, the acid preservation of vegetable products, the leavening of bread, the production of vinegar and the production of multiple cheeses and cured meats. Once the multitude of kinds of yeasts found in nature has been established, the choice almost always falls on Saccharomyces cerevisiae; commonly called "brewer's yeast" is the most important technological yeast used in the manufacture of wine, beer and bread.

While yeasts are fundamental for some technological processes, they are also responsible for food spoilage, often causing unwanted changes in them. These changes manifest themselves according to two mechanisms: a purely aesthetic one, due to the physical presence of yeasts (eg film on the surface of liquids), the other resulting from their metabolism (eg development of unwanted particular aromas).