EXAMINATION OF MILK 

 FOR PUBLIC HEALTH PURPOSES 



CHAPTER I 

 CONSTITUENTS OF MILK 



MILK is the opaque white fluid which is secreted by the 

 mammary glands. It consists essentially of an emulsion of 

 fat and a colloidal solution of caseinogen in water containing 

 lactose and traces of mineral matter. 



Milk fat, with which is associated small quantities of 

 cholesterol, lecithin, and a trace of colouring matter, consists 

 of a mixture of triglycerides of various fatty acids. These 

 acids are mixtures of the straight chain series C n H 2n +iCOOH 

 and C n H 2 n-iCOOH, the less saturated acids being, accord- 

 ing to the best information, entirely absent. The relative 

 proportions of the various acids are by no means constant, being 

 dependent upon various factors such as foodstuffs, seasonal 

 variations, breed of cattle, and climatic conditions. 



The fat is present in milk as enormous numbers of very 

 small globules and it is the reflection of light from these par- 

 ticles and those of caseinogen that produces the character- 

 istic white opaque appearance of milk. Although it was for- 

 merly held that the fat globules were surrounded by albuminous 

 membranes which preserved the form, it is now generally 

 accepted that this is due to surface tension and that the size 

 of the globules can be altered by physical methods. 



The size of the fat globules in milk varies from 0.8/i to 20/i 

 with an average of about 2.7/z and the number of globules 

 from 19X10 8 to 60X10 8 per cubic centimeter. Although no 



