EXAMINATION OF MILK. 9 



3. A portion is ground with water and some calcium car- 

 bonate, and filtered. The filtrate, which is usually not quite 

 clear, contains casein, as may be shown by acidifying with 

 acetic acid. Casein therefore has the character of an acid: 

 it drives out the carbonic acid and forms a soluble salt with 

 calcium. 



4. With a portion of the substance try the reaction with 

 the alkaline solution of lead hydroxide described under Albu- 

 min. Only a faint gray color results. Casein contains only 

 a little of the unoxidized sulphur; the greater part is in the 

 oxidized form. In order to show more exactly the difference 

 between casein and albumin in this reaction, we proceed as 

 follows: the solution of caustic soda containing the lead is 

 diluted with several times its volume of water and the 

 dilution is continued until some of the diluted solution 

 when boiled with albumin is only slightly blackened. The 

 casein is then tested with this solution. It should react 

 negatively. 



5. Casein is not a simple proteid, but may be split up by 

 appropriate means (digestion in the stomach) into a proteid 

 and paranuclein. Since paranuclein, like nuclein itself, con- 

 tains organically combined phosphorus, the casein also 

 contains phosphorus. In order to prove the presence of 

 phosphorus, we grind about 0.2 g. of the casein, which has 

 previously been treated with alcohol and ether, with 6 g. of 

 the oxidizing mixture (see above under Detection of Sulphur), 

 heat to fusion, dissolve the fused mass, after cooling, in 

 nitric acid and heat the solution in order to drive out the 

 nitrous acid formed. A part of the solution is added, drop 

 by drop, to about 5 cc. of the molybdate solution; yellow 

 color, cloudiness, and then a yellow precipitate prove the 

 presence of phosphoric acid, formed from the phosphorus by 

 the fusion with the niter. The reaction is only to be relied 

 on to prove the presence of phosphorus when the substance 



