250 THE NATURE OF YEAST 



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 CO2 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. These forms show the 

 same range of variability in the work which they perform as 

 noted in the bacteria of decay. In the case of cider our govern- 

 ment experts have separated several forms of yeast that are now 

 offered for distribution and which give distinctive flavors to 

 cider. So in brewing and bread making enough has been done 

 to show that there are numerous forms of yeast that differ 

 materially in their power to induce fermentation as well as in 

 the nature of the fermentations that they bring about. 



(&) 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 into 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 CO2 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 

 poroUs. 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 differ- 

 ent purposes. Bread yeast is a form that has been selected be- 

 cause 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. 



