494 ENZYMES 



mixed with lOO c.c. of water and subjected to pressure. The 

 extracts are united, shaken up with a httle kieselguhr, and 

 filtered. The filtrate contains the zymase, but in an impure 

 condition ; it may be purified by precipitating with alcohol 

 and dissolving the precipitate in water. The aqueous solu- 

 tion rapidly loses its ability to ferment owing to the destructive 

 action of a tryptic enzyme. It may, however, be preserved 

 for a longer time — but not indefinitely — by drying the extract 

 under reduced pressure, the solid substance so obtained 

 being kept in a cold desiccator and dissolved in water as 

 occasion demands. 



In preparing extracts of yeast, it must be remembered that 

 the potency of the extracts depends upon the physiological 

 state of the yeast used. Thus, if brewers' yeast be taken from 

 the wort whilst fermentation is at its height, a high quality 

 zymase will be obtained ; if, however, fermentation of the wort 

 be over, the yeast taken from it will yield an extract of little 

 or no fermenting power. 



A more stable preparation than zymase is zymin, which is 

 prepared by stirring a bottom fermentation brewer's yeast 

 with acetone for some minutes, filtering, treating again with 

 acetone, draining and finally extracting with ether. The 

 material is dried and kept at a temperature of 45° for twenty- 

 four hours. This zymin is more active than yeast juice but 

 is less active than living yeast, fermenting at about one-eighth 

 the rate of an equivalent weight of the living cells. The 

 optimal temperature of zymase is between 28-30° C, and in 

 solution it is destroyed at 40-50° ; its optimal reaction lies 

 between P^ 6-2 and 6-8. 



ROLE OF PHOSPHATE IN YEAST JUICE FER- 

 MENTATION. 



Wroblewski * was the first to observe that the addition 

 of an alkaline phosphate increased the rate of fermentation by 

 yeast juice, which fact he attributed to the reaction of the 

 medium. The work of Harden and Young,t however, provides 



* Wroblewski : " J. prakt. Chem.," 1901, [2], 64, i. 



t See Harden : " Alcoholic Fermentation," London, 1923. 



