SECRETARY'S REPORT. 89 



ible in view of the fact that when acids are added to milk in suf- 

 ficent quantity, the casein separates — that is, the milk curdles or 

 coagulates. So it does when by exposure to the air it becomes 

 sour, which souring is due to the conversion of the milk sugar into 

 lactic acid. But this theory is inconsistent with other facts, for it 

 is equally true that freshly drawn milk is often neutral, and some- 

 times it has an acid reaction. I have myself several cows whose 

 milk, when drawn, distinctly reddens litmus paper, and I have 

 known scores of others to do the same, thus showing an apprecia- 

 ble amount of free acid when quite new ; nor does this milk curdle 

 or become appreciably sour to the taste sooner than other milk. 

 If we are to be governed in our theory by a part of known facts, 

 we might in this case as well say that casein is held in solution by 

 free acid as in the other by free alkali ; but neither view will ac- 

 count for its solution in a liquid perfectly neutral. Again, if it 

 were true that the casein of milk is held in solution by virtue of a 

 free alkali, it would be precipitated — that is, the milk would 

 curdle just as soon as this alkali was neutralized. If this were the 

 case, newly drawn milk would never present an acid reaction, for 

 before this could be the case the milk would be changed to curd in 

 the udder. 



Experience has demonstrated that whether the casein of milk is 

 separated spontaneously after exposure, and in connection with the 

 development of lactic acid from the conversion of milk sugar, or by 

 the addition of other acids to the milk, as is practiced in some 

 countries, good cheese is not the result. To this end the action of 

 rennet is indispensable ; an action the precise nature of which is not 

 known, although it is probably what is called " catalytic," or an 

 " action of presence," a name given to an action which we know 

 little about beyond the fact. 



Kennet is an infusion of an animal membrane. Various mem- 

 branes of different animals will serve to curdle milk, but the stom- 

 ach of the young sucking calf cleaned, salted, dried and kept a 

 year or more, is greatly preferred. The infusion, as usually pre- 

 pared, is slightly acid, but that it is not the acid in it which effects 

 coagulation is proved by the fact that it may be made slightly alka- 

 line by the addition of potash or soda, and still be effective. Milk 

 which is slighly alkaline I have found may be curdled by rennet 

 which has purposely been rendered alkaline, and the whey pro- 

 duced in such case showed an alkaline reaction, so that in cheese- 

 making we may conclude that the coagulation is not effected by 



