244 



GENEPx AL BOTANY 



When the juice of such fruits is pressed out, as in the making 

 of cider, grape juice, or homemade wines, the wild yeast plants 

 are certain to get into the expressed juice and cause the fermen- 

 tation which is so common under such circumstances. Sweet 

 solutions of all kinds, when exposed to the air, become infected 

 with wild yeasts, which causes them to ferment. 



The cultivated yeasts used in the making of yeast cakes and 

 in the brewing industries are simply selected varieties of wild 



yeasts which have 



been found especially 

 adapted to the rais- 

 ing of bread or to 

 the making of various 

 sorts of fermented 

 liquors. These culti- 

 vated yeasts are also 

 of many kinds, each 

 particular kind hav- 

 ing its peculiar char- 

 acteristic effect on 

 sugary solutions in 

 which they are placed. 

 Thus, in making beer 



& 



FIG. 130. Various kinds of wine and beer yeasts 



a, 6, wine yeast (S. ellipsoideus) (a, young and vigo- ^he people of some of 



rous; 6, old (1) and dead (2)) ; c, d, beer yeast (S. cere- 



visite) (c, bottom yeast; <7, top yeast). After Marshall the European COU11- 



tries use what is called 



bottom yeast (Fig. 130), which is capable of producing only a 

 moderate supply of alcohol by fermentation. Top yeast, which 

 is used in making English beers and compressed yeast cakes, 

 produces a larger amount of alcohol than bottom yeast. Dis- 

 tillers' yeasts, used in making rum and brandy, are also capable 

 of producing large quantities of alcohol by fermentation. The. 

 various kinds of wines likewise owe their peculiar flavors to the 

 kind of yeast used in them. All these cultivated yeasts have been 

 gradually selected by man for particular uses, just as various 

 kinds of fruit trees, cereals, and flowers have been selected 

 and improved by cultivation. 



