6 BULLETIN" 973, U. S. DEPARTMENT OE AGRICULTURE. 
With vat pasteurizers it is not necessary for the operator to remain 
constantly at the steam valve, but it is very important that the 
steam be shut off at the proper time. Usually the steam must be 
shut off just before the milk in the vat reaches 145° F. The coil 
may then be shut down, although some operators favor allowing 
the coil to revolve during the holding process injorder to keep the 
temperature more nearly uniform and prevent the cream from rising. 
If the coil is run intermittently, there is much less danger of injuring 
the cream line than when it is run continuously. The outlet valve 
should not be opened to let the milk out of the vat to the cooler 
until 30 minutes have elapsed from the time the temperature of 
the milk reached 145° F., and no milk must have been added to 
the vat in the meantime. When continuous-flow pasteurizers are 
used with no temperature control, the operator must be at the 
steam valve constantly, and even then it is very difficult to prevent 
considerable fluctuation in temperature. 
When an automatic device for controlling the temperature is 
used, it is important that the device be properly adjusted at all 
times. 
Temperature recorders should always be used. They should be 
attached so as to show the temperature to which the milk has been 
heated. When vat pasteurizers are used the time of holding can 
be determined from these recorders provided the time it takes to 
empty the vat is known. Usually the apparatus is attached at a 
point near the bottom of the vat, and the record will show the total 
period from the time the milk first reaches the pasteurizing tem- 
perature until the vat is practically empty. The time required to 
empty the vat must, therefore, be deducted from the time of hold- 
ing, as shown on the record chart, in order to determine the length 
of time all the milk was held at the pasteurizing temperature. When 
all or a part of the cooling is performed in the vat the record chart 
will show the actual time of holding at the pasteurizing temperature 
for the entire vat of milk. 
With continuous-flow pasteurizers it is difficult to record accu- 
rately the time the milk is held at the pasteurizing temperature, 
and this can best be determined by means of an accurate timepiece 
and by means of accuracy tests. 
The charts from the recording instruments are a record of how 
the milk has been pasteurized, and are often valuable to show health 
officials as well as customers how the process was performed, 3 
CHECKING ACCURACY OF HOLDERS. 
The accuracy of all continuous-flow holders should be tested 
from time to time. Several different tests are used for this pur- 
pose. Automatically operated compartment holders should also be 
checked often to determine their accuracy. It is essential that the 
valve of the compartment which has been emptied should close 
tightly before the next lot of heated milk begins to flow in and that 
none of the valves leak at any time. 
3 For further information on pasteurization see U. S. Department of Agriculture Bulletins No. 342, 
Present Status of Pasteurization, and No. 890, Milk -Plant Equipment. 
