BACTERIA IN MILK. 81 



bad flavor or the milk has been allowed to stand in some place 

 where it has absorbed foul odors. 



I have since determined the number of bacteria in milk from 

 several hundred dairies and find on the whole that they are rather 

 lower than could be expected in view of the numbers present 

 in most of the city milk. 



In one lot of sixty-seven dairies examined during the sum- 

 mer, 12 per cent showed less than 100,000 bacteria per c. c, 50 

 percent 100,000 to 1,000,000 per c. c„ 25 per cent 1,000,000 to 

 5,000,000 per c. c, and 13 per cent 5,000,000 and over per c. c. 



As I have said, however, there is some milk passed by the 

 tasters that contains large numbers of bacteria. It is my opinion 

 that if dairymen did not of necessity have to mix the milk from 

 different dairies to even up the amount of butter fat present, they 

 would be much better off as regards the keeping quality and 

 healthfulness of the milk, since a single can of poor milk will 

 spoil a dozen cans of the best when mixed with them. 



Another source of the bacteria in city milk is the can in which 

 it is delivered. Most milk cans are washed with hot suds and 

 rinsed in water. I have examined some fifty milk cans to deter- 

 mine to what extent they may infect the milk and find on the 

 average there is a sufficient number remaining in a washed can to 

 seriously affect the keeping quality of milk placed in it. The 

 method of examining cans was to place 100 c. c. sterilized water 

 in a can, shake it up thoroughly and empty back into a bottle. In 

 this way a part of the bacteria were removed with the water and 

 their numbers could be estimated by the usual process of plating. 



The following table shows the numbers of bacteria found per 

 c. c. of water for rinsing. A large proportion of the bacteria 

 found were of the lactic acid producing type. 



1 68,000 



2 560,000 



3 13/000 



4 728,000 



5 798,000 



6 2,240,000 



7 110,000 



8 180,000 



This means that if a quart milk-can like the above were 

 filled with sterilized milk, the milk would have added at once 

 from 2,600 to 224,000 bacteria per c. c, which number alone 

 would be large enough to cause it to change, if the milk received 

 improper care afterward, as is often the case. 



In butter-making, however, some of the bacteria are use- 

 ful rather than detrimental and play an important part in the 

 fermentation of cream previous to churning. Formerly it was 



