FILTERING MILK. 157 



although perhaps not quite so efficient as centrifugal force 

 (187, 1 88). Filtered milk certainly tastes cleaner and 

 sweeter than unfiltered milk. In removing the bacteria, how- 

 ever, the filter is practically of no value. In many tests there 

 has actually been found an apparent increase of bacteria after 

 the filtering. This fact is probably due to two causes. 

 First, many of the bacteria in the milk are likely to be 

 clinging together in masses, and each mass, when planted 

 upon a gelatin plate (see Chapter IX.), will appear as a single 

 colony, even though there be several in the mass. The pas- 

 sage through the filter tends to break up these masses 

 so that each bacterium may float freely. Each of these would 

 produce a colony, and there would appear to be an increase 

 in numbers, although there is really none. Second, it is 

 extremely difficulty to clean and sterilize thoroughly the sand 

 in the filters. If some bacteria and a minute amount of 

 organic matter are left in the filters, the bacteria are quite sure 

 to grow and presently become numerous in the sand. The 

 next lot of milk to pass through is actually contaminated with 

 bacteria during the filtering. If the sand is thoroughly 

 sterilized by heat after use this apparent increase in bacteria 

 during the filtering process becomes reduced or disappears. 

 But even then the filtering process cannot be looked upon as 

 a means of reducing the bacteria or increasing materially the 

 keeping property of the milk. The removal of the filth is 

 certainly desirable ; but filtering, by any device yet invented, 

 has no value in rendering milk less dangerous from the stand- 

 point of disease bacteria. Nor indeed does it seem possible 

 that such should be the case. The bacteria are very much 

 smaller in size than the fat globules, and a filtering machine 

 that would take out the bacteria would also take out the fat, 

 leaving only the poorest of skim milk. If we wish the milk 

 to pass the filter as milk, the pores in the filter must be large 

 enough to allow fat to pass, and then the bacteria will pass 



