236 MILK AND ITS HYGIENIC RELATIONS 



of the milk. It is generally believed by those who have investi- 

 gated the question that the acidity of milk falls somewhat as a 

 result of heating. This is believed to be due to the passing off of 

 carbonic acid as a result of heating. 



Jensen and Plattner found a decrease in the total acidity with 

 the employment of moderate heat, and a rise of acidity when the 

 temperature rose to 120 C., or even when a temperature of 100 C. 

 is employed for a prolonged period. They believe that the fall 

 of acidity, which is slow, is due to the giving off of carbon dioxide, 

 and they point out that the calcium falls out from solution when 

 the acidity has been reduced to the lowest point reached. They 

 state, however, that the falling out of the calcium does not always 

 occur, and they believe that this depends upon the amount of semi- 

 combined carbon dioxide present in the milk, the rise of acidity 

 consequent on heating to a high temperature being due to a slight 

 disintegration both of the lactose and of the caseinogen. 



Splittgerber found that there was a fall of total acidity when 

 the milk was heated for about two hours in a Soxhlet apparatus. 

 He pointed out that lactic acid, when present, passes off very readily 

 on the application of a low temperature, and appears to suggest 

 that the reduction in acidity may be connected with small quantities 

 of lactic acid in the milk. In the course of heating it is very likely 

 that changes take place in the method of combination of the calcium 

 with the phosphorus in the milk, and possibly with the carbon 

 dioxide present. The possibility of similar changes has been 

 mentioned in connection with the effect of heat upon calcium 

 citrate. Any such changes, if they occur, would tend to reduce 

 the acidity of the milk and at the same time cause the calcium 

 salt to fall out of solution. 



It has been shown above, and it is a well-known fact, that 

 reduction in the calcium content of the milk delays the time of 

 clotting. The degree of acidity appears also to exercise influence 

 upon the time of clotting. Some difficulty has been experienced 

 in the making of cheese with boiled or pasteurised milk, owing to 

 the delay which occurs in the subsequent process of clotting with 

 rennet. Sammis and Bruhny have shown in recent investigations 

 upon this subject, that the difficulty can be readily overcome by 

 the addition of a small quantity of hydrochloric acid to the milk 

 after it has been heated. It seems likely, therefore, that milk, even 

 when the coagulation has been indefinitely delayed as a result of 

 heating to a high temperature, will undergo coagulation when it 

 reaches the stomach. Moreover, there appears to be subtle differ- 

 ences between the rennets occurring in the stomachs of different 

 animals. Fuld and Noeggerath found, for instance, that the rennet 

 obtained from calves and goats showed a greater activity on the 

 caseinogen of the same species. It is possible, therefore, that 

 although human milk does not clot readily with cows' rennet, 

 clotting may occur in the stomach of the human infant. 



