THE CHEMICAL CONSTITUENTS OF MILK 309 



or " milk-sugar." It belongs to the dihexose group (p. 20), 

 and is represented by the formula C 12 H 2 2O n . It occurs in 

 the milk of all animals, and nowhere else in nature. Cow's 

 milk contains from 3 to 6 per cent. The average quantity is 

 about 4*5 per cent. 



Compared with cane sugar, it is not so sweet or so readily 

 soluble in water. It is more easily oxidised, and it reduces 

 Fehling's solution. On hydrolysis (p. 21) it yields dextrose 

 and galactose. Milk sugar is very easily caramelised. On 

 prolonged boiling milk acquires a brownish colour, owing to 

 the formation of lactocaramel. Traces of this substance are 

 produced when milk is heated to the boiling-point, even for a 

 short time, and impart the peculiar, disagreeable (cooked) 

 flavour to the milk. 



Fermentation. Apart from the interest it possesses as a 

 nutrient, the sugar of milk is chiefly of importance on account 

 of the readiness with which it undergoes fermentative changes. 

 It can undergo direct alcoholic, and probably also lactic fer- 

 mentation, but usually, in milk, it is first hydrolised. 



C 12 H 22 O n + H 2 -> C 6 H 12 6 + C 6 H 12 6 



(Lactose.) (Dextrose.) (Galactose.) 



The hexose sugars are subsequently converted into alcohol 

 or into lactic acid, according to the type of fermentation. In 

 the latter case the change may be represented by the following 

 equation : 



C 6 H 12 6 ->2C 3 H 6 3 



The acid produced by the fermentation of sugar is called 

 by chemists a hydroxy-propionic acid, to distinguish it from an 

 isomeric compound called /? hydroxy-propionic acid. They 

 are often called a lactic acid and j8 lactic acid respectively, 

 but only the former occurs in milk. The difference between 

 the two compounds is shown by the formulae : 



CH 3 CH(OH) CO.OH CH 2 (OH)CH 2 CO.OH 



(Lactic acid.) 03 Hydroxy-propionic acid.) 



Under the most favourable conditions not more than 90 



