Yeasts

Yeasts are probably the oldest cultivated organisms: Their use dates to 3000 b.c. or earlier. They are eukaryotic organisms, eukaryotes having by definition a defined nucleus surrounded by a nuclear membrane and organized cytoplasmic organelles (mitochondria, peroxisomes, vacuoles, etc). The recognition of yeasts as a group of microorganisms and of their role in fermentation generally preceded their taxonomic classification. They are fungi that exist as single cells during at least some part of their life cycle. They have no photosynthetic ability and are not motile. All the major groups of fungi, Basidiomycetes, Ascomycetes, Phycomycetes, and the Fungi imperfecti, include species that have adopted a yeast phase. Yeasts usually reproduce vegetatively by budding or fission, but parasexual and sexual reproduction occur as well in most yeasts. There are extensive and authoritative texts on yeast taxonomy and identification. Among the yeast listed, the genus Saccharomyces is of greatest practical and economic importance for the baking, beer, and wine industries, as well as for the production of biomass. Due to its importance in the study of basic cell biology, reviews of Saccharomyces genetics and molecular biology appear regularly. This article emphasizes the biology and industrial use of yeasts, particularly Saccharomyces cerevisiae.