THE MILK CONTRACTOR 



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bacteria. Since the disease-producing organisms and viruses that are 

 commonly carried in milk have thermal death points below the tempera- 

 ture of pasteurization, the assumption is generally correct but it is entirely 

 possible that heaters that heat the milk unequally, or holders that do 

 not hold all of the milk the full period, or retarders that permit the passage 

 of portions of the milk more rapidly than others, or slack operation of the 

 plant might show bacterial reductions and yet there might be failure to 

 kill all pathogenic bacteria. Low bacterial counts backed by tempera- 

 ture record cards and by faithful inspection are a safer guarantee of 

 efficiency than the counts alone. It may seem to some that the logical 

 way to test the efficiency of pasteurization would be to look for disease 

 germs in the pasteurized milk and so it would be, if bacteriological tech- 

 nique was developed to the point where this could be quickly and cer- 

 tainly done, but it is not. 



Method of Cooling. After pasteurization milk must be cooled. It 

 was formerly held that sudden cooling was essential for it was believed 

 that it helped to destroy the bacteria. Ayres and Johnson made some 

 experiments Table 84 which show that this is not so. The value of 

 the sudden cooling lies in this; that after pasteurization the milk is pre- 

 vented from slowly cooling through temperatures around 100F. where 

 bacterial multiplication is sure to be rapid. 



TABLE 84. EFFECT OF SUDDEN COOLING ON THE BACTERIAL CONTENT OF PASTEUR- 

 IZED MILK (AYRES AND JOHNSON) 



Later these authors studied the question of how quickly milk must be 

 cooled in order to check bacterial growth and in the course of their inves- 

 tigation made three experiments to show respectively the effect on the 

 bacterial content of cooling quickly, slowly and not cooling at all. Milk 

 was pasteurized in bulk and the hot milk filled into three steamed hot 

 quart bottles. One bottle was cooled in ice water in % nr - to 50F. and 

 refrigerated at 45F. Another was cooled in a blast of air at room 

 temperature for J^ hr. during which time the temperature of the milk 

 dropped from about 145F. to 80F. in 5 hr. after which it was placed in a 

 refrigerator at 45F., where it cooled slowly in still air. The third bottle 

 was cooled for J^ hr. in an air blast at room temperature and held 

 at about 75F. throughout the experiment. The results are given in 

 Table 85. 



