86 PHYSIOLOGY OF YEASTS 



which is folded in limited space and may become rather thick. In 

 such a form the myco-yeast is a strong oxidizing agent and acts like 

 an aerobic mold. It does not produce alcohol. If the veil or pellicle 

 is transferred to a flask filled to the neck with a carbohydrate me- 

 dium, an alcoholic fermentation results. From this moment, the 

 myco-yeast acts like a yeast enzyme; its development is slow and 

 almost the same weight is maintained which it had at the beginning. 

 This yeast does not produce as much alcohol as ordinary yeasts. 

 It never exceeds 3 per cent. Monilia Candida, a fungus, intermediate 

 between the yeasts and molds, acts in the same way. It produces 

 a veil on the surface of the medium and grows aerobically; at the 

 bottom of the flask it may appear as a deposit which decomposes 

 the sugar. Hansen found that it yielded 1.1 per cent of alcohol in 

 the time interval in which S. cerevisiae would yield 6 per cent. 



Excepting these species, most yeasts, especially the industrial 

 yeasts, are very energetic agents in alcoholic fermentation. These 

 are distinguished from the myco-yeast and Monilia Candida by the 

 fact that they vegetate almost solely at the bottom of the culture 

 flasks and form no veil at the surface. They almost always grow 

 under conditions of restricted aeration and possess the ability to 

 adapt themselves to anaerobic life which distinguishes them from 

 other plants. These yeasts may then be regarded as true agents of 

 alcoholic fermentation. We have stated above that the industrial 

 yeasts may form about six times as much alcohol as the intermediate 

 forms. 



Fermentable Sugars 



It is to the renowned researches of Fischer and Thierfelder that 

 we owe our knowledge with regard to the laws which govern the fer- 

 mentation of sugars. We have said a little about this, but it is of 

 sufficient importance to receive more extended treatment. 



From the investigations of these authors, it has been established 

 that only those sugars are fermentable, in which the carbon atoms 

 are in multiples of three. The series begins with glycerol (C 3 H 6 O 3 ), 

 the tetroses not being fermentable, neither the pentoses. Finally come 

 the hexoses which are very fermentable. These are made up of the 

 dextroses, levuloses, fructoses, galactoses and mannoses. There is 

 also a fermentable nonose (C 9 Hi 8 O 9 ); it is mannonose which is fer- 

 mented by yeasts as easily as is glucose. After these come the 

 bisaccharides (C^H^On) and the trisaccharides, melitrioses, or raf- 

 finoses; the melitrioses have the formula (CigH^Oie). The rule 

 then seems to be general. There seem, however, to be certain excep- 

 tions. Thus it is that the fermentation of glycerose, always feeble, 



