MILK 321 



solution of "trikresol" 1 acts as a sensitizing agent in the peroxidase 

 reaction and offers the following test which is based upon this fact. 



Procedure. To 2-5 c.c. of raw milk in a test-tube add 0.1-0.3 c.c. of M/io 

 hydrogen peroxide and i c.c. of a i per cent solution of "trikresol." A slight 

 though unmistakable yellow color will be observed to develop throughout the 

 solution. Repeat the test using milk which has been boiled or heated to 

 80 C. for 10-20 minutes and cooled, and note that no yellow color is produced. 



The color reaction in the case of the raw milk probably results from 

 the oxidation of the cre^ols by the hydrogen peroxide. The first 

 product of this oxidation 2 then oxidizes the leuco compound, when such 

 is present, and causes the color observed. 



(b) Benzidine Peroxidase Reaction (Wilkinson and Peters).* To 10 c.c. of the 

 milk to be tested add 2 c.c. of a 4 per cent alcoholic solution of benzidine, suffi- 

 cient acetic acid to coagulate the milk (usually 2-3 drops) and finally 2 c.c. of a 

 3 per cent solution of hydrogen peroxide. Raw milk yields an immediate blue 

 color. In adding the peroxide it is best to permit it to flow slowly down the 

 wall of the vessel containing the mixture instead of allowing it to mix with the 

 milk. Milk which has been heated to 78C. or above remains unchanged. 



12. Saturation with Magnesium Sulphate. Place about 5 c.c. of milk in a 

 test-tube and saturate with solid magnesium sulphate. What is this precipitate? 



13. Influence of Gastric Rennin on Milk. Prepare a series of five tubes as 

 follows : 



(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. 4 



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



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



(e) 5 c.c. of fresh milk + 5 drops of 02 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 in 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 in 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- 



1 "Trikresol" is the trade name of an antiseptic which contains the three cresols in ap- 

 proximately equal proportions. 



2 Probably some organic peroxide or quinoid compound. 



3 Wilkinson and Peters: Z. Nahr-Genussm., 16, No. 3, p. 172. 



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

 of the pig may be employed. 



