132 PHYSIOLOGICAL CHEMISTRY 



man milk about 6-7%. Milk fat, about 2-4% in both cow's and 

 human milk is characterized by its content (about 8% of the 

 total fats) of lower fatty acids, butyric, caproic, caprylic and 

 capric. There- also are traces of phosphatids. There are at 

 least three proteins, (0.7-1.5% in human milk and 2.5-4% in 

 cow's milk). Casein makes up about 80% of the total protein in 

 cow's milk, the remainder is mostly lactalbumin, with a small 

 amount of lactoglobulin. Milk also contains a variety of salts 

 including those of calcium, magnesium, sodium, potassium and 

 iron; it contains phosphates and chlorides. Milk contains about 

 87% water. 



It has recently been shown that milk is a valuable source of 

 vitamines. This will be discussed in the section dealing with 

 these substances. 



If milk is allowed to stand, bacteria decompose lactose form- 

 ing organic acids. The milk becomes disagreeable in taste, and if 

 the process goes far enough the casein is precipitated, and the 

 milk is said to be sour. By properly sterilizing or pasteurizing 

 milk, the bacteria are destroyed, and if it then is kept in closed 

 vessels and not exposed to the air it will keep for a much longer 

 time than otherwise. Half emptied milk bottles should on no 

 account be left uncovered in a room or in the family ice chest, 

 as bacteria from the air, falling into the milk, will thrive and 

 multiply, and the milk if used may thus become a menace to 

 health. 



When milk stands for some time a portion of the fat rises 

 to the surface. If this layer is skimmed off the liquid is called 

 cream. Average cream contains 18-20% fat. 



Buttermilk differs from sweet milk chiefly in its lower fat 

 content (about 0.5% fat), since most of the fat has been re- 

 moved as butter. As butter is usually made from old cream, 

 buttermilk often is sour, that is, contains some lactic acid pro- 

 duced by the fermentation of lactose. 



Butter. Butter is a food of very high fuel value, for it 

 contains 80% or more of fats. The composition of this fat 

 already has been discussed (see under fats). Butter contains 



