INORGANIC SOURCES OF NITROGEN. 211 



to obtain reliable data regarding the nitrogenous nutriment of 

 yeast is by the aid of pure cultures, no others being worth the 

 trouble of undertaking. 



The problem set out above is still too fresh for a final solution 

 to have been found ; and each day may reveal some new observa- 

 tion opening up a quite unexpected perspective ; for which 

 reason the matter has been very briefly treated here. However, 

 one point is now well established, namely, that, in a saccharified 

 nutrient mineral solution containing ammonia as the sole form 

 of nitrogen, cell reproduction and fermentation can only com- 

 mence when the number of cells introduced does not fall below a 

 certain minimum, the absolute dimensions of which have not yet 

 been definitely ascertained, and which is probably dependent on 

 the other conditions of the experiment. 



This, however, does not imply that yeasts unconditionally 

 reject ammonium salts when the other conditions essential to 

 development are present. On the contrary, they then exhibit a 

 certain preference for ammoniacal nitrogen. This was already 

 observed by DUCLAUX (XVII.) during his experiments on the 

 fermentation of wine must in 1866, the nitrogen content falling 

 from 120 mgrrns. to a very small proportion per litre in conse- 

 quence of its consumption by the yeast present. The question 

 has also been studied by Miintz and Rousseaux, then by Roos 

 and Chabert, and finally by J. Laborde, the last-named stating 

 that, at 28 C., ammoniacal nitrogen is taken up more extensively 

 than organic nitrogen, though the reverse was found to occur at 

 36 C. According to ARTARI (I.), Sacch. Zopfii is even satisfied 

 with ammonium sulphate as the sole source of nitrogen. Further 

 particulars respecting the behaviour of Mycoderma species toward 

 ammonia salts will be given in chapter Ix. 



The nitrates, which are the best, and in some cases the only, 

 sources of nitrogen for higher plants, are of no value for yeasts, 

 except in a few cases, typified by BEYERINCK'S (XXI.) Sacch. 

 a^etethylicus. This was first proved by AD. MAYER (I.) ; and the 

 converse opinion expressed by DUBRUNFANT (III.), was disproved 

 by E. LAURENT (VI.). The injurious effect produced on yeast by 

 the presence of such salts in otherwise favourable nutrient solu- 

 tions seems due to the reducing action of the cells causing the 

 formation of highly poisonous nitrites. One of the reasons of 

 fermentation disturbances in molasses distilleries is certainly to 

 be found in the presence of nitrates, which sometimes occur in 

 large quantities in molasses. Similar results have also been found 

 by L. BRIANT (II.) in breweries employing water rich in nitrates ; 

 and this worker mentions about 75 grains per gallon as the 

 highest permissible limit of these salts. The influence of nitrates 

 on attenuation has also formed the subject of experiments by 

 EVANS (I.). In a patented process for cultivating races of yeast 

 capable of thoroughly fermenting dextrins (see chapter Ixv.), 



