SAPROPHYTES 395 



to the dough as it is mixed. The yeast that is mixed with 

 the dough ferments this sugar and produces both alcohol and 

 carbon dioxide. The flour also contains a considerable quan- 

 tity of a protein substance, GLUTEN (Art. 418). This gives the 

 dough its sticky character and makes it more or less impervious 

 to gases. Consequently, as the yeast produces carbon dioxide 

 throughout the dough, this gas collects and forms little 

 cavities in the dough and is thus responsible for the char- 

 acteristic porous quality of light bread. You have seen (Art. 

 12) that alcohol is a very volatile substance. Consequently, 

 in baking the bread, all the alcohol is caused to escape by the 

 heat. As the bread is baked, the gluten of the dough is so 

 changed in character as to become quite pervious to gases and 

 it thus allows both the alcohol and the carbon dioxide to escape. 

 You thus see that we use yeast in making bread, primarily, 

 for a sort of mechanical effect which it produces on the bread, 

 making it light and porous, and probably on this account more 

 easily digested. In addition to this, the yeast adds a nut-like 

 flavor to the bread which we like very much. 



If the knowledge of the role that yeast plays in bread 

 making is of any practical value, it lies in the fact that the 

 bread maker will realize that she is dealing with living organ- 

 isms. If she has this fact in mind, she is likely to be more 

 intelligent in providing favorable conditions for the growth 

 of the yeast. In general, it may be said that the best bread 

 results from a moderately rapid fermentation, but the details 

 of managing yeast for bread making are too numerous for us 

 to enter into here. ^ 



449. Wild and Cultivated Yeasts. All the different kinds 

 of yeast used in bread making except that used in making salt 

 rising bread, and all that are used in the breweries and dis- 

 tilleries, are what may be called CULTIVATED YEASTS. These 

 consist of several distinct species and varieties which vary in 

 their usefulness for different purposes and all have been in use 

 by man for so long a time that their origin from the wild state 

 cannot be traced. In addition to these, WILD YEASTS of 



