11 THE SOURCES OF BACTERIA IN MILK 25 



The masking effect caused by multiplication of the bacteria 

 already present makes calculations as to the extent to which 

 organisms are added on these premises difficult. Orr (loc. cit.) 

 has attempted with some success to measure the bacteria added 

 to milk from outside sources while the milk is in the hands of the 

 retailer and the consumer. Samples of milk were collected into 

 sterile bottles from milk in the ordinary mi Ik -containers in the 

 dairies selected, and then kept for a variable number of hours 

 on the counter in locked wire cages by the side of the con- 

 tainers. During the time of exposure, the milk from which the 

 sample had been taken was sold and dealt with by the retailer 

 in the usual way, covered or uncovered. After some time 

 samples were collected from the containers, and these and the 

 control samples were at once transmitted (in ice boxes) for 

 examination. The control samples had been kept at the same 

 temperature as the milk in the container but not exposed to 

 contamination, so that the added organisms could be estimated 

 by comparing the results of the two samples. Eleven such 

 experiments were carried out with the results shown in the 

 following table : 



Average increase due to contamination = 22 '7 per cent. 

 Average time in premises = 7 '7 hours. 



This table shows that the average increase is 22'7 per cent, 

 and Orr counts this as the percentage increase due to con- 



