330 PHYSIOLOGICAL CHEMISTRY 



milk serum. Their classification of milk constituents in a slightly 

 modified form follows: 



CONSTITUENTS OF MILK 



In true solution in milk Partly in solution and Entirely in suspension or 



milk serum. partly in suspension or col- colloidal solution. 



loidal solution. 



1. Lactose. i. Albumin. i. Fat. 



2. Citric acid. 2. Inorganic phosphate. 2. Casein. 



3. Potassium. 3. Calcium. 3. Fat-soluble A. 



4. Sodium. 4. Magnesium. 



5. Chlorine. 



6. Water-soluble B. 



7. Water-soluble C. 



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. 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 



I I 

 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. 5 6C. 



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

 acidi 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 On + H 2 0- C 6 H 12 6 + C 6 Hi 2 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 



