21 



3. Separation and determination of casein, casein monolactate, and casein dilactate in milk. 

 (a) The total amount of nitrogen precipitated by acid is determined by the official 

 method prescribed for determining casein in milk. (Bui. 46, p. 55.) 



(b) Heat 10 grams of milk diluted with 90 cc of water to 40 C. for fifteen or twenty min- 

 utes, filter the precipitate formed, and wash with distilled water. Transfer the precipitate 

 to a small Erlenmeyer flask, provided with a stopper, treat with 100 cc of a 5 per cent solu- 

 tion of sodium chlorid and heat at 55 C. for two hours, with frequent agitation. Filter the 

 mixture, wash the remaining precipitate with water, and determine the nitrogen in both 

 the precipitate and filtrate. The nitrogen in the precipitate represents casein dilactate; that 

 in the filtrate, casein monolactate. The sum of these two subtracted from the total nitrogen 

 found by precipitation with acid gives the amount of nitrogen as casein. 



Twenty-second Convention, 1905, Bui. 99, Cir. 26. 



The following changes were adopted in the proposed provisional 

 methods offered in 1902, as explained above: 



The amount of water used in making the extract is increased from 

 500 cc to 1,000 cc. 



After the fat and insoluble nitrogenous bodies have been removed by 

 the absorbent cotton, pass the nitrate through asbestos. 



SEPARATION OF VEGETABLE PBOTEIDS. 



Twenty-first Convention, 1904, Bui. 90, Cir. 20. 



(No attempt shall be made, on the results of the following method, to 

 separate the proteids of wheat into individuals, and only such names 

 are to be applied as describe the process of separation, viz, (a) proteids 

 soluble in alcohol; (b) proteids insoluble in alcohol; (c) proteids solu- 

 ble in dilute salt solution.) 



PROVISIONAL METHOD FOR THE SEPARATION OP THE PROTEIDS OF WHEAT. 



A. Total Proteids. Determine total nitrogen by the Gunning method. Nitrogen X 

 5.68 = total proteids. 



B. Proteids Soluble in Alcohol. 1. Weigh out 2 grams of flour into a small flask 

 holding only slightly more than 100 cc, about 110 cc. Add 100 cc of 70 per cent alcohol 

 and shake thoroughly for fifteen minutes. Allow to stand eighteen hours and filter into a 

 Kjeldahl digestion flask. Wash the residue with 100 cc of alcohol. Add to the total 

 filtrate 5 to 10 cc of concentrated sulphuric acid and distil off the excess alcohol, continuing 

 the distillation until fumes appear. Determine nitrogen by the Gunning method: 



Nitrogen X 5.68 = proteids soluble in water. 



2. Determine proteids soluble in alcohol by Snyder's polariscopic method. (J. Amer. 

 Chem. Soc., 2 : 263.) 



C. Proteids Insoluble in Alcohol. Subtract proteids soluble in alcohol from total proteids 

 to obtain proteids insoluble in alcohol. 



D. Proteids Soluble in Dilute Salt Solution. Use a 5 per cent solution of potassium 

 sulphate. Place 4 grams of the flour in a flask such as is used in B 1, and introduce exactly 

 100 cc of the salt solution. Shake thoroughly and allow to stand, with frequent shakings, 

 for eighteen hours, or, better still, agitate in a shaker for six hours. After standing to 

 settle, filter off 50 cc of the liquid, pouring back the first portions of the filtrate until it filters 

 clear, and determine the nitrogen therein by the Gunning method. The amount multiplied 

 by 2 gives the nitrogen in the original flour. This result multiplied by 5.68 gives the amount 

 of proteids in the flour soluble in a 5 per cent potassium sulphate solution. 



