PHYSICAL AND CHEMICAL EXAMINATION OF MILK 187 



per minute has been attained. A range of 50 revolutions per min- 

 ute on either side will not introduce serious error. After a lapse 

 of eight minutes the power is turned off, and when the centrifuge 

 has stopped rotating the tubes are removed and placed in the 

 rack. They should rest there for at least ten minutes before read- 

 ings are taken. The casein will form a well-defined mass above the 

 chloroform and the percentage can be read from the scale. Some- 

 times a film forms below the layer of casein, but this should not 

 be included in the reading, as it is usually the result of violent 

 shaking. 



In order to obtain accurate results with this test several con- 

 ditions should be carefully considered. These conditions are: 1, 

 The acidity of the milk; 2, the influence of prolonged shaking; 3, 

 the temperature of the ingredients; 4, the influence of preserva- 

 tives. 



The Acidity of the Milk. — The accuracy of the test is not 

 materially altered by acidity up to 0.36 per cent. This is shown by 

 the following figures given by Hart : 



SUCCESSIVE DETERMINATIONS OF CASEIN IN MILKS OF VARYING ACIDITY 



2. 3. 



When milk is curdled the test cannot be made. 



The Influence of Prolonged Shaking. — When the shaking takes 

 more than 20 seconds a ragged line forms at the juncture of the 

 casein and chloroform, and readings become inaccurate. 



Influence of the Temperature of the Ingredients. — The proper 

 temperature of the ingredients is 21° C. (70° F.). If it varies 5 

 degrees F. on either side, the influence is negligable, but greater 

 variation vitiates the test. When the temperature falls below 

 70° F. the reading will be too high ; when above 70° F. the reading 

 will be too low. This is illustrated by experiments made by Hart 

 and given in the following table : 



INFLUENCE OF TEMPERATURE ON THE DETERMINATION OF PERCENTAGE OF CASEIN 



IN MILK 



No. 68°-70°F. 68°-60°F. 65°-66° F. 74°-75° F. 80° F. 95° F. 



per cent. per cent. per cent. per cent. per cent. per cent. 



1 2 80 3,20 2.90 2.88 2.65 2.20 



2 3.10 3.58 3.10 3.00 2.85 2.30 



3 2 00 2.25 2.00 2.10 1.80 1.70 



4 2 05 2.20 2.10 2,00 1.90 1.50 



5 2.75 2.80 2.75 2.70 2.45 1.90 



6 2 40 2.60 2.45 2.30 2.25 1.80 



