ALCOHOLIC FERMENTATION OF SUGAR BY YEAST. 593 



fermentative process is that a complex proteid substance, 

 when in a state of decomposition, transmits its chemical 

 movements to other molecules, such as sugar. 



Nageli's more recent researches, however, show most dis- 

 tinctly that, in the self-fermentation of yeast, the process 

 which takes place is not limited to, and only dependent on ? 

 the yeast ; but that, in the former experiments, without doubt 

 bacteria also acted and took part in the decomposition of the 

 substance of the yeast. In fact the very circumstances under 

 which the self-fermentation was observed were such that of 

 necessity an active development of bacteria must occur ; these 

 lived and multiplied at the expense of the dead yeast cells, and 

 probably also set up fermentative processes in certain of the 

 constituents of the yeast substance which had been rendered 

 soluble by ferments given off by them. A portion of the 

 nitrogenous derivatives found in the extract may result from 

 the activity of these bacteria, which can probably also take 

 part in the production of carbonic acid and alcohol. Nageli 

 so arranged the experiments that when, for example, by the 

 addition of citric acid, the development of bacteria was 

 rendered difficult, only minimal traces of alcohol were found, 

 probably derived from the small quantities of saccharine sub- 

 stances which are contained in the yeast cells, and which 

 underwent fermentation when no saccharine nutrient material 

 was at the disposal of the yeast. This process would be quite 

 analogous to the decompositions which occur in starving 

 animals. But we have no grounds for believing that the 

 proteid substance of the exhausted yeast cells can be utilised 

 hy other living yeast cells ; because the ferments necessary 

 for the solution of these substances are not found in yeast. 

 Further experiments, in which attention is paid to the 

 purity of the cultivation, are therefore required to give definite 

 information as to the real extent of the self -fermentation of 

 the yeast, and as to the products of tissue change. 



Cochin * has attempted to ascertain the time required The time 

 for fermentation, by continuous measurement of the J equire ? . or 



J fermentation. 



amount of carbonic acid developed. He found that in 

 the first instance ten to twenty minutes always elapsed 

 "before active fermentation commenced ; and the time 

 was still longer in dilute solutions. From that time 

 onwards the course of the fermentation may be expressed 

 by a steeply ascending line, which finally ends in the 

 form of a parabola. This period of incubation does not 

 mean that the saccharine solution must first penetrate 



* Cochin, Cumpt. rend., vol. 06. 



38 



