88 THE MILK QUESTION 



higher than this, say 148° F., or for a longer time, the 

 cream layer will blend with the milk below. If the milk is 

 heated at still higher temperatures or for a very long time 

 the cream scarcely rises at all. 



It is very easy to destroy the cream line if milk is shaken 

 or roughly handled : even if milk is poured from one can to 

 another three or four times it tends to destroy the sharp 

 cream line. 



The emulsion of the fat in milk is peculiar in more than 

 one particular. Ordinarily when a fat or an oil is mixed 

 with water, the more it is shaken the finer and finer become 

 the individual droplets and the more permanent is the 

 emulsion. The opposite is the case with milk, for as soon as 

 it is shaken, the fat droplets coalesce into butter. One rea- 

 son for this is believed to be the fact that each fat droplet in 

 milk is surrounded by an albuminous envelope. So long as 

 this envelope is intact, the emulsion is secure, but as soon 

 as it is broken by shaking, or softened and dissolved during 

 the process of souring, the fat droplets readily run together 

 and destroy the emulsion. For this reason it is much easier 

 to make butter from cream that is "ripe," or partly fer- 

 mented. 



In normal milk the larger proportion of fat droplets 

 group themselves together into tiny clusters or masses. 

 This is known as the phenomenon of agglutination. At a 

 temperature of 65° C. or above, these clusters are broken 

 up and the globules are more homogeneously distributed 

 throughout the liquid. This is one of the tests by which 

 heated milk may be distinguished from raw milk. 



When milk is subjected to a pressure of about three 

 thousand pounds to the square inch and a temperature 

 of about 75° C, the individual fat globules are broken up 

 into exceedingly fine particles which remain as a uniform 

 and permanent emulsion. This is known as " homogenized " 

 milk. When this process is applied to cream it increases 

 its viscosity, so that cream containing twenty per cent of 



