252 MICROORGANISMS AND FERMENTATION. 



As to the concentration of the liquid, the largest amount 

 of carbon dioxide is obtained by fermentation of liquids 

 containing 30 to 40 per cent, of sugar, doubtless because in 

 such concentrations the action of endotryptase is restricted. 

 The fermentation lasts longer under these circumstances, and 

 to secure a rapid fermentation, 10 to 15 per cent, of sugar 

 should be employed, but under these conditions the action 

 soon comes to an end. 



Amongst the many attempts that have been made to 

 isolate zymase from the juice, we may mention that by pre- 

 cipitation with alcohol-ether the whole of the zymase can be 

 converted into a dry form without loss of activity. By the 

 treatment of the dry substance with water and glycerine, the 

 liquid, even when filtered, possesses the full fermentative 

 power. This constitutes, therefore, a true solution of the 

 active substance ; a further treatment with alcohol-ether 

 does not yield any increase of zymase in the precipitate. 



The living yeast cells contain varying quantities of zymase. 

 Thus the content of zymase often increases perceptibly in 

 quiescent pressed yeast when kept at low temperatures. It 

 is a remarkable fact that yeast cultivated in a strong sugar 

 solution with inorganic salts exhibits a comparatively small 

 amount of zymase at the moment of greatest fermentative 

 activity with the greatest production of froth. If, however, 

 the yeast is removed at this stage, washed, pressed, and stored 

 for a few hours at a low temperature, it will be found that the 

 zymase content has considerably increased. In the same way, 

 yeast taken fresh from the brewery shows an increase of zymase 

 in some cases after storing. In other cases no such increase 

 is observed. These facts can be explained on the assumption 

 that the endotryptase is influenced by the low temperature, 

 even when the other conditions are favourable. 



A short review of the chemical changes that take place 

 during the fermentation of yeast- juice follows. 



The first problem is to discover whether the phenomena 

 caused by the addition of yeast-juice to sugar solutions are 

 identical with those of alcoholic fermentation of sugar. The 

 attempts to solve this have led to remarkable results. The 

 chemical action of the enzymes already discussed invertase, 



