CARBOHYDR.\TES 1 19 



Lactose, Milk Sugar, C12H22O11 



Lactose or milk sugar. — The third disaccharose, as its name 

 signifies, is found in the milk of mammals. It is not found in 

 any plants. In milk, it is present to the extent of about 4-5 

 per cent. When both the fat and the protein have been sep- 

 arated from the milk, the remaining liquid on evaporation yields 

 lactose (see Experiment XXIX). This separates as a crystalline 

 substance, often in large crystal masses resembling cane sugar 

 crystals or rock candy. The crystals contain one molecule of 

 water. It also crystallizes with three molecules of water. It 

 is soluble in six parts of water, and is slightly sweet. Like 

 maltose, lactose reduces Fehling's solution, and both of these 

 sugars may be determined quantitatively by means of this 

 reagent, as in the case of glucose and fructose. It is optically 

 active, being dextro-rotatory. When hydrolyzed by enzymes or 

 boiling acids it yields one molecule of glucose and one of galactose, 

 both being also dextro-rotatory so that no inversion takes place. 

 It is not fermented hy zymase. With yeast alcoholic fermenta- 

 tion occurs slowly after the lactose is first hydrolyzed by another 

 enzyme of the yeast, probably lactase. 



Lactose differs from the other two disaccharoses in readily 

 undergoing a bacterial fermentation by which lactic acid is 

 produced. As explained in connection with lactic acid, lactose 

 is fermented by the lactic acid bacteria and inactive lactic acid, 

 or fermentation lactic acid, is obtained. These bacteria are 

 naturally present in milk, and after standing fermentation 

 takes place, the lactic acid is formed and the milk, as we say, 

 sours. The chemical reaction in the conversion of lactose into 

 lactic acid is first one of hydrolysis into glucose and galactose, 

 and then one of simplifying the molecule without hydrolysis, 

 oxidation or any other chemical reaction. 



C12H22O11 + H2O -> 2 C6H12O6 



Lactose Glucose and 



Galactose 



CeHisOs -> 2 C3H6O3 



Glucose Lactic acid 



