34 Composition of Cheese. 



Scientific and practical writers on milk have stated that the 

 casein is held in solution by a small quantity of alkali ; that 

 when in warm weather milk curdles, lactic acid, which is 

 always found in sour milk, is formed from a portion of the sugar 

 of milk ; and this lactic acid, by neutralizing the alkali which 

 holds the casein in solution, causes its separation from the milk. 

 Rennet is supposed to act as a ferment, which rapidly converts 

 some of the sugar of milk into lactic acid. Whether, therefore, 

 milk coagulates spontaneously after some length of time, or more 

 rapidly on the addition of rennet, in either case the separation of 

 the curd is supposed to be due to the removal of the free alkali 

 by lactic acid. 



This theory, however, is not quite consistent with facts. The 

 casein in milk cannot be said to be held in solution by five 

 alkali ; for, although it is true that milk often has a slightly alka- 

 line reaction, it is likewise a fact that sometimes perfectly irrsli 

 milk is slightly acid. We might as well say, therefore, that the 

 casein is held in solution by a little free acid, as by free alkali. 

 Newly drawn milk, again, is often perfectly neutral ; but, 

 whether milk be neutral, or alkaline, or acid, the casein exists 

 in it in a state of solution, which cannot, therefore, depend 

 on an alkaline reaction. We all know that milk, when it 

 turns sour, curdles very readily. It is not the fact that a good 

 deal of acid curdles milk which I dispute, but the assumption 

 that the casein in milk is held in solution by free alkali. The 

 action of rennet upon milk, then, is not such as has been 

 hitherto represented by all chemists who have treated of this 

 subject. Like many other animal matters which act as ferments, 

 rennet, it is true, rapidly induces the milk to turn sour ; but free 

 lactic-acid, I find, makes its appearance in milk after the curd 

 has separated, and not simultaneously with the precipitation of 

 the curd. Perfectly fresh and neutral milk, on the addition of 

 rennet, coagulates, but the whey is perfectly neutral. I have even 

 purposely made milk alkaline, and yet succeeded in separating 

 the curd by rennet ; and, what is more, obtained a whey which 

 had an alkaline reaction. 



What may be the precise mode in which rennet acts upon 

 milk, I do not presume to explain. I believe it to be an action 

 sui generis, which as yet is only known by its effects. We at 

 present are even unacquainted with the precise chemical cha- 

 racter and the composition of the active principle in rennet, and 

 have not even a name for it. Finding the effect of rennet upon 

 milk to be different from that which I expected, I made a number 

 of experiments, which may here find a place. 



1st Experiment. To a pint of new milk, slightly alkaline to 



