MILK-PLANT EQUIPMENT. 
11 
in the same vat for 30 minutes. The vat should be well insulated to 
prevent loss of heat during the holding process. The coils are some- 
times allowed to revolve during the holding process so that the cream 
will not rise and the milk will be held at a more uniform temperature. 
One of the dangers of the vat system is that the milk may be 
started out of the vat before it has been held for 30 minutes. At one 
plant it was noted that the milk was started out of the vat 11 minutes 
after it was brought up to 145° F. It was then allowed to go over 
a tubular cooler. The cooling process took about 35 minutes, so that 
while the last of the milk was held for 45 minutes the first part was 
held for only 11 minutes. 
Milk may be cooled in the vat by forcing cold water and brine 
through the coils, or by water jackets, or it may be run over a separate 
cooler. Where it is cooled in the vat the process is very slow, and the 
milk will not show a clear cream line so readily as when cooled 
quickly over a separate cooler. Cooling in the vat also may cause 
leaks in the coils, due to the repeated sudden changes in temperature. 
A continuous system may be obtained with the batch system of 
pasteurization by using a battery of three or more vats. While the 
third vat is being filled and heated the milk in the second vat is 
being held and that in the first vat is being drawn off to the cooler. 
The time required to empty the vat depends somewhat on the capacity 
of the cooler, so it is important that the cooler be of ample capacity. 
The capacity of the vats may be increased by preheating the milk 
by some form of heater before it enters the vats. If the milk is 
slightly cooled in the vats before it is allowed to go to the cooler the 
° F. will be made uniform for all the milk 
Fig. 5. — Pasteurizing milk by the batch or vat system. 
