208 IMPORTANCE OF YEAST PLANTS 



tions of rye, corn, barley, potatoes, etc., and slightly warmed, the 

 growth of the yeast produces a vigorous fermentation. Carbon 

 dioxide rises to the surface, forming a frothy scum while the alco- 

 hol accumulates in the fluid. Beers and ales are fermented bever- 

 ages of this nature, while whiskies, brandies, alcohol, etc., are 

 obtained from the fermented mass by removing a part of the 

 water by distillation. Wines and cider are weak alcoholic bev- 

 erages formed by the yeast plant from the sugars in the juices 

 expressed from grapes and apples. In this case, the yeast is 

 generally allowed to find its way naturally into the fluids. So 

 minute are these plants that they are distributed in the air 

 throughout the world and no weak sugar solution can be exposed 

 for any length of time without being inoculated by them a fact 

 that has. been apparently taken advantage of by man in all times 

 and places. When these fermented beverages are bottled before 

 the decomposition of the sugar is complete, then a further gen- 

 eration of CO 2 sets up a pressure in the bottle or cask that causes 

 the popping of the cork when the bottle is opened and the sparkle 

 of the fluid owing to the escaping gas. 



(b) Bread Making. The most important use of the yeast plant 

 is its application to the " raising " of bread. Flour contains, in 

 addition to starch, a little sugar, and this amount is increased in 

 bread making by a ferment, diastase, in the flour which changes 

 starch to sugar. The flour is mixed with water containing yeast 

 plants into a dough and placed in a warm place, when it begins 

 to rise. This means that the yeast plants begin to grow and 

 decompose the sugar into CO 2 and alcohol. The dough prevents 

 the escape of the gas which collects in bubbles in the dough, 

 causing it to swell. Baking further expands the gas and also 

 drives off the water and alcohol, leaving the bread light and por- 

 ous. There are many forms of yeast plants which differ in their 

 power of producing fermentations and in the flavor which they 

 impart. Consequently, different forms are used for different pur- 

 poses. Bread yeast is a form that has been selected because of 

 the quickness of its action and the flavor that it imparts to bread. 

 Bread yeast is grown in large vats and put up with starch in 

 cakes, known as compressed yeast. This product is not pure and 



