i MILK 3 



When milk is heated, the only apparent change is the 

 formation of a skin on the surface when the temperature 

 rises above 50 C. This skin consists of casein, which, in 

 consequence of the rapid evaporation, has lost a portion of the 

 water which it contains ; on stirring the milk the skin again 

 dissolves. 



Milk is naturally intended for the nourishment of newly-born 

 animals, but from the earliest times man has understood by 

 various means how to prolong the period of milk production 

 with some domestic animals such as the cow, goat, and sheep. 

 Thus it is that now the production of milk is not limited to 

 the time of suckling the young, but continues much longer, often, 

 in fact, until shortly before the birth of the next calf, kid, &c. 



If milk is to serve exclusively for nourishing the young, it 

 must, of course, contain all the nutrients required by the 

 animal organism for its maintenance and growth, and these are 

 protein, fat, carbohydrates, and inorganic salts. In addition, 

 milk also contains small quantities of various substances which, 

 although they possess no direct feeding value, have neverthe- 

 less an important physiological effect. The milk of all species 

 of animals contains the whole of the constituents mentioned 

 above, but the quantities in which they are present vary in 

 different cases. 



Cow's milk contains almost nine-tenths of its weight of 

 water ; the other components, which may be got by evaporating 

 off the water and are known together as " dry matter " or " total 

 solids," represent slightly more than one-tenth the weight 

 of normal milk. According to Fleischmann, 1 the average 

 composition of milk, based upon a large number of investiga- 

 tions, is : 



Average numbers. Limits of variation. 



Water 87'75 per cent. 86'5 89'5 per cent. 



Fat 3-40 27 4*3 



Protein 3*50 3'0 4'0 



Milk sugar 4'60 3'6 5'5 



Mineral substances... 0'75 0'6 0'9 



100-00 per cent. 100 '00 per cent. 



The above figures apply to mixed milk from a large number 

 of cows ; with single animals the limits of variation are often 

 considerably greater. 



1 Lehrbuch der Milchwirtschaft, Fourth Edition. Leipsic, 1907. 



B 2 



