62 PHYSIOLOGY OF YEASTS 



able. These are the glyceroses (C 3 H 6 8 ), tetroses (C 4 H 8 4 ), hex- 

 oses (C 6 Hi 2 O 6 ), nonoses (C 9 Hi 8 O 9 ), the sugars of Ci 2 and C 18 which are 

 bisaccharides and trisaccharides and finally the polysaccharides 

 (starch, inulin, glycogen, etc.). It is known that the bisaccharides and 

 trisaccharides must be changed to hexoses in order to be fermentable. 

 Fermentation, then, consists in a molecular splitting, in the course 

 of which large molecules of sugar are changed into molecules which 

 are much simpler, the bi- or trisaccharides into hexoses, alcohol and 

 carbonic acid. 



Polysaccharides: Glycogenase, Amylase, Inulase: According to 

 the results of Wroblewsky, Cremer, Kohl and Hosaceus, and Geret 

 and Hahn yeast juice contains a hydrolytic enzyme for glycogen, 

 glycogenase. Yeasts do not act upon glycogen when it is given them 

 as food because the glycogenase is an intracellular enzyme, and 

 glycogen is not able to pass through the cell membrane. This glyco- 

 genase is able to act only upon the glycogen which is made by the 

 yeast itself on the interior of the cell. 



Starch, in order to be fermented, must be transformed into dex- 

 trine and maltose; then the dextrine itself is changed into maltose. 

 This is transformed by maltose into glucose l which is then fermented. 

 The exact mechanism of saccharification is not known. However, 

 according to the investigations of Maquenne and Roux starch is com- 

 posed of from 90 to 92 per cent of amylose and from 8 to 10 per cent 

 of amylopectin. The change of starch into maltose seems to demand 

 the action of three enzymes, amylase, amylopectinase and dextrinase. 

 The amylase changes amylose into maltose, the amylopectinase 

 changes amylopectines into dextrines and dextrinase changes dex- 

 trine to fermentable maltose. 



Some yeasts are able to ferment starch, as S. exiguus thermanti- 

 tonum, acetethylicus, Sch. Pombe, mellacei, octosporus, the yeast of 

 Logos and some yeasts of Saaz and Mycoderma sphaeromyces. 



Inulin differs from starch by its composition. Certain yeasts are 

 able to saccharify it and ferment it on account of an inulase which 

 produces levulose and not maltose, as from starch. This enzyme 

 has been encountereed in Sch. Pombe and mellacei, S. marxianus 

 and ihermantitonum, certain species of the type of Saaz, and the 

 yeasts E and F of Rose. 



Trisaccharides: Raffinase, Melibiase and Melizitase: Raffinose 

 or melitriose (CisH 3 20i 6 ) is capable of decomposition by certain yeasts. 



+ H 2 = CeH^Oe 

 Raffinose Levulose Melibiose 



By glucose we shall mean d-glucose or dextrose* 



