CHAPTER 20 



The Fungi — Yeasts 



DEFINITION AND BRIEF DESCRIPTION 



CLASSIFICATION OF YEASTS 



MORPHOLOGY AND REPRODUCTION 



DISTRIBUTION IN NATURE 



ECONOMIC IMPORTANCE 

 Industrial applications 

 Spoilage 

 Disease 



DEFINITION AND BRIEF DESCRIPTION 



A completely satisfactoiy definition of a yeast is difficult to formu- 

 late if it is to include all of the considerations involved in describ- 

 ing these particular fungi. The word yeast like the w^ord mold 

 has no taxonomic significance. According to Henrici, yeasts are 

 fungi that permanently maintain a unicellular growth form, not 

 developing mycelia. Many fungi may temporarily fulfill this de- 

 scription, but since the condition is not permanent, these organisms 

 are excluded as yeasts. Yeasts are generally considered to be those 

 unicellular, chlorophyll-free fungi which possess a demonstrable 

 nucleus, which multiply by budding and ferment simple sugars 

 with the formation of carbon dioxide and ethyl alcohol. Not all 

 fungi classified as yeasts exactly fit this definition or the one pro- 

 posed by Henrici. 



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