MILK-PLANT EQUIPMENT. 7 
CLARIFIER. 
The clarifier (fig. 4) is a centrifugal machine used to remove visi- 
ble dirt from the milk. There are on the market several clari tiers 
that do very efficient work. 
PASTEURIZATION. 
Pasteurization is quite generally recognized as necessary unless milk 
has been produced and handled under special conditions that make 
its use safe in the raw state. The proper pasteurization of milk con- 
sists in heating it to 145° F., and holding it at that temperature for 
30 minutes. All disease-producing bacteria which may occur in milk 
are destroyed by proper pasteurization, and a considerable quantity 
of other bacteria are also destroyed, so that, provided the milk is 
properly taken care of after pasteurization, its keeping quality will 
be improved. 1 
Formerly milk was pasteurized to a large extent by the " flash " 
system. By this system the milk is heated to 155° F. or higher and 
then cooled. Exhaustive laboratory investigations, as well as actual 
Fig. 3.— Milk-storage tanks used in large plants to store milk at low temperature before 
it is pasteurized. 
practice on a commercial scale, have shown that this system is not 
satisfactory, and that positive holding is necessary. 
1 For detailed information on pasteurization see Bureau of Animal Industry Circular 
184 Bureau of Animal Industry Bulletin 161, and Department Bulletin 342. 
