168 Milk and Its Products 
that temperature for at least thirty-six hours. 
Further, since the milk must remain for so long a 
time, it is essential that the atmosphere to which it 
is exposed be pure and free from currents of air 
and particles of dust. These conditions can be 
secured in a clean, cool, well ventilated cellar. 
In the shallow pan system, the depth of milk should 
be from 2 to 4 inches. Occasionally water, usually 
running water, is used to secure a quick cooling 
and more even temperature surrounding the milk, 
and when this is done the depth of the milk may 
be increased to 4 or 6 inches. Under the shallow 
pan system, the cream is separated from the milk 
by removing it from the surface. with a thin flat: 
instrument, usually made of tin, and ealled a 
skimmer. In this method of removal, it is not 
possible to completely remove all the cream that has 
risen to the surface, and in removing the cream 
more or less of the milk is taken with it, so that 
in the process of skimming there is a considerable 
loss of fat and a thin cream is always obtained; 
but even if the losses in the skimming operation 
could be obviated, the separation of the particles of 
fat in the shallow pan system is less complete than 
in either of the others. Under ordinarily good con- 
ditions, 20 per cent of the fat in the milk is lost 
when the shallow pan system is used. Skimmed milk 
containing: less than .5 of 1 per’ cent of fat is 
rarely obtained under this system. 
Deep setting system.—About thirty years ago it 
was discovered that-if milk could be set iu vessels 
when first drawn, and, rapidly cooled to a tempera- 
