16 BULLETIN SCO, U. S. DEPARTMENT OF AGRICULTURE. 
Fig. 8. — Pasteurizer and cooler of the internal-tube type. 
6. Easy sterilization. If the ends of the tubes are closed steam can be 
introduced under pressure and the apparatus thoroughly sterilized. 
7. High temperatures of the heating medium not necessary. 
Some of the disadvantages of internal tube heaters are : 
1. A large quantity of piping to be cleaned. 
2. The milk may be agitated considerably by passing from one pipe to 
another. 
3. Some forms are quite expensive. 
4. Some types are not adapted to increase in capacity by adding addi- 
tional units. 
It is very important that pumps supplying milk to heaters of this 
type are not run too fast and that a uniform quantity of milk is 
allowed to flow to the pump. All machines should have automatic 
temperature controls for maintaining a uniform temperature. 
HOLDERS. 
After the milk is heated to the desired temperature (145° F.) it 
must be held at that temperature for 30 minutes. The process of 
holding is usually carried on in an apparatus separate from the 
heater or cooler. 
In general, there are two types of holders, the positive holder and 
the continuous holder or retarder. 
