802 
The Fermentations of MWi, 
the neutral solution of the curd cannot be curdled at all by addition 
of rennet. Something is necessary to this kind of curdling which 
was present in the milk and is not present in the precipitated curd. 
This is proved by adding the whey to the neutral solution of the 
curd, when it becomes precipitable by rennet, like fresh milk. The 
necessary substance appears to be the very small amount of lime 
compounds present in milk and in rennet curd but not present in 
acid curd. 
The active principle in rennet is considered by all authorities to 
be a chemical ferment, which has been variously called lab, chymosin, 
pixine, and rennet diastase. This last name is in allusion to the 
diastase of malt, the earliest and best known of the class of chemical 
fermeiats, which has for its special function the transformation of 
the starch of malt into sugar and dextrin. 
These chemical ferments are readily distinguished from the 
organised or living ferments. They are easily soluble in water, and 
their solutions can be filtered through so fine a material as unglazed 
porcelain without losing their fermentative power, whereas bacteria 
and other living ferments cannot pass through such a filter. Moreover, 
a definite quantity of a chemical ferment will do only a definite, 
although very large, amount of work, whereas a single bacterium is 
capable of transforming any quantity of fermentable substance, 
because it is capable of indefinite multiplication. 
There is, however, an intimate relation between the living 
ferments and th^ chemical ferments, or, as they are called, enzymes. 
The enzymes themselves are products of the growth of living or- 
ganisms, bacterial or other. In fact, it is a general rule that the 
cultivation of a species of organised ferment is accompanied by the 
production of at least one definite chemical ferment, and it is some- 
times a matter of difficulty to determine whether a given change is 
due to bacteria themselves or to the enzymes which they produce. 
In the case of rennet it is very probable that the rennet ferment 
proper is produced by the multiplication of bacteria during the time 
the veils are hung up in the air as well as during the period of their 
maceration in brine. The alcoholic preparations known as essences 
of rennet contain the ferment in a stronger and purer condition than 
ordinary dairy rennet, and by precipitation of these with more alcohol 
and drying at a low temperature Duclaux has even succeeded in 
making a dry or solid rennet extract, but the pure ferment itself has 
never been isolated. It is estimated by Duclaux that one part of the 
pure ferment would coagulate as much as 800,000 parts of casein. 
The exact manner of its action is not fully made out, but according 
to the most probable account it acts on the caseinogen (the substance 
in the milk from which casein is produced by curdling), breaking it 
up into two albuminoids, one of wliich is easily curdled, whilst the 
other is curdled only with great difficulty. Tlie former is readily pre- 
cipitated from its solution by calcium salts ; and since these are 
always present in the milk, the result of rennet action is always to 
throw down the curd. The other portion of the original caseinogen, 
being soluble, goes into the whey and is lost to the cheese-maker. 
