126 



THE ESTIMATION OF FAT. 



fat. In the first case, there is a tendency to be slightly low, 

 owing to the fact that the ether, which dissolves in the aqueous 

 portion, retains a minute proportion of fat ; in the other, the 

 tendency is to be somewhat high, as the water which dissolves 

 in the ether dissolves a small amount of substances other than 

 fat ; in either case, however, the error introduced is very small 

 and usually may be neglected. 



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 off 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. 125). 

 Smetham has devised an extractor on a principle similar 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. 13), 

 10 c.c. of alcohol, 1 c.c. of ammonia (sp. gr. 0-96), and 25 c.c. of 

 ther, and mixing ; on adding 25 c.c. of petroleum ether a layer 

 of about 50 c.c. separates. In the original method it is recom- 

 mended 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 



Fig. 13. 



Farnsteiner 



Tube. 



