34 CHANGES IN MILK CAUSED BY BACTERIA. 



at various stages in the handling, and watching for the appear- 

 ance of ropiness after the milk has stood a day in a cool place. 

 Frequently it will be found that the milk as it comes from the 

 stable is normal and that the source of contamination is in the 

 milk room. If such is not the case, the dampening of the 

 udders of the cows, before milking, will be necessary (41, 42). 



Germicidal activity. When samples of freshly drawn milk 

 are examined at frequent intervals for numbers of bacteria, the 

 fact becomes evident that the total numbers do not immediately 

 begin to increase. The numbers may decrease or remain sta- 

 tionary for a period before the increase of bacteria begins. 

 Immediate increase of total numbers is distinctly the exception. 

 The phenomenon varies in degree in the milk of individual 

 cows (20). 



Kunze (22) determined the number of bacteria in milk when 

 freshly drawn, after thorough cooling, after sixteen hours at 

 6-8°C. and after forty hours at 6-8°C. He averaged the results 

 of 35 samples collected at periods covering a whole year. ^ The 

 freshly drawn milk was very clean, averaging only about four 

 hundred colonies per cc. Thorough cooling alone reduced the 

 original count by 11.4% . After sixteen hours at 6-8°C. it was 

 reduced by 16.8%. At forty hours the count was 33.09% of 

 the original, a reduction of 21.69% from the count taken just 

 after cooling. 



The length of the period of decline, in any case, is governed 

 by the temperature at which the milk is kept. For instance, 

 at 37°C. (98°F.) the period-lasts about three hours ;' at 21°C. 

 (70°F.), three to twelve hours ;" atl3°C. (55°F.), about twenty- ' 

 four hours; at4°C. (40°F.); forty-eight hours; at I'C. (34°F.), 

 six to eight days. ' 



The temperature goveriis the extent of decrease. At 70°F. 

 the decrease is marked, frequently 50% or more in market 

 milk. At 55°F. it is less marked, and at 40°F. still less. 

 These observations are presented in a graphic way in Fig. 7. 

 The chart (Fig. 7) shows the changes in the number of bac- 

 teria in three portions of the same original sample of milk, 

 kept at different teniperatures. The sample Xvas drawn from 

 the cow with the strictest aseptic precautions. 



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