FERMENTABLE SUGARS . 87 



has been contested by Emmerling. On the other hand, Saccharomyces 

 thermantitonum ferments the pentoses (arabinose and xylose) and it 

 seems to be true with S. Ludwigii according to Lindner. This author 

 found that this yeast would ferment sorbose and tagatose and that 

 Sch. octosporus would ferment xylose. 



We have seen that yeasts are not able to ferment polysaccharides, 

 but they are broken up by hydrolysis under the action of a special 

 enzyme in each case. It is thus that starch is transformed by amylase 

 to maltose, by maltase to glucose, saccharose into glucose and levu- 

 lose by invertase, trehalose into glucose, and lactose into glucose 

 and galactose, etc. Thus the sugars which have a more complicated 

 structure are split into C 6 sugars by enzymes and these in their turn 

 are decomposed into alcohol and carbon dioxide. Alcoholic fermenta- 

 tion offers, then, one of the best examples of molecular simplification 

 which the enzymes are able to accomplish, and suggests the important 

 role which these bodies play in cellular life. We have stated before 

 that the a-methylglucosides and the /3-methylglucosides are ferment- 

 able by certain yeasts after having been decomposed by maltase and 

 methylglucosases. 



It has also been established from the work of Thierfelder and 

 Fischer that the sugars with C 6 , Ci 2 and CM atoms are not ferment- 

 able ,to the same degree. Thus trehalose ferments more slowly and 

 only by certain yeasts. Lactose is fermented only by a small number 

 of yeasts. 



The same thing is true with regard to the hexoses which are also 

 not fermentable to the same degree. Among the ketohexoses, for ex- 

 ample, levulose and d-fructose are alone fermentable, and among 

 the d-glucoses only the d-glucose (grape sugar), d-mannose and d- 

 galactose are fermentable. 



According to Thierfelder and Fischer, a relation exists between 

 the fermentability and the structure of the molecule. In other words, 

 the enzymes have a direct relation to the stereochemic constitution 

 of the molecule. This relation has been compared to that relation 

 which exists between a key and its lock. The aldohexoses l suggest a 

 very important example of this relation. Of the nine known aldo- 

 hexoses, there are fermentable, as we have seen, only d-glucose, d- 

 mannose and d-galactose with the last possessing much less fermenting 

 possibilities than the other two. The chemical formulae of the aldo- 

 hexoses are the same and differ only in their molecular grouping. 

 Some of these will be given. 



1 The term aldohexoses refers to the hexoses which have an aldehyde group 

 CHO, while the term ketohexoses refers to those which possass the keton group 

 C=0. 



