THE ALCOHOLIC FERMENTATION. 215 



leaven. The next step, taken a long time afterward, was to discover 

 that it is the yeast in the leaven which produces the raising of 

 the bread, and then to separate the yeast from other undesired 

 materials in the leaven, and use it in pure cultures. This finally 

 gave the yeast that has been used for half a century or more. The 

 use of leaven has not altogether disappeared, but yeast is quicker 

 and more reliable. 



The action of the yeast in bread-raising is very simple. The 

 dough contains a considerable quantity of starch and also a small 

 quantity of diastase, an enzyme capable of converting starch into 

 sugar. The yeast acts upon the sugar thus produced and forms 

 from it alcohol and carbon dioxid. The latter, being a gas, 

 forms bubbles in the dough, causing it to swell and become lighter. 

 When subsequently baked, these gas bubbles leave their traces in 

 the numerous holes that one finds in raised bread. 



In the raising of bread the practice of depending on the wild 

 yeasts of the air has long since disappeared and yeast cultures are 

 now almost universally used. These commercial yeasts have been 

 chosen from the considerable variety of yeasts known, and are, 

 of course, the ones that have been found to produce the best results 

 in bread-raising. They are not the same varieties as those found 

 best for brewing. The yeast is cultivated in large quantity and 

 then put up in a convenient form for distribution, sometimes dried, 

 in which condition it will keep alive for weeks, and sometimes 

 compressed into a moist cake, compressed yeast, in which condition 

 it will keep only a few days. 



Bacterial- Impurities in Bread-raising. The yeast cakes are 

 never pure yeast, but may contain undesired bacteria, which then 

 get into the dough and sometimes produce trouble. Occasion- 

 ally, too, such bacteria may get into the dough from other sources 

 than yeast, such as dirty water or dirty utensils in the kitchen. 

 During the raising, lactic acid bacteria always grow, and they 

 seem to be necessary in order to prevent tire growth of other species 

 of more troublesome organisms. Sometimes such bacteria grow 

 too vigorously and may cause trouble. At least two different 

 faults in bread-making are known to be caused by undue growth 



