ABSTRACT
Yeasts are fungi that grow vegetatively as single-celled organisms (see Ref. 47 for a review). The distinction between yeasts and other fungi is sometimes murky; fungi such as rusts and smuts grow as single cells during part of their life cycle and as typical mold-like mycelia during other parts. All yeasts grow vegetatively by budding or by binary fission. Ascomycetous yeasts are distinguished by the fact that during sexual reproduction spores are produced inside a specialized structure known as an ascus, whereas basidiomycetous yeasts produce external spores on a structure known as a basidium. Although a yeast lifestyle has been adopted by members of three of the four the major fungal groupings (Basidiomycetes, Ascomycetes, and Fungi imperfect, but not Phycomycetes), most yeasts of interest to the baking industry are biologically members of the ascomycetous genus Sacchar omyces. However, because fungal taxonomy relies on the identification of reproductive structures, yeasts that have lost the capacity for sexual reproduction are automatically classified among the Fungi imperfecti. Consequently, the San Francisco sourdough yeast, although biologically an isolate of Saccharomyces exiguus, is also classified variously in the Fungi imperfecti as Torulopsis holmii or Candida milleri.
