118 PASTEURIZATION OF MILK. 



hold with kitchen utensils, it being necessary to purchase only 

 a thermometer. The bottles of milk for individual feeding of 

 the infant are placed in a covered vessel of water provided with 

 some simple device, like a perforated pie tin, to keep the milk 

 bottles from touching the bottom. The vessel and contents 

 are heated on the stove until the water reaches 155"F. The 

 whole is removed and allowed to stand for half an hour, after 

 which the bottles of milk are rapidly chilled. 



High-temperature pasteurization. The demand for speed 

 and economy has been a factor in the development of pa,steur- 

 ization at high temperatures for a short time. The heating is 

 accomplished in various styles of continuous-flow machines, 

 in which the milk passes over heated surfaces in a thin layer. 

 The range of temperature extends from 176°F. (80°C.) to 185°F. 

 (85°C.), and the time of exposure varies from a few seconds 

 upwards. The momentary exposure to 176°F. (80°C.) is con- 

 siderably above that necessary to satisfy the requirement for 

 killing tubercle bacilli. The highet limit has been found 

 necessary to compensate for defects in the thoroughness with 

 which the continuous-flow machines do their work. As a class 

 the machines are defective, in that all the milk passing through 

 may not be heated to the desired temperature. This is attribu- 

 table to a fundamental characteristic of the behavior of fluids. 

 The parts of the stream not in actual contact with the heated 

 surfaces will flow more rapidly than those portions influenced 

 by friction. The employment of a temperature as high as 

 176°F. (80^C.) violates conditions required for market milk 

 in the United States, but the process is unobjectionable in milk 

 used for butter-making and is extensively employed in Den- 

 mark for this purpose. 



Commercial pasteitrizaiion . A modification of the foregoing 

 method, by the use of the same types of continuous-flow 

 machines, is quite generally used for the purely commercial 

 purpose of preventing the souring of milk. It hides, and par- 

 tially undoes, the results of filthy methods of production and 

 poor care in shipment. 



The temperatures vary from 140° to 165°F. (60° to 74°C.), 

 more often the lower temperature, followed by rapid cooling. 



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