ELEMENTARY CHEMICAL PHYSIOLOGY 217 



such as beer and ale, are obtained. In this process the maltose is first of all 

 inverted into two molecules of dextrose by the invertase contained in the 

 yeast, and the dextrose then undergoes alcoholic fermentation. 



It reduces metallic oxides in alkaline solution, but is feebler in this 

 regard than dextrose. It rotates the plane of polarised light more 

 strongly than dextrose ( (a)D = -f 138). After hydrolysis, there- 

 fore, the reducing powr shows an increase and the rotatory power 

 a decrease. 



EXPERIMENT. Boil lactose or maltose solution with Barfoed's 

 reagent. There is no reduction. This reagent is not reduced by 

 disaccharides. 



Isomaltose. This sugar is closely related to maltose, differing from it in the 

 fact that its osazone melts at a much lower temperature, 158 C. It has been 

 prepared by pure chemical synthesis e.g. the condensation of dextrose by 

 strong acids. It is of special interest because it is probably the sugar produced 

 as a result of the reversible action of maltase. 



CHAPTER VI 



CARBOHYDRATES Continued 

 III. Polysaccharides. 



A polysaccharide is the condensation product of more than two 

 monosaccharide molecules, and has accordingly the general formula 

 (C 6 H 10 5 ) n , where n stands for a variable number. 1 Polysaccharides 

 can be hydrolysed, in which process they yield, first of all, poly- 

 saccharides (dextrines) of lower molecular weight (i.e. with n of 

 less value), then disaccharides and, finally, monosaccharides. 



Thus, when acted on by diastatic ferments, dextrines (poly- 

 saccharides of lower molecular weight) and maltose (disaccharide) 

 are formed. When boiled with acid, on the other hand, the hydro- 

 lytic cleavage goes further, and, although dextrine and maltose 

 occur as intermediate products, the final product is monosaccharide. 



The most important members of this group are the starches, the 

 dextrines, glycogen, the celluloses, and the gums. They are very 

 widely distributed in vegetables, and constitute a most important 

 class of food-stuffs. 



General Characters. -They do not form crystals, nor, with few 

 exceptions, are they soluble in cold water. Few possess any sweet 

 taste. As a rule they do not diffuse through parchment and are 

 therefore colloids. Their solutions are optically active. They do 

 not reduce metallic oxides in alkaline solution, they do not form 

 osazones and they cannot be fermented with yeast. Like other 

 colloids, they are precipitated when their solutions are saturated 



1 It is impossible to give a definite value to n because the molecular weight is 

 unknown. The symbol n signifies that the formula within the brackets is to be 

 multiplied an indefinite number of times. 



