82 MILK 



4. Unsaturated acids are diminished by the formation of 

 hydroxyacids. 



The results of rancidity in milk are somewhat different from 

 those in butter, inasmuch as the milk contains a larger amount 

 of water than butter, and soluble products are dissolved. In con- 

 tact with the air some volatile products escape. 



The consistency of the fat changes when it becomes rancid. 

 The texture is more hke tallow and the taste is bitter. 



The exact chemical processes which occiu' when fat becomes 

 rancid are not thoroughly understood and probably are not 

 always the same. 



The Chemistry of Casein 



Casein is not a clearly defined chemical substance, but probably 

 consists of several compounds. It is a weak acid and occurs in 

 milk in combination with calcium as calcium caseinate, and per- 

 haps contains some calcium phosphate. Casein is an important 

 constituent of all kinds of milk, but, although casein from dif- 

 ferent sources has important properties in common, there are 

 also material differences. Therefore it would be more proper to 

 speak of "caseins" rather than of "casein" as a substance of uni- 

 form composition. The complex constitution of casein is illus- 

 trated when milk is passing through a porous filter. Citrates 

 and chlorids are removed by a filter and casein is split so that 

 part of it passes into the filtrate as a soluble protein. The whey, 

 therefore, contains more soluble protein than the milk. Further- 

 more, precipitation by salts used for protein precipitation gives 

 results which are similar to those obtained when the proteins of 

 blood-serum are precipitated. That is to say, the precipitated 

 proteins vary according to the kind of salt employed and accord- 

 ing to the concentration of the solution. 



Casein obtained from the milk of one species has a fairly defi- 

 nite composition, but the caseins from different animals are not 

 identical. 



Casein occurs in milk in a colloidal condition. This is proved 

 by the following facts: The colloidal particles have been clearly 

 seen under the ultramicroscope. Furthermore, casein is thrown 

 into the bowl sediment in centrifugal machines, so that after sev- 

 eral hours of centrifugation nearly all the casein can be removed. 

 Finally, casein is withheld by porcelain filters. 



Halliburton suggested the name caseinogen for casein as it 

 occurs in milk, and casein after it has been precipitated. The 

 term "caseinogen" is not generally accepted; instead, casein is 

 used to designate casein as it exists in milk, and paracasein the 

 precipitate which is formed by rennet coagulation. 



