118 



ANALYSIS OF MILK. 



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Stokes has devised a modification of this method, by which 

 the total solid residue is treated with hydrochloric acid, and the 

 fat and total solids estimated in one portion of milk. 



This method is eminently adapted for the estimation of fat in 

 sour milk. A weighed portion of the well-mixed milk should be 

 placed in a graduated tube, and diluted with an equal bulk of 

 water ; a quantity of hydrochloric acid, slightly greater than the 

 total volume of the diluted milk, should be added, and the whole 

 boiled till clear. After cooling, ether should be added, the tube 

 shaken, and the ether allowed to settle for ten minutes. Before 

 taking out the stopper of the tube, it is an advantage to cool the 

 upper portion of the tube to as low a temperature as possible, so 

 that any ether which may have collected round 

 the stopper may be drawn inwards. An aliquot 

 portion of the ether should be drawn oS, and 

 evaporated and the fat weighed. Owing to the 

 presence of lactic acid in sour milk, which is 

 soluble in ether, and gives a non-volatile lactone- 

 on heating, the results have a tendency to be 

 slightly high. For this reason also, the whole 

 of the fat should not be extracted by repeated 

 shaking with ether, as a greater amount of lactic 

 acid is thereby extracted ; the ethereal layer may 

 be, however, washed with water to remove this 

 (see ante, p. 117). 



Smetham has devised an extractor on a similar 

 principle to Soxhlet's for extracting liquids with 

 ether ; the fat may be removed in this apparatus 

 after boiling with hydrochloric acid. 



Gottlieb's Method. — This method consists 

 in adding to 10 c.c. of milk in a special tube 

 devised by Farnsteiner (Fig. 10), 10 c.c. of 

 alcohol, 1 c.c. of ammonia (sp. gr. 0'96), and 

 25 c.c. of ether, and mixing; on adding 25 c.c. 

 of petroleum ethei a layer of about 50 c.c. sepa- 

 rates. In the original method it is recommended 

 that an aliquot portion be evaporated and the fat 

 weighed, but the author finds that the layer is not homogeneous, 

 and it is advisable to draw off as much as possible, and add 

 further quantities of ether and petroleum ether to extract the 

 whole of the fat. By mixing and allowing to separate several 

 tames a homogeneous layer is obtained, but this procedure renders 

 the method a slow one. 



Popp has shown that the strength of the ammonia used is 

 immaterial, and that no saponification of the fat occurs. 

 The author finds the following modification to work well : 



Fig. 10. 



Farnsteiner 



Tube. 



