The Work of Acidity in Cheese-maicing. 
281 
flocks. It has. in fact, all tlie characteristics of what is called 
a flocculent curd, and if any attempt is made to sepai'ate it from 
the whey, the latter will become turbid from the minute frag- 
ments of curd which will separate with it, forming the '• white 
whey "' which is so familiar to cheese-makers with a defective 
curd in the milk- vat. 
It will thus be seen that whilst the presence of acidity in 
milk is not es.-^eiitial to enable rennet to exercise the peculiar 
power which it possesses of coagulating it, a certain amount of 
acidity in the milk when the rennet is added to it gives to the 
curd a degree of tenacity which renders it firm and enables it to 
be handled without loss of any material portion of its structure 
in the whey. 
We have already noted that, in the case both of the milk 
coagulated spontaneously by souring, and in that coagulated by 
rennet in the presence of a small degree of acidity, the curd 
possesses the power of spontaneous contractility (which is really 
an indication of eladicitij) ; but if we compare with it in this 
respect the curd of the alkalised milk, we shall find that the 
contractility of the latter is very feeble. Instead of the rifts 
produced by the knife steadily widening, and the blocks of curd 
as steadily contracting, the curd remains an inert mass, which 
does in time show some slight amount of contractility, but not 
enough to give it anything like the firmness of the other samples, 
or to free it from the whey, as has occurred with them . 
The indication thus afforded of the effect of a small amount 
of acidity in communicating elasticity and tenacity to the curd 
formed in milk by rennet may be still further illustrated by 
taking yet another sample of fresh milk and adding to it enough 
of a solution of lactic acid (the acid produced in milk spon- 
taneously during souring) to give it from ten to twelve degrees 
of acidity, as determined by the acidometer. If the milk thus 
artificially soured be treated with comparatively the same 
quantity of rennet used in the previous experiments, and under 
the same conditions of temperature, &c., it will be seen to 
coagulate in a much shorter time than before ; and if the curd 
be sliced, as before, the readiness with which the blocks will 
contract, and the comparative firmness which they will present 
when handled, will give proof of a high degree both of elasticity 
and tenacity. 
From this short series of experiments, which are so simple 
that any person of ordinary intelligence can perform them with 
the aid of a few cups or basins, of solutions of soda and lactic 
acid, and of common litmus-papers (if an acidometer be not 
available), it is easy to learn in a thoroughly practical way what 
VOL. II. T. 3.— G U 
