CONTAMINATION OF MILK 57 



position as during milking. Wetting the inner surface of 

 the pail prevents contamination from it. The Exposure should 

 not be continued for more than thirty seconds. The results 

 should be finally expressed in numbers of bacteria that would 

 fall into a 12-inch pail under the conditions obtaining in the 

 trial during the average time of milking (five minutes). 



Animals in varying degrees of cleanliness should be selected. 

 Exposures should also be made in a similar manner under the 

 same animals immediately after their flanks and udder have 

 been thoroughly moistened with a clean damp cloth. 



Exercise. Each student will prepare four plates of lactose gelatin 

 and allow them to solidify : (1) expose two plates for thirty seconds, 

 as described, under animals which have not been previously treated ; 

 (2) expose the remaining two under animals whose udders arid flanks 

 have been thoroughly moistened with a clean damp cloth. 



Contamination from manure. The amount of contamina- 

 tion which the milk receives from animals whose udders and 

 flanks are soiled with manure or mud is not determined with 

 any degree of accuracy by the exposure of plates in the manner 

 described, since each particle of dirt may cause the develop- 

 ment of a colony, although it may have carried hundreds of 

 bacteria which would, in large part, be scattered through the 

 milk, on account of the solubility of the manure. To gain a 

 more correct idea of the extent of contamination that may 

 come from this source, a quantitative determination of the 

 bacteria in manure should be made by collecting some, if pos- 

 sible, from the flanks of animals. One gram should be weighed 

 out on a piece of sterile filter paper. Place this in a definite 

 amount of sterile water and shake until the manure is dis- 

 integrated and dissolved as completely as possible. Prepare 

 plates with varying dilutions, as in the quantitative analysis 

 of milk. 



