246 MILK ! ITS ORIGIN AND COMPOSITION 



separated milk if the heated milk is passed through the 

 separator. 



Little change, if any, occurs in the chemical constitu- 

 tion of the fats, and cream can be heated to a higher 

 temperature than milk before the cooked flavour is pro- 

 duced. The small groups or clusters of fat globules 

 which are found in fresh milk are largely disintegrated 

 into separate globules by heating, and the cream, which 

 rises slowly, is thin. 



The milk sugar is slightly oxidized, and acids are 

 split off from it, the alkaline phosphates of the serum 

 assisting the change ; the milk becomes distinctly acid 

 if it is boiled for three or four hours, and yellowish or 

 brownish in colour, the altered tint, however, being more 

 likely due to alteration in the proteins than in the 

 sugar. 



Very striking changes are made in the proteins of 

 the milk by heating. The soluble albumin becomes 

 coagulated and probably decomposed to some extent 

 with the evolution of a small quantity of sulphuretted 

 hydrogen at temperatures from 70 C. to 1 00 C. The 

 pellicle or skin which forms when milk is heated in an 

 open vessel consists of coagulated albumin and dried 

 caseinogen, with fat and other constituents also. 



The power of curdling by rennet .and acids is very 

 much reduced, a point of great importance to the cheese- 

 maker. For the coagulation of the caseinogen more 

 rennet is required and the curd is too crumbly and soft 

 for the manufacture of hard pressed cheeses. The altered 

 character of the caseinogen appears to be chiefly con- 

 nected with the precipitation of insoluble calcium phos- 

 phate, and although some improvement in the curdling 

 property can be made by the addition of dissolving agents 



