432 ENGINEERING ASPECTS OF PASTEURIZATION 



are also desirable to stop the flow of milk whenever the temperature falls below a 

 certain point which is well within the margin of safety for effective pasteurization 

 with the equipment being used. 



Unsatisfactory thermometers. — The use of a recording thermometer alone as a 

 guide for temperature control, as practiced extensively in commercial operation, does 

 not give satisfactory accuracy for positive pasteurization. This is for the reason that 

 recording thermometers frequently get out of adjustment, owing to the delicate 

 mechanism, jarring, and rough handling they receive. It is not uncommon to find 

 one registering from i° to 4° off. Many of the charts used have small 2° spacing, and 

 at times the ink line made by the recorder pen may be a full space wide. These factors 

 combine to make the practice of relying on the recording thermometer for tempera- 

 ture control unsatisfactory from the point of view of both the health officer and the 

 milk dealer. If registering too high, the pasteurization temperature is not being 

 reached. If registering too low, the cream volume may be reduced. The use of a small 

 mercury thermometer or floating dairy thermometer to control the temperature of 

 the milk is unsatisfactory also, since it is very difficult to read accurately the fine 

 degree spacings on them when immersed to the proper point in a vat with steam ris- 

 ing from the milk. 



The remedy is to provide an accurate mercury-indicating thermometer on each 

 holder for temperature control with the scale at the side or above the holder. Scale 

 divisions of at least -/(•- inch per degree eliminate guesswork and make reading easy. 

 The scale should be installed and tilted in such a manner that the operator can read 

 the thermometer easily from a standing position. 



Both a recording and an indicating thermometer should be provided for each 

 vat used for pasteurizing milk. On continuous-flow and pocket-type holders these 

 two thermometers should be provided on both the inlet and outlet headers. The re- 

 cording thermometer chart should preferably have wide scale divisions equivalent 

 to at least -jV inch per degree on a twelve-hour chart, so as to give a clear picture for 

 the health officer to judge how satisfactorily the requirements as to time and tem- 

 perature for pasteurization are being fulfilled. The health officer should insist on the 

 complete pasteurizing operation being shown on each chart. The recording tempera- 

 ture line should start and end at room temperature and should be continuous. An 

 empty space may indicate that the pen was lifted to avoid recording some unsatis- 

 factory point in operation. 



FLASH PASTEURIZATION 



As previously mentioned, the flash method of pasteurization consists in rapidly 

 heating milk to the desired temperature followed by immediate cooling without an 

 appreciable holding period. It was used extensively prior to 1914, but rejected by 

 health officers due to failure of the equipment to heat the milk uniformly to the same 

 temperature. 



While bacteriologists have made several determinations showing that a tempera- 

 ture of 160° F. for sixty seconds would destroy tubercle bacilli, milk-heaters and 

 temperature-controllers had not been sufficiently perfected prior to 1914 to secure 

 satisfactory results in commercial operation. Whether they have been sufficiently 

 perfected at this time is still questionable, although enormous progress has been made. 



