ENZYMES. FERMENTATION 403 



V. Zymase (Yeast). 



Yeast contains a mixture of several enzymes. Its principal 

 enzyme is zymase, which acts upon the four natural hexoses. It 

 contains also maltase and invertase, but it does not contain lactase. 

 Lactase is only present in special yeasts, such as kefir. It is owing to 

 the presence of maltase and invertase that yeast is able to ferment 

 maltose and cane sugar and convert them into alcohol and carbon 

 dioxide. Lactose is not fermented as it is not hydrolysed into its 

 constituent monosaccharides. Before alcoholic fermentation can occur 

 hydrolysis into monosaccharides must take place. 



The action of yeast upon the sugars is most conveniently demon- 

 strated with a series of Einhorn fermentation tubes (p. 233). They are 

 filled with I per cent solutions of glucose, fructose, galactose, maltose, 

 cane sugar and lactose, and a small piece of yeast is added to each. 

 Fermentation proceeds slowly, but in 12 hours it will be observed 

 that all the sugars except lactose have been fermented and that galac- 

 tose is fermented more slowly, as shown by the smaller volume of 

 carbon dioxide evolved. 



The presence of zymase in the yeast can be shown either by preparing, 

 yeast juice by Buchner's method (p. 392) or by preparing maceration extract 

 by Lebedeffs method. Fresh yeast is carefully dried. 100 gm. of the dried 1 

 material are treated with 300 c.c. of water for 2 hours at 37. The mixture 

 is filtered rapidly on a large folded filter paper and the filtrate is collected in 

 a vessel in ice. 



Portions of 5 c.c. or 10 c.c. are added to i per cent, solutions of glucose, 

 fructose and the other sugars in Einhorn fermentation tubes. After some 

 hours the formation of carbon dioxide will become visible. 



Specificity of the Action of Enzymes. 



5 c.c. of diastase solution are added to 5 c.c. of cane sugar 

 solution and kept at 40 for some time. There is no conversion of 

 cane sugar by diastase into glucose and fructose as shown by testing 

 for reducing sugar with Fehling's solution. 



The same experiment is performed with 5 c.c. of salicin solution 

 instead of cane sugar. Again there is no reduction of Fehling's 

 solution. 



In the same way the action of 5 c.c. of the invertase solution is 

 tested upon 5 c.c. of starch solution at 40. There is no hydrolysis 

 of starch by invertase. If any action occurs it is due to impurity in 

 the invertase solution, i.. to its containing a little diastase. It is very 

 difficult to obtain enzyme solutions which contain only one enzyme. 

 Most cell contents contain a mixture of enzymes. 



5 cc. of emulsin solution will not hydrolyse 5 cc. of starch 

 solution or 5 c.c. of cane sugar solution. 



In the experiments with yeast neither invertase, nor maltase, nor 

 zymase acted upon lactose. 



