FERMENTATION 209 



thus, for example, the enantiomorph of rf-glucose, i. e. /-glucose, has the follow- 

 ing configuration : 



OH H OH H H H a OH 



C O C C C C 



OH H OH OH 



Many, but not all, of these sixteen different bodies are known, and it has 

 been found that only those which rotate polarized light to the right, and even 

 not all of these, are fermentable. Besides rf-glucose, ^-mannose and rf-galactose 

 alone among the hexoses are fermentable, i. e. those whose asymmetric carbon 

 atoms are united to the H and OH groups in the following way : 



H H OH OH OH H H OH 



d-mannose c c c c and d-galactose o c c c 



III! I I I I 



OH OH H H H OH OH H 



In contrast to rf-glucose it is the first carbon atom in d-mannose and the 

 third in d-galactose which is exchanged, but these exchanges do not prevent 

 fermentation ; if, however, as in d-talose, the first as well as the third carbon 

 atoms are exchanged : 



H H H OH 



I I I I 



C C C O 



I I I I 



OH OH OH H 

 then fermentation is impossible. 



Of all the known ketohexoses only one, d-fructose ( = laevulose), is ferment- 

 able ; its configuration is as follows : 



H 2 OH O H OH OH H,OH 



I I I I I I 

 C C C C C C 



I I I 



OH H H 



So far as regards the behaviour of its three carbon atoms it agrees entirely 

 with d-glucose, and hence may be explained the fact that it is nearly as easily 

 fermentable as that sugar ; mannose also closely resembles glucose, and ex- 

 perience shows that it is more easily fermentable than galactose, whose configura- 

 tion differs more widely. In individual cases we have not as yet learned what 

 factor determines this capacity for undergoing fermentation. Further, all 

 Saccharomycetes do not behave in the same way towards galactose ; S. 

 Pasteurianus I causes it to ferment almost as quickly as it does the three other 

 hexoses ; S. ellipsoideus induces fermentation in it only slowly, 5. productivus 

 and S. apiculatus do not cause it to ferment at all. 



The disaccharides are, as already mentioned, not directly fermentable, 

 they must first of all be hydrolysed into hexoses by means of enzymes. Thus 

 ordinary beer and wine yeasts give off enzymes which break down cane sugar 

 very quickly outside the cells into equal parts of dextrose and laevulose. One 

 of these, invertase or saccharase, we have met with before. The products 

 of decomposition are, for the most part, not acted on with equal rapidity; 

 many yeasts consume the dextrose first, some attack the laevulose first. This 

 may depend on a difference in fermentative power, but probably other 

 considerations may play a conspicuous part, e. g. powers of diffusion (KNECHT, 

 1901). In certain cases, Monilia Candida for example, we may assume a direct 

 fermentation of cane sugar, for it disappears during fermentation without any 

 invert sugar ( = dextrose + laevulose) appearing in its place. Careful research 

 has shown that in this case also an invertase comes into play ; but since it 

 cannot diffuse out of the cell its activity is intra-cellular only. 



