x] TESTING OF MILK 159 



difference in the conditions under which the tests 

 were made. 



2. Optical Method of Determination of Fat. 



A variety of instruments have been devised for 

 applying this test. It is based on the fact that the 

 opacity of milk is dependent on the milk-fat globules. 

 As, however, this is only partly the case, and as the 

 same weight of fat retards more light when it is in 

 the form of small globules than when it is in the 

 form of larger globules, this method of testing is 

 very unreliable. 



3. Determination of the Fat by Churning. 

 Apparatus has also been devised for the determina- 

 tion of the fat in milk by the amount of butter it 

 yields on churning. This method, since it involves 

 the use of very expensive apparatus, is not suited for 

 general use. 



4. Determination of the Fat by the Addition 

 of Reagents. Of the various methods for the deter- 

 mination of the fat by the addition of reagents, 

 Soxhlet's is the best known. This method consists 

 in dissolving the fat from a measured quantity of milk, 

 to which some potash or soda has been previously 

 added, by ether, and calculating its amount from the 

 quantity of fat dissolved in the layer of ether which 

 separates out, from the specific gravity of the latter. 

 The apparatus . by which this test is carried out is 

 known as the ' areometer' 



/^eSE LIBR^\ 



f OF THE r \ 



(UNIVERSITY 



V^C 



