THEORIES OF FERMENTATION. 247 



molecular structure of a given sugar and the sugar-inverting 

 enzyme of a yeast cell ; if a sugar comes into contact with the 

 albuminoids of a yeast cell, which play the most important 

 part among the agents utilised by the living cell, the sugar is 

 decomposed only if its configuration, the geometrical structure 

 of its molecules, does not deviate too much from the con- 

 figuration of the molecules of the albuminoid. Thus, accord- 

 ing to Fischer's theory, the function of the living cell depends 

 much more upon its molecular geometry than on the com- 

 position of the nutritive material. 



Another way in which Fischer, as well as Thierfelder, 

 obtained confirmation of his fermentation theory was by 

 examining the behaviour of Hanson's and other yeast species 

 towards the artificial sugar species, synthetically obtained 

 by Fischer. They found, indeed, that the yeasts are quite 

 fastidious regarding the geometrical configuration of the sugar 

 molecule, whilst they often remain unaffected by other altera- 

 tions in its composition. 



Among the various synthetically-prepared sugars examined 

 by Fischer with regard to their behaviour towards yeasts, 

 melibiose is especially mentioned. It is fermented by brewers' 

 common bottom-fermentation yeasts, but not by many brewers' 

 top-fermentation yeasts. In harmony with this, Fischer 

 found that bottom-fermentation yeast contains an enzyme 

 capable of extraction from the dried yeast in aqueous solution, 

 which decomposes melibiose, converting it into glucose and 

 galactose ; but in a corresponding treatment of the brewers' 

 top-fermentation yeasts no decomposition of this sugar could 

 be observed. As brewers' top-fermentation yeast contains 

 invertase, it follows that the ferment which splits up melibiose 

 cannot be identical with invertase. 



C. J. Lintner and Fischer showed, by methods devised by 

 the latter, that natural maltose is split up into two molecules 

 of glycose, if acted upon by an aqueous extract of dried yeast, 

 or by cells, the membrane of which has been torn by grinding 

 with powdered glass, and that there is a marked difference 

 between this enzyme and invertase which hydrolises cane- 

 sugar. The former enzyme is termed yeast-glycase or yeast- 

 maltase. Its optimum temperature is about 40 C., whilst 



