PHYSIOLOGICAL CHEMISTRY 287 



Lactose (C 12 H 22 O n ) is the sugar found in milk, and it has been 

 detected in the urine of nursing mothers. 



It is not very soluble in water, and is quite insoluble in alcohol and 

 ether. It has only a slightly sweet taste. It does not ferment with 

 yeast in 24 hours, but it undergoes a special fermentation with the 

 bacillus acidi lactici which develops in sour milk. This fermentation 

 results in the production of lactic acid. 



OH. 



C 12 H 22 O n + H 2 = 4CH 3 - OH/ 



COOH. 



Lactose. Lactic acid. 



By hydrolysis it yields dextrose and galactose. It reduces metallic 

 oxides in alkaline solution. It is dextro-rotatory ((a)D = 52'5). By 

 oxidation with nitric acid it yields mucic acid (cf. p. 278). 



Maltose (C 12 H 22 O 11 ) is important physiologically because it is the 

 sugar produced from starch by the action of ptyalin (the ferment of 

 the saliva), of amylopsin (a ferment in the pancreatic juice) and of 

 glycogenase (a ferment in the liver, blood serum, etc.). Maltose is 

 therefore mainly an intermediary substance in the animal body. 



Maltose is also produced by the action of malt diastase, which is 

 obtained by moistening barley and allowing it to germinate in heaps at 

 a constant temperature. The diastase acts on the starch of the grain 

 and produces maltose The product when dried is called malt. When 

 malt is dissolved in water, and the yeast plant allowed to grow on the 

 solution, malted liquors, 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 = -I- 137 -04). After hydrolysis, therefore, 

 the reducing power shows an increase and the rotatory power a 

 decrease. 



EXPERIMENT VIII. Boil lactose or maltose solution with Barfoed's 

 reagent (Experiment VI. p. 275.) 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. 



