THE CHEMICAL CONSTITUENTS OF MILK 305 



Nitrogenous Matter. The nitrogenous matter of milk 

 consists entirely of compounds classed as albuminoids or 

 proteins. It includes, at least, three different types previously 

 described (Chap. VI.) as albumins, globulins, and phospho- 

 proteins. The lactalbumin and lactoglobulin are very similar 

 in composition and properties. They are distinguished chiefly 

 by the difference in solubility (p. 73). Together they form 

 about one-fifth of the total nitrogenous matter. The remain- 

 ing four-fifths consist of caseinogen, the calcium salt of a 

 phospho-protein (p. 74). 



The " spontaneous " curdling of milk is caused by the 

 action of lactic acid, which gradually accumulates in the milk 

 by the natural process of fermentation (pp. 85, 309). The acid 

 reacts with the calcium caseinate, and liberates the free 

 phosphoprotein as an insoluble curd. This curd may be 

 pressed and made into cheese, but its physical properties are 

 not very suitable, and its strongly acid character is not 

 favourable to the development of the more desirable kinds 

 of bacteria. Such sour milk cheese is probably the original - 

 it is certainly one of the oldest kinds of cheese but, for the 

 reasons mentioned above, it is now rarely or never made in this 

 country. 



The coagulum produced by the action of rennet (p. 331) is 

 of different consistency. It is not strongly acid, and forms 

 a suitable medium for the growth of many different kinds of 

 bacteria. It is the basal substance from which all the different 

 kinds of cheese (except certain cream cheeses) are now made. 



Fat. The illustration (Fig. 26) shows the appearance of a 

 drop of milk under the microscope. The innumerable globules 

 which ^are seen suspended in the serum consist of fat. It will 

 be noticed that they are spherical in shape and vary con- 

 siderably in size. The largest are about 2soo tn an d tne 

 smallest only about 16 Q 00 th of an inch in diameter. The 

 former are, therefore, more than six times the size of 

 the latter. It is evident also that the smallest globules are the 

 most numerous, and that the largest are of comparatively rare 

 occurrence, but there is no proof of any constant relation 

 between the size and the number of fat globules. It has been 



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