84 MILK 



is selected. If several flasks yield water-clear filtrates, the fil- 

 trates should be boiled, filtered again, and tested with potassium 

 ferrocyanid and acetic acid. The flask which shows the smallest 

 turbidity is selected, and the quantity of acetic acid necessary to 

 precipitate the casein from the skimmed milk is calculated on the 

 basis of the amount used in this flask. The casein is precipitated 

 and allowed to settle, the supernatant fluid is decanted, and the 

 precipitate washed with water. The precipitate is gathered on 

 filters, pressed between cloth, and dissolved in 0.01 N.NaOH. 

 After solution the casein is precipitated again with acetic acid and 

 washed with water until HCl-phosphotungstic acid produces but 

 a slight turbidity. A small precipitate will always form with the 

 addition of HCl-phosphotungstic acid, since water decomposes 

 casein slowly and takes up part of it. The precipitate is then 

 redissolved and reprecipitated twice. Finally, the gathered pre- 

 cipitate is washed with alcohol and ether to remove water and fat 

 and then desiccated in vacuo. 



Richmond gives the following method of preparing casein: 

 "Milk is diluted with water to. about five times its volume and 

 sufficient acetic acid added to make 0.1 per cent, of the mixture. 

 The fat is carried down with the casein. The precipitate is well 

 washed by decantation some ten times, collected on a cloth filter, 

 washed on the filter, and then dried, as far as possible, by pressure. 

 The precipitate is dissolved in the least possible" excess of am- 

 monia, the solution allowed to stand for some time to allow the 

 fat to rise, then siphoned off, and filtered. The filtrate is pre- 

 cipitated as before with acetic acid, the precipitate washed, and 

 redissolved in ammonia. This treatment is repeated three or 

 four times. The casein is now rubbed in a mortar with 80 per 

 cent, alcohol and the alcohol poured off. It is then treated two 

 or three times with ether which has been freshly distilled with 

 some reagent which removes aldehyds, and then extracted for 

 some hours in a Soxhlet extractor to remove the fat. The ether 

 is evaporated at as low a temperature as possible. If a very 

 pure product' is required the casein is redissolved, reprecipitated, 

 and treated again with alcohol and ether. Finally, the casein is 

 dried at 100 to 105 C. If casein containing water is dried it 

 forms a horny mass." 



Hill has published a method of precipitating casein by the use 

 of colloidal iron. The milk is boiled and then the colloidal iron is 

 added. Holt, Courtney, and Fales, working' with human milk, 

 used colloidal iron as suggested by Hill, and added a few drops of 

 a saturated solution of magnesium sulphate. By this method the 

 filtrate is made clear and the casein precipitated entirely. 

 " The casein of human milk is more difficult to precipitate 



