FERMENTATION. II9 



is, under such conditions, very complete if sufficient time is 

 allowed for the process, the yeast-cells experience a certain 

 difficulty in wresting the nitrogenous elements from the 

 inorganic ammonia salts, and that all other conditions must 

 be extremely favourable, in order to allow of their taking up, 

 and utilizing for their own use, inorganic nitrogenous material. 

 The presence of other organisms, for example, that have a 

 stronger affinity for nitrogen in this form, i.e., organisms 

 which are better adapted to exist under such conditions, 

 and which can, by their vital activity, interfere with the 

 growth of the yeast-cells, remove from the sphere of action 

 of the yeast-cells material that is absolutely necessary for their 

 rapid and perfect morphological and biological development, 

 as a result of which fermentation and hydration of the sugar 

 do not take place in the ordinary way ; other substances or 

 bye products are formed, and the decomposition of the sugar 

 is incomplete, or is irregularly carried out. 



What, then, are the conditions necessary for the growth of 

 the fermenting organism in a fermentable fluid ? In the pro- 

 cess of wine-making it is a well-known fact that the fermenta- 

 tion is set up by some organism, which, though present 

 either as young cells or as spores on the outside of the grape, 

 cannot attack the juice so long as the skin remains 

 unbroken — a fact brought out by Davaine at a very early 

 stage of his researches. When, however, the grapes are 

 plucked, the skins are bruised and the juice is set free, the 

 fermenting organism, — the wine yeast or Saccharomyces 

 eUipsoideus, or some similar variety — utilizing the grape 

 juice, which contains not only sugar but also all the ele- 

 mentary constituents necessary for its nutrition, — grows 

 vigorously and sets up the vinous fermentation. It is 

 a remarkable fact, but one well known to wine pressers and 

 fermenters, that for the commencement of this vinous fer- 

 mentation there must be an access to free oxygen, or oxygen 

 mixed merely mechanically with nitrogen of the air to the 

 fluid that has to be fermented, as without this the yeast 

 spores and old yeast-cells are utterly unable to develop or to 

 give rise to active and vigorous yeast-cells. Pasteur insists 

 that we have evidence of the necessity for the presence of 

 such free oxygen in the fact that the fermentation of grapes 

 takes place much more rapidly and completely when the 

 grapes are left attached to the stalks of the bunches, by 



