GENERAL CHEMISTRY OF MILK 133 



important to know in what respect milk is changed by the process. 

 It may be expected that there is some separation of all suspended 

 substances in milk, and this is actually true. The slime in the 

 separator bowl contains 2 to 3 per cent, more ash than the milk 

 and a considerable quantity of casein is thrown out. The longer 

 centrifugation continues, the greater is the amount of casein 

 thrown out, so that, according to Van Slyke, if centrifugation is 

 continued for several hours practically all casein can be removed. 

 The increase in ash is due largely to the removal of suspended 

 phosphates and to calcium and phosphoric acid from the casein. 

 The centrifugal slime further contains insoluble dirt, bacteria, 

 and body cells of great variety. 



Dilutions of Milk. When milk is diluted the reaction changes 

 so that less N.NaOH is required for neutralization. The follow- 

 ing figures adapted from Soldner show the change in reaction 

 quite clearly (Sommerf eld's Handbuch) : 



CHANGE IN REACTION OF MILK BY DILUTION 



Milk in Water in N.NaOH 



cubic centimeter. cubic centimeter. required. 



100 1.50 



100 100 1.25 



100 200 1.10 



100 500 0.925 



100 1000 0.625 



Dialysis. By dialysis soluble calcium salts are removed and 

 casein is precipitated. 



Electricity. There is a popular belief that during thunder- 

 storms milk decomposes more rapidly than under ordinary condi- 

 tions. Experiments with passing the electric current through 

 milk have shown that acids are formed and that the casein under- 

 goes some change. This change consists chiefly in separating the 

 calcium from the casein, so that it is precipitated. It is question- 

 able, however, whether these experiments have bearing on the 

 influence of thunderstorms. It is claimed by some that the 

 humidity in the atmosphere and the high temperature are respon- 

 sible for rapid changes, and that if milk is kept cold these changes 

 will not take place even when thunderstorms prevail. According 

 to this view, the influence of thunderstorms consists merely in 

 favoring bacterial growth. It is further asserted that conditions 

 at the dairy during thunderstorms are favorable to bacterial 

 growth, and that subsequent rapid decomposition of milk may be 

 due to this cause. If the utensils are not sterilized the atmospheric 

 conditions during thunderstorms permit enormous multiplication 

 of bacteria in the vessels between milkings, and the milkers may 

 be careless due to the wearing effect of heat and humidity. Exact 

 observations on these points are still lacking, and the effect of 

 thunderstorms on milk must be considered an open question. 



