102 A MANUAL OF BACTEEIOLOGY 



Exercise 143. The Zymase of Yeast 



Zymase is the enzyme of yeast which accomplishes the alcoholic 

 fermentation of sugars, but it is only capable of fermenting 

 hexose sugars, chiefly d-glucose, d-fructose, d-mannose, and 

 d-galactose. The action may be represented by the equation 



C,H^A = 2C,HpH + 2CO,. 



Zymase is classed as an endoenzyme, because it occurs only 

 in the living cell and does not exosmose into the surrounding 

 medium. The isolation of the enzyme from yeast cells is de- 

 scribed in exercises 144 and 145, but its action may be demon- 

 strated by the following simpler method : 



1. Prepare a 4 per cent solution of dextrose in tap water and 

 fiU three flasks about one-third full ; also fill several fermentation 

 tubes. Sterilize all in the Arnold sterilizer. 



2. Inoculate two of the flasks and all of the tubes with active 

 yeast. Weigh the flasks and record the exact weight of each. 

 Keep the cultures in the incubator at 25° C. As fermentation 

 progresses, note the changes in weight of the flasks in com- 

 parison with the control. To what are the changes due ? After 

 a week compare the specific gravity, of the fermented solution 

 with that of the control. 



3. Examine the fermentation tubes at the end of twenty-four 

 hours. When the closed arm of the tube is about half full of 

 gas, test it with 10 per cent NaOH. How much of the gas is 

 carbon dioxide ? 



4. For further studies of the action of zymase see exercises 

 146, 147, 148, and 149. 



5. State as exactly as possible the characters of invertase 

 and zymase. What is the relation of one to another in causing 

 fermentations ? 



Exercise 144. The Preparation of Yeast Juice 



In 1897 Buchner succeeded in mechanically breaking down 

 the yeast cell and obtaining a yeast-free extract containing zymase 

 which would cause alcoholic fermentation in sugar solutions. The 



