366 ENZYMES 



invertase secreted by the yeast converts the sucrose, which is 

 not directly fermentable by yeast, into invert sugar. 



Yeast, or yeast-juice, can set up fermentation in other sub- 

 stances besides glucose, such, for example, as galactose,* 

 mannose, fructose,! sodium lactate, ;{: and, according to Neu- 

 berg and Tir,§ common plant acids, fatty acids, glycerol and 

 lecithin. 



This fermentation, however, may not take place immedi- 

 ately on the introduction of the yeast to the particular sub- 

 stance : for instance, before yeast can ferment galactose, it 

 must be educated with regard to this material by being culti- 

 vated for some time in a solution containing it. A yeast so 

 educated yields a juice which can ferment galactose, the fer- 

 menting mixture, according to Harden and Norris, reacting 

 with phosphate in a manner exactly similar to yeast-extract 

 and glucose (see below) ; further, the process is accelerated 

 by the addition of a small quantity of sodium arsenite. 



In addition to ordinary alcoholic fermentation, yeast also 

 exhibits the power of auto-fermentation. || This is brought 

 about at the expense of the reserve food-materials of the 

 plant, chiefly glycogen, two enzymes being concerned in the 

 process. Glycogenase changes the glycogen into sugar, 

 which is then converted by zymase into alcohol and carbon 

 dioxide, the rate of fermentation being dependent on the rate 

 of sugar production by the glycogenase. Harden and Paine 

 also found that the rate of auto-fermentation is greatly in- 

 creased by the removal of water from the cell, which means, 

 of course, a concentration of the cell sap. This may be ac- 

 complished by partial desiccation or by the use of dissolved 

 substances which plasmolyse the cells. Alcohol in solutions 

 above lo per cent also have the same effect; on the other 

 hand, salts which do not produce plasmolysis, even in con- 

 centrated solutions, such as urea, have no such accelerating 

 effect. 



* Harden and Norris: " Proc. Roy. Soc, Lond.," B., igio, 82, 645. 



t Harden and Young : id., 1909, 81, 336. 



jKohl: " Beih. Bot. Centrlbl.," 1910, 29, 115. 



§ Neuberg and Tir : " Biochem. Zeitschr.," 191 r, 32, 323. 



II Harden and Paine: " Proc. Roy. Soc, Lond.," B., 1912, 84, 448. 



