30 MODERN METHODS OF TESTING MILK 



bichromate. (3) Lactic acid in milk considerably re- 

 duces the efficiency of bichromate in preserving milk, 

 (4) Samples of milk preserved with bichromate are 

 apt, when exposed to light, to form a tough skin on 

 the surface, which interferes with proper sampling; 

 this can be largely decreased by keeping samples in 

 bottles of amber or brown-colored glass. (5) The 

 presence of bichromate in milk interferes with other 

 tests, such as acidity, specific gravity, detection of 

 formalin, enzyms, freezing-point and refractive index. 

 The amount of potassium bichromate to be used in 

 composite samples is about 8 or 10 grains for half a 

 pint to a pint of milk. The bichromate is put up for 

 sale in tablets of convenient size, ready for use in pre- 

 serving milk samples. Bichromate can be satisfactor- 

 ily used even in hot weather, if the samples are kept 

 in a dark, cool place most of the time. 



CARE OF COMPOSITE SAMPLES 



In caring for composite samples of milk or cream, 

 some special precautions must be observed. ( i ) Com- 

 posite sample jars must be kept covered tight to pre- 

 vent evaporation of water, which would result in giv- 

 ing a test for fat higher than the correct amount. (2) 

 They should be kept in a cool place, so that the small- 

 est possible amount of preservative will need to 

 be used. (3) They should be kept in the dark 

 most of the time, since direct sunlight may cause 

 the formation of a tough cream, rendering difficult 

 the taking of a good sample for testing. (4) When 

 the daily sample of milk is added to the composite 



