200 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



LACTOSE. 



Lactose, or milk sugar, occurs in the milk of all animals, but has 

 not been found in plants ; about 4 per cent, is present in cow's milk, 

 from 6-8 per cent, in human milk. 



Lactose is prepared and manufactured from whey. The whey is 

 evaparated and the lactose crystallises out ; it is punned by recrystalli- 

 sation from water. 



Lactose forms a white crystalline powder, soluble in water, but in- 

 soluble in alcohol. In taste it is less sweet than glucose or cane sugar. 



Reactions. 



Lactose resembles glucose in its reactions. 



(i) Formation of Esters. Lactose forms esters with eight hydroxyl 

 groups. 



(4) Oxidation. On oxidation of lactose with nitric acid, a mixture 

 of mucic and saccharic acids is formed. 



(5) Action of Alkali. Lactose gives Moore's test. 



(6) Action of Hydrochloric Acid. Lactose is hydrolysed by boil- 

 ing with dilute acid into a mixture of glucose and galactose. Strong 



acid has the same effect 

 as upon glucose. 



(7) Reduction of Me- 

 tallic Hydroxides in Alki- 

 line Solution. The re- 

 ducing power of lactose 

 is less than that of glu- 

 cose. (See under esti- 

 mation.) After hydrolysis 

 by acids the mixture of 

 glucose and galactose has 

 a greater reducing power 

 than lactose. 



Lactose does not re- 

 duce Barfoed's reagent. 

 FIG. 4 o.-Lactosazone. * ^ Formation of Osa- 



zone. Lactose forms an osazone with phenylhydrazine in acetic acid 

 solution in the same way as glucose. Lactosazone is soluble in boil- 

 ing water ; the compound separates as the solution cools. Its crystal- 

 line form (Fig. 40) is different from that of glucose and it is thus most 

 easily distinguished from glucosazone and also maltosazone. 



* (10) Fermentation. Lactose is not fermented by yeast. 

 (n) Molisctis Test. Lactose gives Molisch's reaction. 



* (12) Rotation. Lactose is dextrorotatory. After hydrolysis by 

 acids into a mixture of glucose and galactose the rotatory power of 

 the solution is greater than before. 



