28i 
The Work of Aciditij in Cheese-making. 
From all of these facts it follows that the curding of milk 
by rennet as practised in cheese-making is by no means a simple 
phenomenon. We will dismiss from our consideration here the 
effect on the process of variation of temperature and of variation 
in the character of the casein itself (dependent on variation of 
food, period of lactation, &c.), and will confine our attention 
exclusively to the relations which exist between the rennet 
and acidity. It is obvious that, so far as the mere coagulation 
of the casein is concerned, these two agents are capable of 
replacing one another, but in each case the curd obtained will 
differ materially both in its primary and in its secondary charac- 
teristics. By using excess of rennet we may, it is true, develop 
a considerable contractile power in the curd, but then the 
secondary effects of such an excess in promoting premature 
decomposition render its employment for that purpose fatal to 
the keeping quality of the cheese. Hence the need of a certain 
amount of acid to co-operate with the rennet and to give the 
curd a firmness which it would not otherwise possess. 
The contractility which the curd exhibits in its earlier 
stages is easily destroyed, depending as it does on the main- 
tenance of a proper degree of warmth. If the curd is chilled, 
and in proportion as it is so, its contractility will be paralysed, 
the whey being retained in it in a degree which will defy all 
the powers of the jjress to extract it, and then good-bye to any 
prospect of making a sound cheese. 
It is often asserted that it is during the development of 
acidity in the whey, before the latter has been drained from 
the curd, that the most important influence of this agent is 
produced. But this is a mistake. For, if the acidity of the 
whey be tested whilst it is in the vat, it will be found to increase 
very slightly, if at all, during the time for which it is safe to 
leave it there. The error has arisen from mistaking the effect 
of the heat of the whey for that of its acidi'li/. It is after the 
whey has been lifted from the curd, and the latter has been ex- 
posed to the combined effect of heat and air, that its acidity 
increases so rapidly. 
lleference may hero be made to a rather paradoxical fact 
connected with tlie development of acidity in milk, which )nuy 
be noted in making the experiments described above. If, after 
the curd has been formed in each of the samples of renneted- 
milk (assuming that they have been meanwhile kept at a suit- 
able teniperature), we test them at intervals we sliall find 
that the acidity in tlie whey of the sample that had been 
artificially soured at starting, though greater ])y sevei-al degrees 
tlian it was at the outset, is not appreciably more (and may 
