Composition of Milk and Some of its Products 1639 



Fat exists in milk in the form of small droplets, or globules, forming an 

 emulsion — that is, a liquid with the fat in a state of fine suspension; 

 the fat is not in solution. This condition is favorable for digestion. Each 

 globule is spherical in form and it keeps this form by reason of its power 

 to condense about it the liquid part of the milk. 



The fat globules are very tiny, being about 1 / 1 ,500 to 1 /2 5,000 of an inch 

 in diameter. A high-power microscope is required in order that they 

 may be seen, and some idea of their minuteness may be obtained when it 

 is stated that in one cubic centimeter of milk (about 22 drops) there are one 

 to two billion fat globules. 



The fat is lighter than the other constituents of milk, and when milk 

 is allowed to stand the fat globules rise to the surface, forming what is 

 called a "cream line." This layer of cream can usually be seen distinct 

 from the rest of the milk. The presence of a thick cream line indicates 

 that the milk is rich in fat. The absence of a cream line is not, however, 

 a positive proof that there is a small percentage of fat in the milk, although 

 it indicates that such is the case. The writer has sometimes found milk test- 

 ing 4.2 per cent fat which showed almost no cream line. Just why the 

 fat does not always rise readily is not fully understood. 



Milk fat is not one fat alone, but is made up of about nine different 

 fats. These all have different properties and the character of the butter 

 is influenced by the kinds of fat of which it is composed. One of the fats 

 in milk is called " olein " and another is called " stearin." Olein has a 

 melting-point of about 43 ° F. and stearin a melting-point of about 143 F. 

 The melting-points of the other fats lie between these limits. The melting- 

 point of butter is therefore variable according to the amount of the different 

 fats present, and is between 92 and 96 F. One of the fats in milk, called 

 " butyrin," is the characteristic fat of butter because it imparts the flavor 

 and odor that are recognized as butter flavor and butter odor. 



All the fats in milk fat have the same basis, which is glycerin. They are 

 formed by the union of glycerin with the corresponding fatty acid. For 

 example, butyrin is made up of glycerin and butyric acid; olein is composed 

 of glycerin and oleic acid. Nearly every one has had experience with 

 strong, or rancid, butter. Such rancidity is due to the splitting up of 

 butyrin into butyric acid and glycerin. The butyric acid produces the 

 strong, disagreeable flavor and odor. It is a strange fact that no chemist 

 can make in the laboratory any of the fats of milk fat. The cow alone 

 can do this. A chemist might be given glycerin and butyric acid, with all 

 laboratory facilities, but he could not make butyrin. 



Fat is the most variable constituent of milk. There are several con- 

 ditions under which it varies, the reasons being not always understood. 

 Some of the reasons are as follows: 



