MILK PRODUCTS 



323 



follows that the separated milk must contain a smaller per- 

 centage of fat. Separated milk is also perfectly sweet, whereas 

 that which remains after shallow setting is often appreciably 

 acid. Deep-set skim milk should also be sweet, or only very 

 faintly acid. The composition of these products also depends 

 upon that of the original milk. If the same milk were treated 

 in each case, the composition of the products would be 

 approximately as follows : 



It will be seen that the only difference of any importance 

 is in the percentage of fat which remains. For practical 

 purposes the percentage of non-fatty solids may be regarded 

 as the same as in the original milk. 



Churning. The fat globules exist in milk in a state of 

 superfusion (p. 307), and do not readily coalesce, owing either 

 to their great surface tension, separation by films of water, or 

 to both of these causes combined. The latent energy of the 

 liquid globules is discharged by violent agitation, causing a 

 rise of temperature, and the fat solidifies. Solidification of the 

 fat can also be effected by cooling the milk to near its freezing- 

 point. On solidification the fat globules lose their spherical 

 form and become more or less angular. This appearance has 

 been observed both in milk that has been frozen and in that 

 which has been churned for some time. Further agitation 

 causes the solidified globules to cohere together and form the 

 " grains " of butter. 



