Chemical Analysis of Milk and Its Products. 243 



dehyd in 500 cc. G. P. H 2 S0 4 ) is poured down the side 

 of the tube so it will form a layer under the milk. If 

 nitrates or nitrites are present, a violet ring will form 

 at the place of contact. This is Hehner's test for for- 

 maldehyd reversed, see (304). 



292. Besides by the methods given in the preceding 

 (pp. 109-115), watering or skimming of milk may be de- 

 tected by determining the specific gravity of a, the skim 

 milk, b, the milk serum, and c, the whey. 



a.' Specific gravity of skim milk. The milk is set in a flat 

 porcelain or glass dish for 12-24 hours in a cold room; the layer 

 of cream formed is then skimmed off, and the sp. gr. of the skim 

 milk determined at 60 F. Skim milk has a sp. gr. of .002 to 

 .0035 (2 to 3.5 lactometer degrees) above that of the correspond- 

 ing whole milk; a smaller difference than this indicates that the 

 milk was skimmed. If both skimming and watering had been 

 practiced, the difference given above might be obtained, but the 

 analysis of the milk would in such case easily disclose the adul- 

 teration. 



b. Specific gravity of the milk serum. To 100 cc. milk 2 

 cc. of 20 per cent.-acetic acid are added, and the mixture heated 

 in a covered beaker or closed flask for 5-10 min. on a water -bath 

 at 55-65 C. After cooling, the milk serum is filtered off and its 

 sp. gr. determined at 60 F. In case of pure milks, the sp. gr. 

 of the milk serum (at 60) will come above 1.0270. Serum from 

 normal milk contains 6.3 to 7.5 per cent, solids and .22 to .28 

 per cent, fat; by the addition of 10 per cent, of water, the 

 solids in the serum are lowered .3 to .5 per cent., and the sp. gr., 

 .0005. 1 



c. Specific gravity of whey. 500 cc. of milk are warmed in 

 water of 40-50 C. until its temperature is 35 C.; one-half cc. 

 of rennet extract (12-15 drops) is added, and the milk stirred 

 thoroughly. After allowing the curd to solidify for 10 minutes, 

 it is cut and the whey filtered off through several layers of cheese 

 cloth. The whey must be clear; it is cooled to 60 F. and its 



1 Konig, Menschl, Nahrungsmittel, II, p. 276. 



