Yeast is some unicellular fungi, not a unit of phylogenetic classification. Ascomycetes, basidiomycetes and other single-celled fungi, some of which are pathogenic bacteria, can be used in wine production 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 1000 kinds of yeast. According to the ability to produce spores (ascospores and basidiospores), yeast can be divided into three types: the strains that form spores belong to ascomycetes and basidiomycetes. Those fungi that do not form spores but propagate mainly through budding are called incomplete fungi, or "false yeast" (similar to yeast). At present, few yeasts are known to be classified as ascomycetes. Yeast is widely distributed in nature, mainly growing in acidic, humid and sugary environments, and is also very important in wine brewing.