212 METABOLISM 



difference in the activity of enzymes and zymase appears to us to be sufficient 

 to separate these bodies from each other. Since also it is possible that we may 

 find substances associated with other fermentative processes analogous in their 

 action to zymase, we had better employ the name 'zymase' as a collective term 

 for all substances produced by organisms which have the power of inducing 

 fermentative decompositions, and designate Buchner's zymase by the term 

 alcoholase. There can be no doubt that the determination of zymases is one of 

 the greatest advances as yet made in the study of the theory of fermentation; it is 

 therefore worthy of mention that E. Traube (1858) believed ' enzymes ' to be 

 the cause of fermentation, although he was unable to separate them from the living 

 substance. The successful isolation of alcoholase has as yet not been effected. 



Although the existence of alcoholase has been proved it by no means follows 

 that alcoholic fermentation is to be regarded as a vital phenomenon, even 

 although zymases as well as enzymes generally, in their origin and activity, are 

 entirely dependent on organisms. This is all the more apparent when we 

 remember the dependence of fermentation on external conditions. We will 

 confine ourselves to the consideration of two factors only, temperature and 

 oxygen. With regard to temperature we may note that an optimum of 25° C. 

 is necessary for fermentation while the optimum activity of zymase lies much 

 higher. We must go more carefully into the question of the effect of oxygen 

 on fermentation. Doubtless, when zymase operates in a test-tube it is quite 

 immaterial whether oxygen be present or not. From our experience of Fungi 

 and higher plants also, we must expect yeast to produce alcohol only in the 

 absence of oxygen ; but in the cases mentioned the formation of alcohol takes 

 place only in intra-molecular respiration and is due to the presence of a zymase ; 

 at least, Stoklasa (1903) has found such an enzyme in beet, although 

 GoDLEWSKi (1901) was unable to determine its presence in seeds which pro- 

 duced alcohol. [Maze (1904) has shown that Stoklasa's arguments in favour 

 of general distribution of zymase are not valid.] The formation of zymase in 

 beet must come to an end in the absence of oxygen. Yeast, however, behaves 

 quite otherwise ; it forms alcohol in presence of oxygen as easily as in its absence. 

 [According to Wehmer (1905) species of Mucor behave in the same way.] This 

 fact, obviously of so much importance to the theory of fermentation, deserves 

 more detailed investigation, for it is impossible to believe that alcoholic fer- 

 mentation is quite independent of oxygen. 



Yeast, in addition to its power of inducing fermentation, can also respire in 

 the ordinary way, and if oxygen be present in the fermentable medium part 

 of the sugar will be respired and the rest fermented. A definite quantity of 

 yeast, however, generates all the more alcohol the less normal respiration is 

 permitted, and if oxygen be entirely absent the whole of the sugar disappearing in 

 the course of metabolism will be fermented. Since the energy evolved from 

 the respiration of the sugar is much greater than from its fermentation one 

 comprehends that more sugar will be used up in the latter case. In Giltay 

 and Aberson's experiments (1894) i g. of yeast (dry weight) transformed 7 g. 

 of sugar in presence of oxygen and 14 g. in its absence in the same time. The 

 maximum formation of alcohol in no way corresponds to the optimum vital 

 conditions of yeast. Growth and increase of yeast are largely dependent on the 

 presence of oxygen ; when it is withdrawn vegetative activity ultimately ceases, 

 although fermentation still goes on. There is, unfortunately, no unanimity 

 among the different authorities as to how far increase of the yeast is possible in 

 the absence of oxygen ; according to some authors, it ceases entirely, according 

 to others (Beijerinck, 1894), twenty to thirty times the original mass may be 

 produced. Whichever view is correct, growth is always limited when alcoholic 

 fermentation only occurs, while it is unlimited when respiration begins. Since, 

 however, the amount of alcohol produced in a unit of time depends obviously 

 on the amount of yeast present, one cannot say a priori whether a minimum 

 amount of yeast will give in the long run more alcohol in a nutritive fluid con- 



