28 CONSTITUENTS OF MILK 



are specific in their reactions and may be used as qualitative 

 tests for milk. Bauer succeeded in detecting as small a quan- 

 tity as 1 c.cm. of cows' milk per litre of human milk by the 

 complement fixation method. The various proteids of milk, 

 caseinogen and albumin, etc., also produce specific antibodies 

 which may be recognised by the precipitin method. The 

 specificity of lactoserum, like those of sera in general, is relative 

 rather than absolute, the lactosera of closely related animals 

 being differentiated by the intensity of the reactions. The 

 phenomenon of anaphylaxis may also be induced by the injec- 

 tion of milk. Arthus and Besredka state that boiled milk, as 

 well as the raw product, is capable of producing the requisite 

 conditions, though Miessner found that a larger number of 

 injections were necessary before sensitisation was satisfac- 

 torily established. Caseinogen and albumin also produce 

 specific anaphylactins which may be used as a basis for differ- 

 ential tests. 



PhysicaL The characteristic appearance of milk is pro- 

 duced by the colloidal suspension of caseinogen complex and 

 the emulsion of fat globules. When milk is allowed to remain 

 quiescent, the fat globules, being of smaller density, rise to the 

 surface and form a layer of cream which is distinctly yellowish 

 in tint, the residual milk being bluish white in colour. The 

 opacity is diminished by the addition of alkali, which dissolves 

 the caseinogen, and is increased by any process that reduces the 

 size of the fat globules. Heat alone, at different temperatures, 

 is capable of reducing the diameter of the fat globules, but it 

 may be more effectively accomplished by forcing milk heated 

 to 60 C. through very small orifices under high pressure. 



The specific gravity of milk bears a definite relation to the 

 total solids it contains (see p. 70), being decreased by the fat 

 content and increased by the solids other than fat. The specific 

 gravity or density varies considerably with variations in season, 

 period of lactation, breed, and character and quantity of food, 

 but 1026.4 to 1037.0 ( water if^-;= 1000 ) may be regarded as 

 the extreme limits. When milk, freshly drawn from the udder, 



