256 Milk and Its Products 
fold and important part. The curd is contracted to 
some extent. by the action of the rennet still con- 
tinuing; also to some extent by the direct applica- 
tion of the heat, but to a greater extent by the 
effect of lactic acid upon the curd, the production 
of lactic acid being greatly favored by the increased 
temperature. It is desirable that this contraction of 
the curd in the cooking stage go on uniformly through- 
out the whole mass. The heat is therefore applied 
gradually and with constant stirring. As the parti- 
cles of curd shrink in size, the tendency for them 
to unite in masses constantly diminishes, and they 
are broken with greater difficulty, so that while at 
first extreme care is necessary to prevent the par- 
ticles of curd from becoming broken, as the temper- 
ature rises, the stirring may become more rapid and 
vigorous. The heat should be raised slowly from 
the setting point of the milk up to 98° F., and it 
should not be more rapid than 2° in five minutes; 
and unless the lactic acid develops very rapidly it 
is better to take even more time than this. When 
the temperature of 98° F. is reached, the stirring 
may cease and the particles of curd be allowed to 
settle upon the bottom -of the vat until the curd is 
thoroughly shrunken and a sufficient amount of 
lactic acid has been developed. At the end of the 
cooking stage the particles of curd should have 
shrunken to less than one-half of their former size,- 
and should have become so hard that when pressed 
together . between the hands, and the pressure sud- 
denly relaxed, they will fall apart, and show no 
