188 MILK 



The Influence of Preservatives. — After a series of tests Hart 

 concludes that potassium bichromate is the only preservative 

 commonly used for preserving composite samples that permits an 

 accurate test to be made. In these tests formaldehyd, mercuric 

 chlorid, toluol, chloroform, and potassium bichromate were used. 

 The readings were always too high except when potassium bi- 

 chromate was used. After the samples had stood for more than 

 three days and a half with potassium bichromate the reading Une 

 was ragged. 



The speed of the centrifuge should be 2000 revolutions per 

 minute when the revolving wheel is 15 inches in diameter. When 

 a hand centrifuge is used the author advises regulation of speed by 

 means of a metronome. 



When carefully executed the Hart centrifugal test for casein 

 gives results which closely approximate those obtained by the 

 Association of Official Agricultural Chemists in their test for de- 

 termining casein. The comparison is shown by the following 

 figures : 



COMPARATIVE DETERMINATIONS OF CASEIN BY THE OFFICIAL METHOD AND BY THE 



NEW METHOD 



Jersey .... 

 Guernsey . 

 Holstein. . 



Breed of cow. Official method. New method. 



[2.45 2.50 



• 3.31 3.20 



3.65 3.70 



2.47 2.60 



2.91 3.10 



' 3.12 3.05 



3.50 3.70 



1.88 1.88 



2.10 2.08 



'2.13 2.13 



2.50 2.60 



„ c . / 2.66 2.65 



crown DW1S3 i 2 70 2 70 



2,56 2^50 



Ayrshire J 2 61 2.65 



I 3 14 3.18 



Hart's Titration Method for Determining Casein. — Another 

 method for rapidly estimating the casein content of milk has been 

 described by Hart. The test is based on the following principle: 

 When casein is precipitated by acid and the acid removed by 

 prolonged washing, a substance remains which has acid proper- 

 ties and displaces CO2 from calcium carbonate to form an opales- 

 cent solution. Lime-water or a fixed alkali dissolves the casein 

 precipitate with production of a slightly milky solution. When 

 the excess of alkali in such a solution is titrated with acid and 

 phenolphthalein as indicator, some alkali remains unaccounted 

 for. This fact shows that the casein precipitate neutralizes a 

 definite amount of alkali. By a series of tests it was ascertained 

 that 0.1 c.c. of N. one-tenth NaOH bound on the average 0.0108 

 gram casein precipitate. Therefore, 10.8 grams of milk are re- 



