50 Agricultural Bacteriology. 



out for two reasons: (1) to prolong the time during 

 which the milk will be fit for use in the household, (2) 

 to destroy any pathogenic bacteria the milk may hap- 

 pen to contain. 



In Germany milk is usually heated in the home. Hot 

 milk is a regular item on the bill of fare of all German 

 restaurants. The milk used for tea and coffee is also 

 heated. This same custom also prevails in many of the 

 southern countries as Mexico and Cuba. The people 

 of these countries are accustomed to the peculiar taste 

 of heated milk, which is objectionable to the American, 

 accustomed to raw milk. In commercial pasteurization 

 it is necessary to produce an article that the consumer 

 will like. Therefore the demand here is for a milk that 

 shows no abnormal flavor such as a cooked taste. 

 Heated milk not only acquires this taste, but it loses 

 its power of creaming to a great extent. Its property 

 of curdling with rennet is also injured. If cream is 

 pasteurized, it appears thinner than raw cream. All 

 of these are objectionable changes from the standpoint 

 of the consumer. 



As commercial pasteurization is done largely to pre- 

 serve the milk, the temperature must be sufficient to 

 kill most of the bacteria present, and especially the dis- 

 ease-producing types. The most resistent of these is the 

 tubercle bacillus and this of all the pathogenic bacteria 

 is most often to be found in milk. It has been deter- 

 mined by careful trials that if the milk is heated to 

 140 F. for twenty minutes, the tubercle bacilli will be 

 destroyed, or if raised to 160 F. and the exposure 

 made for a moment they will also be killed. Both of 

 these ways are used in commercial work. The first is 

 done in the discontinuous pasteurizing machines in 

 which the milk is heated for any length of time to any 



