Biochemical Engineering Fundamentals, Part 1 - Bailey J., Ollis D. 1989

Introduction to Microbiology
Major Cell Types
Yeasts

Yeasts constitute a major Class within the fungal kingdom. Like Bacteria, Fungi are widely distributed in nature, although they typically inhabit soils in regions that are relatively drier than those favored by bacteria. Fungi lack The ability to harvest solar energy and generally exist independently as single-celled organisms. While most fungi exhibit a rather complex Morphology, yeasts are readily distinguishable by their appearance—typically comprising discrete small Cells ranging from 5 to 30 μm in length and 1 to 5 μm in width.

Yeasts can reproduce both asexually and sexually; the scheme of asexual reproduction (via budding and fission) is illustrated in Fig. 1.7. During budding, a small outgrowth first begins to develop on the parent Cell; the detachment of the daughter cell from the parent does not occur immediately, thereby enabling The formation of Yeast cell colonies consisting of multiple generations. Fission results in the formation of two new cells from a single original cell. Sexual reproduction is accomplished through the fusion of two haploid cells (possessing a single set of Chromosomes), which is accompanied by The breakdown of the intervening Cell wall and the formation of a diploid (double-chromosome set) zygote. The Nucleus within the diploid cell may undergo one or more divisions to produce ascospores; each ascospore eventually develops into an individual new haploid cell, which can subsequently reproduce by budding, fission, or sexual reproduction. These ascospores, representing a natural reproductive outcome for these organisms, should not be confused with the endospores discussed above, which are formed as a protective response to a hostile environment.

FIG. 1.7. The photographs at the bottom of the figure depict The process of asexual reproduction in a yeast cell via budding. (The numbers indicate the elapsed time of the process in minutes.) As shown schematically at the top of the figure, sexual reproduction also plays a specific role in the yeast life cycle. (Photographs kindly provided by K. F. Robinov.)

In The production of alcoholic beverages, yeasts represent the sole commercially utilized type of microorganism. In addition to brewing and winemaking, anaerobic yeasts are employed for the industrial-scale production of industrial alcohol and glycerol. Yeasts are also utilized in baking and as protein supplements in animal feed (see Ch. 12).