314 PHYSIOLOGICAL CHEMISTRY 



Fresh milk, both human and cow's, is amphoteric in reaction to 

 litmus and acid to phenolphthalein. The acidity is believed to be due 

 in part at least to soluble acid phosphates. 1 Upon standing for a 

 sufficiently long time, unsterilized milk sours, i.e., it becomes strongly 

 acid in reaction to litmus due to the production of the optically in- 

 active fermentation lactic acid, 



H OH 



H C C COOH, 



I I ' 

 H H 



from the lactose contained in it. This is brought about through 

 bacterial activity. The white color is imparted to the milk partly 

 through the fine emulsion of the fat and partly through the medium of 

 the caseinogen in solution. The specific gravity of milk varies some- 

 what, the average being about 1.030. Its freezing-point is about 

 -o.56C. 



This lactic acid fermentation may be brought about by Bact. lactis 

 and other microorganisms. Certain putrefactive bacteria in the 

 human intestines may also cause lactic acid fermentation. The chem- 

 ical changes in lactic acid fermentation may be indicated thus : 



Ci 2 H 22 Oii+H 2 O^C 6 Hi 2 6 +C 6 H 12 6 



Lactose. Galactose Glucose. 



Galactose Lactic Acid. 

 or Glucose. 



Fresh milk does not coagulate on being boiled but a film consisting 

 of a combination of casein and calcium salts forms on the surface. 

 If the film be removed, thus allowing a fresh surface to come into 

 contact with the air, a new film will form indefinitely upon the applica- 

 tion of heat. Surface evaporation and the presence of fat facilitate 

 the formation of the film, but are not essential (Rettger 1 ). As Jamison 

 and Hertz 2 have shown, a similar film will form on heating any protein 

 solution containing fat or paraffin. If the milk is of a pronounced 

 acid reaction, through the inception of lactic acid fermentation, or from 

 any other cause, no film will form when heat is applied, but instead a 

 true coagulation will occur. When milk is boiled certain changes occur 

 in its odor and taste. These changes, according to Rettger, 3 are due 

 to a partial decomposition of the milk proteins and are accompanied 

 by the liberation of a volatile sulphide, probably hydrogen sulphide. 



1 Rettger: American Journal of Physiology, 7, 325, 1902. 



2 Jamison and Hertz: Journal of Physiology, 27, 26, 1902. 



3 Rettger: American Journal of Physiology, 6, 450, 1902. 



