FERMENTATION PHENOMENA. 433 



however, not impossible that, in certain circumstances, only one 

 or the other of these two factors will come into play ; but at 

 present no systematic investigations have been carried out to 

 decide this question. 



It has already been mentioned (p. 429, vol. ii.) that the repro- 

 duction of Apicalatus yeasts is stimulated by the admission of free 

 oxygen or air ; and all that now remains is to deal with the influ- 

 ence of the oxygen supply on the fermentation process. ROHLING 

 (I.), who experimented with several races, found them to be power- 

 fully stimulated by the admission of oxygen, and enabled to produce 

 much larger quantities of alcohol in grape juice. In the absence 

 of a supply of oxygen the final percentage of alcohol varied 

 between 2.27 and 3.03 per cent, by weight, rising to between 

 5.01 and 5.76 percent, when oxygen was supplied. Since only 

 0.2 per cent, of unfermented sugar remained in the sample fur- 

 nishing the maximum quantity of alcohol, the yeast in question 

 would probably have been able to produce still more alcohol in a 

 stronger juice. These experiments show that the production of 

 alcohol was doubled on the average by the provision of a supply 

 of oxygen, the vital energy and power of resisting alcohol being 

 considerably increased, to a greater extent than had hitherto been 

 observed with elliptical wine yeasts. When oxygen is supplied, 

 the fermentation proceeds more rapidly from the start, and, 

 despite the higher final content of alcohol, is terminated sooner 

 than in the check experiment without oxygen. 



Certain chemical substances met with in vinous fermentation, 

 such as acetic acid, sulphur dioxide, and tannin (see pp. 246-248, 

 vol. ii.), restrict the growth of Apicidatus yeasts, and also probably 

 exert a direct lowering influence on the fermentative energy, 

 in the same way as they do in respect of beer yeasts and wine 

 yeasts. According to the experimental results communicated by 

 ROHLISG (I.), even o.i per cent, of acetic acid exerts a decided 

 influence on fermentation ; 0.5 percent, restricts the fermentation 

 to about one-third the normal, and i per cent, practically arrests 

 it altogether. In the case of sulphur dioxide, as little as 

 0.025 P er cen t- suffices to stop the fermentative activity of Sacch. 

 apiculatus almost completely ; and, indeed, the earlier discoveries 

 of MULLER-THURGAU (XXIV.) show that even 65 mgrms. of this 

 substance per litre, i.e., 0.0065 P er cent., will bring about the same 

 result. Tannin is less energetic, no considerable restriction of 

 fermentation being observed until the tannin content reaches 

 0.5 per cent. 



During the fermentation of fruit and grape juices by Apicu- 

 bUus yeasts, the non-volatile or fixed organic acids, i.e., tartaric acid 

 and malic acid, are also involved in the process of metabolism (see p. 

 205, vol. ii.). This was demonstrated by MULLER-THURGAU (XIV.), 

 in whose experiments the races under investigation reduced the 

 amount of 11011- volatile acids in grape juice from 0.883 to 0.669 per 



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