The Funm— Yeasts 375 



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cows is supplemented with yeasts, the milk produced by these 

 animals contains a high amount of these vitamins. 



Some individuals eat yeast as part of their daily diet. These 

 microorganisms have been recommended as an aid in clearing up 

 skin blemishes and preventing constipation. While yeasts may 

 contain as much as 50% protein and readily available sources of the 

 B complex vitamins, ingestion of large numbers of yeasts as part of 

 the daily diet is not recommended except upon the advice of a 

 qualified physician, because undesirable changes in the intestinal 

 flora of the individual may be brought about by too many of these 

 organisms. 



Brewer's yeasts are usually classified as "top" and "bottom." 

 Top yeasts are active fermenters, and the large amount of carbon 

 dioxide gas which they produce drives the cells to the surface of 

 the vats where the yeasts form a scum. Ales are made with top 

 yeasts, while beers are generally prepared by the slower-fermenting 

 bottom yeasts. Distillery yeasts are high alcohol-producing strains 

 of Saccharomyces. When the alcohol concentration reaches about 

 10%, yeast activity is markedly diminished, and when the strength of 

 alcohol reaches 15%, further production of alcohol virtually ceases. 

 Hard liquors, therefore, must be distilled to concentrate the alcohol 

 produced in fermentation vats. When grains are the substrate 

 employed for the cultivation of yeasts, the enzyme diastase found 

 in the sprouting grain converts the starch into sugar which in tru-n 

 is acted upon by yeasts specifically selected for their pronounced 

 ability to ferment such a substrate. Molasses, potatoes, sugar beets, 

 and rice may also be employed to produce alcohol. 



Wines are made by the fermentation of fruit juices or sugar 

 through the action of particular strains of yeasts, usvially Saccharo- 

 myces ellipsoideus (elliptical-shaped cells). When home-made 

 wines are prepared, it is usually the action of bacteria and yeasts 

 found as the natural flora on the fruit which results in the produc- 

 tion of alcohol. Pure cultures of yeasts may or may not be added 

 to initiate the feiTnentation. 



Another important product which is produced bv the enzymatic 

 action of yeasts is glvcerin. This compound is used as a sweeten- 



