GENERAL CHEMISTRY OF MILK 85 



than that of cow's milk, as it forms very fine flakes and usually 

 leaves the serum opalescent. 



Ash-free casein is prepared according to Van Slyke and Bos- 

 worth as follows: "Separator-skimmed milk is diluted with seven 

 or eight times its volume of distilled water and diluted acetic 

 acid (6 c.c. glacial acetic acid to 1000 c.c. distilled water) care- 

 fully added until the casein has completely separated. After the 

 precipitate has settled the clear supernatant fluid is removed by 

 siphon. Distilled water is then added, the mixture stirred 

 vigorously, and the precipitate allowed to settle, after which the 

 wash-water is siphoned off. Water is again added and the 

 casein dissolved by adding for each hter of milk used 1 liter dilute 

 NH4OH (6 c.c. strong reagent diluted to 1 hter). When the solu- 

 tion is complete, the whole is filtered through a thick layer of ab- 

 sorbent cotton. The casein is again precipitated with dilute 

 acetic acid, the precipitate allowed to settle and then washed, 

 redissolved in dilute NH4OH, and filtered. This process of pre- 

 cipitation, washing, dissolving, etc., is repeated not less than four 

 times. Finally, an excess of strong NH4OH (10 c.c.) is added, 

 and then 20 c.c. of a saturated solution of ammonium oxalate. 

 The mixture is allowed to stand for twelve hours or more. Cal- 

 cium is precipitated as oxalate in a very finely divided condition, 

 too fine to permit its satisfactory removal by ordinary methods 

 of filtration. Better aggregation of the precipitate can be effected 

 by means of centrifugal force. The centrifuged mixture is filtered 

 through double thickness filter-paper. The filtered solution is 

 next treated with dilute HCl (10 c.c. HCl, specific gravity 1.20, 

 diluted to 1 liter) until the casein is precipitated. The precipitate 

 is washed with distilled water free from chlorids and is then placed 

 on hardened filter-paper in a Buchner funnel, as much water as 

 possible being removed by suction. The mass is next transferred 

 to a large mortar and thoroughly triturated with 95 per cent, 

 alcohol. The alcohol is removed by suction in a Buchner funnel 

 and the casein is again placed in a mortar and triturated with 

 absolute alcohol. Most of the alcohol is removed and the casein 

 treated twice with ether in a mortar by trituration, the ether 

 being removed each time by means of suction on a Buchner funnel. 

 The material is finally ground in a mortar until the particles pass 

 a 40-mesh sieve and is dried for two days over H2SO4 in a desic- 

 cator under diminished pressure." 



"Three lots prepared by this method contained the following 

 amounts of ash: 0.10, 0.09, and 0.06. The casein was insoluble 

 in water and 50 per cent, alcohol. One of the lots was shghtly 

 soluble in 5 per cent. NaCl solution, the other lots were not. 

 One gram of the casein treated with 10 c.c. N. 0.1, NH4OH, NaOH, 



