MILK 339 



(a) 5 c.c. of fresh milk + 0.2 per cent HC1 (add drop by drop until a precipitate 

 forms). 



(b) 5 c.c. of fresh milk + 5 drops of rennin solution. 1 



(c) 5 c.c. of fresh milk -f 10 drops of 0.5 per cent Na 2 CO 3 . 



(d) 5 c.c. of fresh milk -f 10 drops of ammonium oxalate. 



(e) 5 c.c. of fresh milk + 5 drops of 0.2 per cent HC1. 



Now to each of the tubes (c), (d) and (e) add 5 drops of rennin solution. 

 Place the whole series of five tubes at 4OC. and after 10-15 minutes note what 

 is occurring hi the different tubes. Give a reason for each particular result. 



14. Preparation of Casein. Fill a large beaker one-third full of skimmed 

 (or centrifugated) milk and dilute it with an equal volume of water. Add dilute 

 hydrochloric acid until a flocculent precipitate forms. Stir after each acidifica- 

 tion and do not add an excess of the acid as the precipitate would dissolve. 

 Allow the precipitate to settle, decant the supernatant fluid, and reserve it for 

 use hi later (15-18) experiments. Filter off the precipitate of casern and re- 

 move the excess of moisture by pressing it between filter papers. Transfer the 

 casein to a small beaker, add enough 95 per cent alcohol to cover it and stir for 

 a few moments. Filter, and press the precipitate between filter papers to re- 

 move the alcohol. Transfer the casein again to a small dry beaker, cover the 

 precipitate with ether and heat on a water-bath for ten minutes, stirring con- 

 tinuously. Filter (reserve the filtrate) , and press the precipitate as dry as possible 

 between filter papers. Open the papers and allow the ether to evaporate spon- 

 taneously. Grind the precipitate to a powder hi a mortar. Upon the casein 

 prepared hi this way make the following tests : 



(a) Solubility. Try the solubility in water, sodium chloride, dilute acid and 

 alkali. 



(b) Mfflon's Reaction. Make the test according to the directions given on 

 page 97. 



(c) Biuret Test. Make the test according to directions given on page 99. 



(d) Glyoxylic Acid Reaction (Hopkins-Cole). Make the test according to 

 the directions given on page 98. 



(e) Unoxidized Sulphur. Test for unoxidized sulphur according to the di- 

 rections given on page 107. The sulphur content of casein is rather low, e.g., 

 about 0.065 P er cent ' 



(f) Fusion Test for Phosphorus. Test for phosphorus by fusion according to 

 directions given on page 128. Casein contains 0.7 per cent of phosphorus. 



15. Coagulable Proteins of Milk. Place the filtrate from the original casein 

 precipitate in a casserole and heat, on a wire gauze, over a free flame. As the 

 solution concentrates, a coagulum consisting of lactalbumin and lactoglobulin 

 will form. Continue to concentrate the solution until the volume is about one- 

 half that of the original solution. Filter off the coagulable proteins (reserve the 

 filtrate) and test them as follows : 



(a) Millon's Reaction. Make the test according to the directions given on 

 page 98. 



(b) Biuret Test. Make the test according to the directions given on page 100. 



(c) Glyoxylic Acid Reaction (Hopkins-Cole). Make the test according to 

 the directions given on page 98. 



1 Any commercial rennin or rennet preparation or an extract of the gastric mucosa 

 of the pig may be employed. 



