Chapter 2 

 CULTIVATED YEASTS 



ORIGIN OF CULTIVATED YEASTS 



Most domesticated plants differ markedly from their natural 

 ancestors. Domesticated plants and animals are specialized, usu- 

 ally at the expense of characters which make for survival under 

 conditions of natural competition. Corn seeds are accidentally sown 

 by the hundreds of thousands, but corn plants are practically never 

 found in any place except a cultivated field, for this domesticated 

 plant is incapable of competing with wild plants under natural con- 

 ditions. Wild plants are in genetical balance with the environment. 

 Domestication has selected specialized variants in which the char- 

 acters essential to competition in the natural state have been di- 

 minished in potency. Wild yeasts are adapted to grow in sugary, 

 sour fluids, such as are found in nature in ripe bursted fruits, like 

 grapes, apricots, and figs, in the nectar of flowers, and other places 

 where sugary, slightly acid solutions are available. They obtain 

 energy largely by the fermentation of sugar, and produce as by- 

 products of growth carbon dioxide and alcohol. 



These two by-products have been the basis for their domesti- 

 cation. Yeasts are used to make bread, because of their ability to 

 lighten the loaf by filling it with bubbles resulting from the produc- 

 tion of carbon dioxide; and the alcohol produced by the fermentation 

 of sugars led to their use in brewing. Yeasts are also adapted to 

 the synthesis of inexpensive protein which may become an import- 

 ant source of human food. They are able to synthesize B-vitamins, 

 of which they are the richest natural source. Their cultivation under 

 modern mass production factory methods may make the production 

 of some sources of human food partially independent of weather and 

 climate. 



There are a variety of genetical processes by which cultivated 

 yeasts may have originated from wild yeasts. Improvement of yeasts 

 has often been achieved by critical study of a great variety of cul- 

 tures followed by selection of the cultures showing desirable char- 

 acters. The early workers recognized that the variability of yeast 

 cultures made it possible to obtain a great variety of yeasts from 

 one original culture, but they found that cultures with desirable 

 characters were sometimes relatively variable. Our ability to re- 

 cognize the variable and stable phases in the life cycle of yeasts 

 enables us to control the stability of our derived stocks. 



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