NUTRITION OF YEASTS 69 



2. Nitrogenous Substances 



The nitrogenous substances may be divided into four groups: 

 ammonia, nitrates, albumins, and their derivatives, such as amides 

 and amines. Since the investigations of Boussingault, it has been 

 known that the nitrates play an important role in the nutrition of 

 higher plants. The investigations of Muntz have shown that ammonia 

 is also assimilated by higher plants, but it is only a substance of 

 medium importance. In the nutrition of yeasts, ammonia salts 

 (phosphates and sulfates), on the contrary, play an important role while 

 the nitrates are generally not assimilated. 



Pasteur was the first to establish that the ammonium salts were 

 good foods for the yeasts. The later investigations by Duclaux and 

 Laborde, and Laurent have confirmed these results. The experi- 

 ments of Laurent have indicated that yeasts do not assimilate ni- 

 trates. According to this author, they would have to reduce the 

 nitrates to nitrites, substances which are toxic. Beijerinck, however, 

 has stated that certain yeasts, such as S. acetethylicus, are able to 

 assimilate nitrates. Since then, the investigations of Kayser, 1 and 

 Fernbach and Lanzenberg 2 have shown that, if nitrates are injuri- 

 ous to multiplication, they have a favoring influence on the zymase 

 in fermentation. 



The relation of albuminoid substances to the metabolism of 

 yeasts is very obscure. According to Pasteur and Ad. Mayer, the 

 yeasts are unable to use egg white or blood fibrin. These substances 

 do not pass through the cell membrane, and the endotryptase of the 

 yeasts is an intracellular enzyme which does not pass easily to the 

 outside of the cell. We have seen that, according to Boullinger, cer- 

 tain yeasts inoculated into milk develop very slowly and produce, 

 after a few months, a curd which slowly liquefies with the formation 

 of ammonium salts, tyrosine, and leucine. There is then a dissolu- 

 tion and digestion of the casein by the yeast. It is known, on the other 

 hand, that certain species of yeasts liquefy gelatin. It must be ad- 

 mitted that, under special conditions, endotryptase may pass through 

 the cell membrane. 



On the contrary, if yeasts do not accommodate themselves to 

 these compounds, they easily assimilate the dialyzable derivatives of 

 them, such as the albumoses and peptones. It is curious to note that 



1 Kayser, E. Influence des nitrates sur les ferments alcooliques. Comp. 

 Rend. Acad. Sci. 150, 1910. 



2 Fernbach, A., and Lanzenberg, A. De 1'Action des nitrates dans la fermen- 

 tation alcoolique. Comp. Rend. Acad. Sci. 151, 1910. 



