52 MILK AND DAIRY PRODUCTS PART 



4. Rose- Gottlieb's Method. 



This method, which is now generally known as Gottlieb's 

 method, is a modification by E. Gottlieb l of the process devised 

 by B. Rose 2 for determining the percentage of fat in milk. 

 The principle of the method is as follows : A certain 

 quantity of milk is shaken with definite quantities of ammonia 

 and alcohol whereby the casein dissolves. Then a known 

 volume of ether and petroleum ether is added, and the whole 

 well shaken in order to bring all the fat into solution. If the 

 mixture is then allowed to stand in a measuring cylinder, a 

 clear ethereal layer separates out, whilst below is an opaque 

 liquid in which the other constituents of the milk, mostly 

 dissolved in dilute alcohol, are contained. After the whole has 

 stood for some time, the volume of ether is read, and an 

 aliquot portion drawn off by means of a syphon into a tared 

 flask, the ether-benzine evaporated off, and the fat which 

 remains then weighed. The presence of benzine (petroleum- 

 ether) is essential, for it prevents the solution of non-fatty 

 substances which otherwise would dissolve in ether as the 

 fat does. 



In its present form the method is carried out as follows : 

 Ten c.c. of milk, which is almost exactly equivalent to 10'27 g., 

 are measured from a pipette into a tall, narrow cylinder, 

 which is graduated to 0'5 c.c. and holds 100 c.c. Then 

 1 c.c. of ammonia (sp. gr. 0'96) is added from a burette, and 

 also, with the help of a pipette, 10 c.c. of 95 per cent, alcohol. 

 The cylinder is closed with a good cork, and then well shaken 

 and frequently inverted. In this way the casein dissolves, and 

 a thick homogeneous fluid is left, to which 25 c.c. of pure ether, 

 not necessarily dry, are added from a burette or pipette. The 

 cork is then replaced, and the shaking and reversing of the 

 cylinder repeated to bring all the fat into solution. Finally, 

 25 c.c. of pure petroleum ether are added and the mixing 

 once more thoroughly done. It should not be forgotten that 

 if the cylinder is not well inverted each time the results will be 

 too low. The petroleum ether must be completely volatile at 



1 Die Landiv. Versuchsstation, Vol. XL, p. 1. 



2 Zeitschrift fur angew. Chemie, 1888, p. 4. 



