114 Dairy Bacteriology. 



It might naturally be supposed that any method by 

 which dirt is removed from milk would improve the 

 keeping quality of milk, due to the reduction of bacteria, 

 yet while the straining of the milk at the time of milking 

 removes dirt of various kinds, it does not appreciably 

 enhance the keeping quality, owing to the fact that the 

 bacteria adherent to the dirt particles are washed off in 

 straining, and pass through the pores of the strainer. 



Filtration of milk. It is possible to remove all bac- 

 teria from water and other fluids and thus render them 

 sterile by passing through filters of unglazed porcelain. 

 This process can not be used with milk for the fat glo- 

 bules are larger than the bacteria (see Fig. 6) and any 

 process that would remove the latter would also remove 

 the former. The term "filtration" is applied to a pro- 

 cess used in some European cities for the removal of the 

 insoluble dirt that has been introduced into the milk. 

 Suitable containers are filled with layers of coarse sand 

 at the bottom and with finer sand at the top. . The milk 

 is introduced at the bottom and is forced upward 

 through the sand. Such a filtering process is a very ef- 

 ficient means of removing the dirt ; but unless the filters 

 are kept scrupulously clean, the bacteria are likely to 

 grow in the filtering material, so that the number of or- 

 ganisms in the milk may actually be increased by the 

 filtering process. It is necessary to remove the sand 

 daily and thoroughly wash and sterilize the same. The 

 extra care required in keeping these sand filters in sani- 

 tary condition has been the great objection to their em- 

 ployment in this country. Filters of other material such 

 as cellulose have been employed but with no marked suc- 

 cess. 



