32 CONSTITUENTS OF MILK 



As the proteins of milk obstruct the carriage of electricity 

 by the moving ions, the conductivity of whey or of serum is 

 greater than that of the milk from which it is prepared. Each 

 1 per cent of protein reduces the conductivity by 2.75 per cent 

 (Rothera and Jackson). The surface tension of milk is lower 

 than that of water, 0.053 as against 0.075 and the specific heat 

 of milk containing 3.17 per cent of fat is, according to Fleish- 

 mann, 0.9457. 



The refractive index of milk cannot be determined on account 

 of its opacity, but that of the serum, after removal of the case- 

 inogen and fat, has been determined on a large number of sam- 

 ples by various observers and is now regarded as a valuable 

 aid in the detection of adulteration by the addition of water. 



This method is of special value on account of the removal 

 of the constituents of milk that are most variable in amount, 

 viz., fat and caseinogen, leaving a serum containing the lac- 

 tose, mineral matter, and albiunin which are generally the least 

 variable. Various methods, which vary somewhat in the 

 completeness of precipitation of caseinogen attained, have been 

 employed, ^^j*" and normal values estabhshed for each. The 

 refractive index of fresh milk serum, prepared by filtration 

 through porous plates, varies from (iUi>20° C.) 1.34200 to 

 1.34275. The specific gravity of milk serum is equally as valua- 

 ble as the refractive index (see p. 79) but on account of the 

 longer time required for its determination it is not generally 

 used as a routine method. The ash of the serum also affords 

 valuable information for the detection of added water. (Lyth- 

 goe,*" and Burr and Berberich.*^). 



BIBLIOGRAPHY 



1. Van Slyke and Bosworth. Bull. 26, N. Y. Expt. Sta. Geneva, 1912. 



2. Schryver. Proc. Roy. Soc, B. 86, 460-i81. 



3. Richmond. Dairy Chemistry. London, 1914, p. 30. 



4. Soldner. Landw. Versuch. Stat. 1888, 35, 351. 



5. Lacquer and Sackur. Beitr. Chem. Phys. u. Path. 1902, 3, 193. 



6. Robertson. Jour. Phys. Chem. 1911, 15, 179. 



7. Osborne. Zeit. Physiol. Chem. 1901, 33, 240. 



