354 THE ANALYSIS OF MILK AND MILK PRODUCTS. XV. 



Another method of precipitation of casein is the following*: 

 Ten grammes of the milk, which must not be curdled, are 

 diluted to 100 c.c. with water and raised to 40. The casein 

 is then precipitated by adding 1-5 c.c. of a 10% solution of 

 acetic acid. The whole is well stirred, allowed to stand for a 

 short time, and the precipitated casein washed three or four 

 times with cold water. The nitrogen in the precipitate is then 

 determined as before. 



Determination of Milk Sugar. This can be done either 

 by the well-known Fehling method or by the use of the polari- 

 meter. In either case previous removal of the fat and proteids 

 from the milk is necessary. If the Fehling method is to be 

 employed, this can readily be done by the method used by van 

 Slyke above described, but in addition boiling the solution and 

 filtering. For polarimetric examination an acid solution of 

 mercuric nitrate may be employed to precipitate the proteids. f 

 It is prepared by dissolving mercury in twice its weight of 

 strong nitric acid and diluting the solution with an equal 

 volume of water. It will serve to clarify fifty times its volume 

 of milk. 



Many methods of conducting the determination of lactose 

 are in use. For details a manual on quantitative analysis 

 should be consulted. One method of performing the Fehling 

 test may be briefly described here. 



Fehling' s solution is best prepared when required by mixing 

 equal volumes of the following solutions : 



(1) A solution of 34-64 grammes of pure crystallized copper 



sulphate in 500 c.c. of water. 



(2) A solution of 173 grammes of sodium potassium tartrate 



(" Rochelle salt") and 51 grammes of sodium hydrate 



in 500 c.c. of water. 



The filtrate from the precipitated proteids is so diluted that 

 it occupies exactly 10 times the volume of the milk taken. 

 50 c.c. of this filtrate are then taken, heated in a water bath, 

 and mixed with a previously boiled mixture of 30 c.c. of the 

 above copper sulphate solution, 30 c.c. of the alkaline tartrate 



* Van Slyke, Jour. American Chem. Soc. 15, 644. 

 t Wiley, American Chemical Journal, 6, 289. 



