MILK-PLANT EQUIPMENT. 
17 
Positive holders. — By positive holding is meant that the milk, after 
being heated to the desired temperature, remains in a certain com- 
partment a definite length of time and is then cooled. Usually the 
milk is drawn from the compartment and cooled over a separate 
cooler. Generally the filling and emptying of the compartments of 
the holder are controlled automatically (fig. 9) , but in the case of the 
batch system, where the vat is used as a holder, the valves are con- 
trolled by hand and it is necessary for the attendant to open and 
close them at the proper time. Some compartment holders also are 
controlled by hand valves. Holders should be so insulated that little 
heat is lost while the milk is being held, and with positive holders of 
the automatic type it is very essential for the valves to be accurate. 
The valve of the compartment to be filled should always be closed 
tight before the milk begins to flow in; otherwise some of the milk 
will pass through without being held. With any holder it is im- 
portant that all pipes or " pockets " in which the milk is held during 
the heating or holding processes be well insulated, so that the holding 
temperature can be maintained. 
Continuous holders. — With continuous holders or retarders the 
milk passes through the apparatus in a continuous flow. This flow 
Fig. 9. — Automatic holding device for the pasteurization of milk. 
40336°— 25— Bull. 890 3 
