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 called 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 in vessels 

 when first drawn, and x rapidly cooled to a tempera- 



