on Milk and Milk-Products. 
377 
gelatinous mass, is such a familiar occurrence that one is almost 
inclined to look at this change as an inevitable and quite 
natural one. If the temperature of the gelatinous mass be 
raised, or if the latter be broken up, a separation takes place, 
the precipitated casein, including the fat, settling down in the 
form of curd in an almost bright liquid, the whey. An ap- 
parently similar occurrence takes place when rennet is added to 
milk at a suitable temperature. The two cases are, however, 
totally different. 
Normal milk when quite fresh exhibits, in consequence of 
some mineral salts which it contains, an amphioteric reaction, 
i.e. it turns red litmus paper slightly blue, and blue paper slightly 
red. The spontaneous coagulation of the casein never takes 
place before the milk has attained a decidedly sour reaction, 
and it is indeed a consequence of the formation of lactic acid, 
the product formed by the decomposition of milk-sugar. 
It used to be contended that also by the addition of rennet, 
lactic acid was first produced, which in its turn precipitated the 
casein. That this, however, is not the case, can be proved by 
the fact that rennet will coagulate milk to which an alkali has 
been added in such quantities that not only the milk employed 
but also the whey produced exhibits alkaline reaction. 
As mentioned before, the " turning " of the milk seems such 
a natural occurrence that formerly it was believed that milk in 
its normal state, and even before it had been drawn from the 
udder, contained the principle or ferment which causes the 
decomposition of milk-sugar into lactic acid, and consequently 
the coagulation of the casein. 
Very curious notions were indeed entertained with regard to 
the nature of this ferment. One investigator, about fifty years 
ago, believed that the milk globules (which in reality consist 
of the finely-divided butter fat) develop or grow into a kind of 
mould, which acts as a ferment ; whilst a well-known chemist 
asserted, only thirteen years ago, that it was the casein in 
milk which caused the lactic fermentation of the milk-sugar. 
If it were the case that milk carried with it, as a natural 
constituent, a ferment setting up lactic fermentation, it would 
be impossible to get a sample which would not curdle in a 
comparatively short space of time. If, on the other hand, the 
ferment is not contained in the milk whilst still in the udder, 
or when drawn from it under proper precautions, but enters it 
from outside after it has left the cow, one ought to be able to 
obtain samples which will keep sweet, under suitable conditions, 
without having to undergo any process of preservation or 
sterilization. 
In opposition to the views previously mentioned, Pasteur con- 
