MONOSACCHAROSES 77 



(b) Milk Sugar or Lactose, C 12 H 22 O n + H 2 O, is 

 found only in the animal kingdom, being an important 

 constituent of the milk of various animals. It occurs in 

 whey obtained in cheesemaking, and is a crystalline com- 

 pound with only a faint sweet taste. Milk-sugar is not 

 so soluble in water as cane-sugar : it reduces Fehling's 

 solution. 



When treated with hot dilute acid it suffers hydrolysis, 

 and splits up into glucose and galactose thus : 



C 12 H 22 O n + H 2 = C 6 H 12 6 + C 6 H 12 O a 



glucose galactose 



The enzyme capable of effecting this change is termed 

 lactase. It is apparently possessed by many kinds of 

 bacteria. 



In the lactic fermentation it is probable that the first 

 action of the bacteria is a hydrolytic process, the two sugars 

 produced being subsequently converted into lactic acid : 

 the chemical changes in lactic fermentation, and the 

 bacteria to which they are due, are discussed at greater 

 length in Chap. XVII. 



Ex. 43. Dissolve some milk-sugar in water and 

 (i) Test its reducing power with Fehling's solution (Ex. 44). 

 (ii) Taste some of the sugar. 



(c) Malt- Sugar or Maltose is a compound with the 

 same empirical formula, C 12 H 22 O u ,'as the two preceding 

 sugars, and is produced by the action of diastase and 

 other enzymes upon starch. Hot dilute sulphuric acid 

 changes it into grape-sugar. Yeast is able to ferment it, 

 with the production of alcohol and carbon dioxide. 



(iii) Monosaccharoses. The most commonly occurring 

 monosaccharoses are glucose, or grape-sugar (formerly 



