116 - ANALYSIS OF MILK. 



for anotter ttree hours ; the quantity obtained in the second 

 extraction is very small. 



The ether is then evaporated and the fat dried at 100° C. in 

 a tared flask ; after weighing, the fat should be dissolved in a 

 little petroleum ether, when any kieselguhr which may have run 

 through will be detected ; this should not be the case, if the 

 plug of cotton wool was properly packed. 



Other media, such as kaolin, plaster of Paris, etc., may be 

 substituted for pumice or kieselguhr, and, so long as the essential 

 point of the method, fine grinding, is adhered to, the results are 

 independent of the medium. Kieselguhr is, however, the most 

 convenient. 



This method may be used with homogenised milk. 



The Werner - Schmid. Method. — This method differs from 

 most others, in that the milk is not reduced to a sohd state by 

 evaporation previous to the extraction of the fat by ether. In 

 order to render the casein, which hinders the extraction of the 

 fat from milk, soluble, Werner-Schmid heated the milk with an 

 equal bulk of hydrochloric acid till the fat floated in a nearly 

 clear layer at the top, shook the resulting solution with ether, 

 and drew off an aliquot portion of the ethereal layer. 



Stokes has devoted much attention to this method, and has 

 studied the effect of slight modifications. 



Werner-Schmid's directions are : — Take a test tube of about 

 50 c.c. capacity, graduated in tenths of a c.c, and introduce 

 10 c.c. of milk ; add 10 c.c. of hydrochloric acid, boil, with shakingj 

 until the liquid turns dark-brown, and cool by placing the tube 

 in cold water ; add 30 c.c. of ether, shake round and let stand ; 

 then measure the volume of the ethereal solution, draw off 

 10 c.c. with a pipette, evaporate the ether, and dry the fat at 

 100° C. 



This method has been thoroughly examined by Stokes, who 

 prefers to draw off 20 c.c. of the ethereal solution. 



T. E. Hill has also examined the method, and considers that 

 the milk should be weighed, not measured. Hill notices that a 

 fluffy-looking stratum is formed beneath the ether, and, following 

 Stokes, adds three-fourths of this to the bulk of the ethereal 

 layer for calculating purposes. He also points out that Werner- 

 Schmid's method is not applicable to the determination of fat 

 in milk to which cane sugar has been added, a conclusion con- 

 firmed later by Dyer and Eoberts, who showed that by boihng 

 cane sugar with hydrochloric acid a substance soluble in the 

 water taken up by the ether was formed. 



Stokes has pointed out that this can be got rid of by extracting 

 the dried residue with dry ether, in which the caramel formed 

 from the sugar is insoluble. Alien recommends petroleum ether. 



