EXERCISE 77 



PHYSIOLOGY OF YEAST 



Materials. Compressed yeast, which must be fresh ; molasses ; 

 bottles ; flasks ; thermometer. 



Directions for work. Mix one fourth of a cake of compressed 

 yeast with water, making a thin paste. Add it to a 10 per cent 

 solution of molasses in water. Add a proportionate amount of 

 yeast to pure water. Set both aside in a warm place for exam- 

 ination the next day. It would be well to divide the molasses 

 solution and yeast into three parts. One of these should be 



reserved for use in the next exer- 

 cise and the other two used as 

 directed below. Cover all three of 

 them loosely in order to prevent 

 evaporation. 



When the yeast culture has 

 stood for twenty -four hours or 

 more, note the bubbles of gas rising 

 through the liquid. Determine as 

 closely as possible their place of 

 origin. In what way, if any, do they appear to be related to 

 the yeast? Collect some of the gas as illustrated. Test with 

 a flame and with limewater. What gas is this ? (See Caldwell 

 and Eikenberry, " Elements of General Science," Rev. Ed., 

 pp. 71, 72.) 



Put a quantity of the solution into a flask arranged as 

 illustrated. Heat until a small amount of distillate has col- 

 lected or until the thermometer goes above 80 C., cooling 

 and condensing the vapors in the receiving flask. Identify the 

 distillate. Note its odor and how rapidly it evaporates, and 

 try to ignite some of it soaked in filter paper. What is the 

 source of the two substances which have been discovered in 

 the solution ? Consider that the three essential materials 

 in the solution are water, sugar, and yeast. Which of these 

 is increasing ? Which is decreasing ? Which is remaining 

 apparently unchanged? 



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