CHEMICAL COMPOSITION 



343 



It is dextrorotatory and reduces Fehling's solution, although 

 to a less extent than dextrose. It is hydrolyzed by enzymes 

 and acids to dextrose and galactose. It does not undergo 

 ordinary alcoholic fermentation except under the influence 

 of certain yeasts. The principal change in lactose is due 

 to the so-called lactic bacteria which hvdrolvze it to lactic 

 acid, thus: 



C 12 H 2 20n + H2O = 4C 3 H 6 03. 



It is this lactic acid to which is due the taste of sour milk, 

 and it is produced so quickly that ordinary milk contains 

 about 0.2 per cent, of lactic acid. The taste is apparent 

 when the proportion rises to 0.4 per cent., and at 0.7 per cent, 

 milk "curdles." Curdling is due to the coagulation of case- 

 inogen (see below). The percentage of lactic acid rarely 

 rises above 2 because in that amount the action of the 

 lactic acid bacteria is inhibited. Lactose is only one-tenth 

 as sweet as sucrose. 



(b) Fat in milk is, of course, like other fixed oils 

 in that it is composed of glycerides of fatty acids. Milk 

 fat differs from other animal fat in that it is composed of a 

 number of lower fatty acids, there having been found the 

 following: 



