184 MILK 



precipitated from it by a solution of tannin. The precipitate is 

 washed and the amount of nitrogen determined. 



"The casein is calculated by difference between the total pro- 

 tein and the albumin." 



Casein may also be determined by precipitation with acetic 

 acid in the following manner: 20 c.c. of milk are diluted with 

 water to 200 c.c. and enough dilute acetic acid (10 per cent.) 

 added to complete precipitation. This is indicated when the 

 filtrate is perfectly clear. The amount of acetic acid necessary 

 for complete precipitation of the casein varies somewhat in differ- 

 ent milks. It is therefore necessary to add the acid cautiously so 

 as to leave no excess of acid which would redissolve the casein. 

 It is advisable to prepare several samples of the milk and place 

 these in different flasks. To each flask is then added a different 

 amount of acid, and that flask selected for operation which shows 

 a clear filtrate with the smallest amount of acetic acid. Intro- 

 troduction of CO2 aids in completing precipitation. The precip- 

 itate is allowed to settle, is washed repeatedly by decantation, 

 and gathered on nitrogen-free filters. The nitrogen is then de- 

 termined by the Gunning method. 



Instead of estimating the nitrogen by the Gunning method the 

 acetic acid precipitate may be washed until free from lactose and 

 albumin and the fat extracted with ether. The precipitate is then 

 gathered on a weighed filter, desiccated, and weighed. 



To determine the albumin the filtrate from the casein deter- 

 mination is boiled, the precipitate washed, and gathered on a 

 weighed filter. After desiccation the filter is weighed again and 

 the weight of the albumin calculated. 



Determination of Nitrogen as Caseoses, Amido-compounds, 

 Peptons, and Ammonia (Van Slyke's Method). — The filtrate ob- 

 tained after the albumin has been removed is heated to 70° C; 

 1 c.c. 50 per cent, sulphuric acid is added, and then chemically 

 pure zinc sulphate to saturation. The mixture is allowed to stand 

 at 70° C. until the caseoses have separated and settled. The pre- 

 cipitate is then cooled and washed with saturated zinc sulphate 

 solution slightly acidified with sulphuric acid and the nitrogen 

 in the precipitate determined. 



Amido-compounds and ammonia are determined as follows: 

 50 grams of milk are placed in a 250-c.c. graduated fiask with 1 

 gram sodium chlorid and a 12 per cent, solution of tannin added 

 drop by drop until no more precipitate is formed. The mixture 

 is then diluted to the 250 c.c. mark, shaken, and filtered. The 

 nitrogen in 50 c.c. of the filtrate is determined and represents the 

 nitrogen of the amido-compounds and ammonia. 



The ammonia nitrogen is determined by distilling with mag- 



