280 The WorJc of Acidity in Cheese-making. 
as well as any others, such as, in the case of milk, the fat- 
globules and certain mineral constituents (earthy phosphates) 
Avhich have been merely suspended in it in a state of minute 
division. In the case of the gelatin, this sponge is possessed 
of no contractility vrhatever, and the effect of heat on it is 
merely to i-esolve it again into a state of fluidity. But, in that of 
the milk jelly, when once this sponge has been organised it goes 
on slowly contracting and squeezing out the liquid whey from 
its interstices, whilst retaining in them the fatty and mineral 
matters, and in so doing it passes into the condition of firm, 
tenacious curd. 
Let us now, whilst the curd is thus being organised in our 
sample of soured milk, take a second sample of fresh milk, of 
about the same degree of acidity as the first, and, after raising it 
to a temperature of about 90° F., mix it with enough' solution 
of rennet to coagulate it in about half-an-hour. We shall then 
obtain a jelly which, in its general appearance, is indistinguish- 
able from that formed spontaneously in the soured milk, and 
which, if divided in the same way as was done with the latter, 
will be found to exhibit the property of spontaneous contractility 
as that did. 
80 far as we have yet gone, therefore, it is evident that the 
primary work of the rennet is to effect the coagulation of the 
casein of the milk in the same way as acidity did, and that it 
will do this in the presence of only a feeble degree of acid. 
Let us now carry the experiment a stage farther, and, 
taking a third sample of fresh milk, add carefully to it, stirring 
the while, enough of an alkaline solution (that formed by dis- 
solving a few crystals of common washing-soda in a teacupful of 
rainwater will answer the purpose) to neutralise any acidity 
that may be present — that is, until the milk produces no effect 
on hhie litmus-paper or gives a permanent purple tint with a drop 
or two of the indicaior solution (see p. 59) of the acidometric test. 
If now the milk, thus slightly alkalised, be treated with solution 
of rennet, under the same conditions as those under which the 
fresh milk was renneted, it will be found that it also will in 
time coagulate, but that it will take much longer to do so, the 
actual length of time required varying with the degree of 
alkalinity of the milk. ]\Ioreover, when the curd lias formed it 
will be seen to differ materially from that of the untreated milk 
in its want of cohesion, breaking up when touched into separate 
' The exact amount necessary for the purpose will depend upon the quantity 
of milk and the strength of rennet employed. One drop of rennet to balf-i>- 
pint of milk will do for a rouffh approximation, which maybe modified accorc- 
injf to the result of experiment. 
