6 BULLETIN OTS, U. S. DEPAHTMENT OF AGEICtTLTUEE. 



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 in order 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,^ 



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. 



