CHAPTER VII. 

 THE CHEMISTRY OF SOME FOODS. 



A. Milk. 



The composition of milk differs considerably in different 

 animals. The percentage composition of average samples 

 of human and cow's milk is as follows : 



Protein. Fat. Lactose. Salts. 

 Human.. .. i '2 27 6*5 o'2 



Cow's .. .. 3*4 4' 4*5 *7 



Other differences are that in cow's milk the proportion of 

 casein to lactalbumin is about 6 to i , compared with 2 to i 

 in human milk. 



Casein, the chief protein of milk, is a phospho-protein. 

 Like all proteins? it is an ampholyte, but it differs from the 

 majority of proteins in having marked acid characters. 

 The iso-electric point of casein (see pages 1 1 and 32) is 

 about P H = 4*6. At this reaction it has its minimum 

 solubility. In solutions alkaline to this it forms salts with 

 bases ; in solutions acid to this it forms salts with acids. 

 These salts are more or less soluble in water. So it can 

 be stated that casein is insoluble in water and dilute acids, 

 but soluble in alkalies and strong acids. Since the reaction 

 of milk is about P H = 7 it follows that the casein is held in 

 solution as a salt with a base. The base is probably 

 calcium, though it is possible that a complex with a phos- 

 phate is the condition in which the casein exists naturally 

 in untreated milk. 



Casein seems to have a molecular weight of about 

 8900. It is readily hydrolysed by alkalies and by proteo- 

 lytic enzymes into two molecules of paracasein, which has a 



