Preservation of Milk. 127 



most important desiderata, but in preparing milk for human 

 use, fulfillment of sanitary conditions is the first requisite. 

 It is to be regretted that milk dealers so frequently lose 

 sight of this requirement in their attempts to secure appa- 

 ratus that will handle large amounts so as to reduce the 

 cost of operation. Pasteurizing involves considerable time 

 and trouble, and it is better not to have the milk treated at 

 all than to have the process imperfectl} T performed. 



The various types of machinery that have been suggested 

 for this use may be grouped as follows, depending upon 

 their method of operation: 1 



1. Continuous- flow machines. 



2. Intermittent machines. 



The continuous-flow pasteurizers were originally designed 

 for the treatment of milk and cream for butter-making, but 

 in many cases they have been applied to the preservation 

 of milk for direct consumption. The difficulty with them 

 is not that the milk cannot be readily heated in the same, 

 but as customarily arranged there is no provision for the 

 retention of the milk at a temperature that would be fatal 

 to the organisms in the same. 



Continuous-flow pasteurizers. Apparatus of this class 

 vary much in detail, but all possess this common principle, 

 that the milk enters the machine in a continuous stream 

 and is generally discharged in the same way. The objec- 

 tion to this type of apparatus is that the time of heating 

 cannot be regulated with any certainty, although the tem- 

 perature can be controlled in part by varying the speed of 

 flow. Recent tests made at the Wisconsin Dairy School 



1 For a more detailed description of pasteurizing machinery, reference should 

 be made to Monrad's Pasteurization of Milk, or Weigmann's Conservierung der 

 Milch. 



