THE MILK CONTRACTOR 301 



dling is likely to be imperfectly pasteurized. After the machine is started 

 and running well interruptions from non-arrival of the milk at the plant, 

 breakdown of the machinery, neglect to keep up steam pressure, and other 

 accidents may cause failure. Besides, if the work is late or there is an 

 extra amount of milk to be pasteurized, the machine may be crowded at 

 the expense of efficiency. Also, since overheated milk is almost certain 

 to cause complaint from customers, there is the constant temptation to 

 pasteurize close to the lower temperature limit which means that the 

 heating it is apt to be insufficient to kill the pathogenic germs. So, it is 

 imperative that every pasteurizing plant shall employ efficient methods of 

 controlling the temperature and of recording that actually used during 

 the entire run. 



Ayres states that a competent man in a small plant by hand control 

 can regulate the pasteurizing temperature within 2 or 3 each way but 

 that in large plants mechanical control is much more certain for there 

 are numerous controllers that will run within 1 each way, of the desired 

 temperature. The underlying principle of automatic control is the regu- 

 lation of the steam flow by a diaphragm valve, operated by air pressure, 

 regulated by expansion and contraction in the controller bulb which is 

 placed in the milk outlet of the pasteurizer. By setting the controller 

 any desired temperature may be obtained. The bulb of the recording 

 thermometer is inserted in the milk as it leaves the pasteurizer and regis- 

 ters on a chart a continuous record of the temperature maintained during 

 the run. Care should be exercised that the controller and recorder bulbs 

 are affected only by the temperature of the milk for if they are exposed 

 to warm surfaces, or escaping steam or if they are placed where the cir- 

 culation of the milk is sluggish, erroneous results will be obtained. The 

 U. S. Department of Agriculture states that relatively few plants are 

 using automatic temperature controllers and urges their adoption, saying 

 that the power used in operating them is not greater than the steam wasted, 

 where hand control is used and estimating the daily overhead expense of 

 the instruments at 11 cts., or less than the wage of one man for an hour. 

 The temperature chart not only shows the temperature used at any period 

 of the process but in all holder systems, except those using retarders, 

 shows the period of time through which any temperature was maintained. 

 Thus these charts are valuable evidence as to the efficiency with which 

 the pasteurization is conducted. So, boards of health in some principal 

 cities of the United Stated require the use of continuous temperature 

 recorders and it would seem wise for other cities to follow their lead, for 

 such records are the best possible guarantee that pasteurization is being 

 properly done. The validity of the Chicago ordinance which requires 

 the use of continuous temperature recorders has been upheld by the courts 

 in Koy vs. the City of Chicago et al., 111., 104, N. E. R., 1,104. 



