Yeast is a kind of unicellular fungi, not a unit of phylogenetic classification. Ascomycetes, Basidiomycetes and other single-celled fungi are the general names, which can be used for brewing production, some are pathogenic bacteria, and are model organisms for genetic engineering and cell cycle research. Yeast is the earliest microorganism used in the history of human civilization. Can survive in an anoxic environment. At present, there are more than 1 kinds of yeasts. According to their ability to produce spores (ascospores and basidiomycetes), yeasts can be divided into three categories: the strains that form spores belong to ascomycetes and basidiomycetes. Those that do not form spores but propagate mainly through budding are called incomplete fungi, or "pseudoyeasts" (yeast-like). At present, it is known that very few yeasts are classified into Ascomycetes. Yeast is widely distributed in nature, mainly growing in acidic, humid and sugary environment, and it is also very important in wine making.