566 Introduction to the Study of Science 



plants and especially their fruits and grains. Whenever they 

 find suitable food, moisture, and temperature, they begin im- 

 mediately to develop, bud, and produce fermentation. 



Wild and cultivated yeasts. It is customary to classify yeasts 

 of the harmless and beneficial kinds as wild and cultivated. 

 The wild yeasts are such as were captured and cultivated in 

 your flour and water mixture and in the fruit juices. The 

 cultivated yeasts are represented by those in the commercial 

 yeast cake, either the -dry or the compressed, and by others 

 mentioned later as the top and the bottom yeasts. 



Industrial uses of yeast. Because of their economic im- 

 portance yeasts have been thoroughly studied and are utilized 

 in various ways. Bread making is practically world wide and 

 supports a great industry of producing and marketing yeast 

 in different forms, such as the compressed cake, the dried cake, 

 or the liquid form. Such industries as manufacturing vinegar, 

 as brewing and distilling make use of different kinds of yeasts 

 and bacteria which are associated with them ; and they have 

 through scientific study acquired control of the different kinds 

 of yeasts and their products. In brewing, for example, there 

 are two kinds of yeast used, one being called top yeast, and the 

 other bottom yeast. The top yeast grows in fermentation on 

 the surface of the brew, while the bottom yeast develops at 

 the bottom of the brew. In their action upon sugary solutions 

 each of these evolves very different and characteristic products. 

 The yeasts employed in the production of distilled liquors 

 are of another kind, producing great fermentation with the 

 evolution of large amounts of alcohol. Yeasts differ greatly 

 in different regions, as may be especially noted in connection 

 with the grape-growing and wine-making districts. In Europe 

 and in America special local varieties of yeasts are cultivated 

 and employed in making wine, and these give the distinctive 

 bouquet and qualities of famous vintages. In this country 

 several varieties of yeast are now available which are utilized in 

 developing distinctive and delicate flavors and qualities in cider. 



