YEAST AS A SOURCE OF CARBON DIOXIDE 73 



is now extremely simple. But it has taken many centuries 

 of experiment and trial to understand the subject well 

 enough to bring it under proper control. 



In all nations, and apparently in all ages, people have 

 been accustomed to make bread from meals obtained from 

 the different kinds of grain. The earliest method of cook- 

 ing such material was simply to mix it with water and 

 then bake it, the result being a rather hard, tough material 

 known as unleavened bread. 



The next step consisted of a spontaneous raising of the 

 dough. If dough is left in a warm place for a number of 

 hours, it becomes somewhat swollen with gas, appears 

 lighter in character, and when baked produces a type of 

 bread more easily masticated, better in flavor, and more 

 easily digested. Flour from most cereals, if mixed with 

 water and kept for a few hours in a warm place, will 

 undergo a fermentation, due to the wild yeasts that may 

 have found entrance to the meal. This method of fer- 

 menting the dough gave the first form of raised, or leav- 

 ened bread. 



Very early, even before historical records, it was dis- 

 covered that a little of the dough thus raised would serve 

 as a starter for a second batch, resulting in a quicker and 

 more satisfactory raising than that obtained by spontane- 

 ous fermentation. This was known as leaven, and as far 

 back as the time of Lot we read of leavened and unleav- 

 ened bread. The Egyptians also knew of this process. 

 Leaven has been used from those early days to the pres- 

 ent time. Even to-day leaven consists of a little dough 

 which has already fermented and hence contains yeasts, 

 and which is saved to be used in fresh dough for the 



