138 . PRACTICAL DAIRY BACTERIOLOGY 



the dust of the barn has a chance to enter is bad. Aeration 

 improves the flavor of milk that is produced under any but the 

 very best of conditions, but it does not decrease the number of 

 bacteria and is of no value to milk that is to be used in butter 

 making or cheese making, except for its rapid cooling of the 

 milk. 



Filtering Milk. The practice of filtering milk to remove the 

 dirt has been adopted in some places, especially in Europe. The 

 filter is commonly made of layers of sand of different degrees 

 of fineness, and in the best methods the milk is forced through 

 the filter from below upwards. These filters are not very widely 

 in use. So far as bacteria are concerned they do not accomplish 

 anything, for they are never fine enough to remove them. They 

 do remove some of the dirt. They are difficult to clean thor- 

 oughly and are not, in general, to be recommended. 1 



Centrifugal Force as a Method of Cleaning Milk. In some 

 dairies the use of centrifugal machines has been adopted for 

 cleaning milk. In doing this the rapidity of the rotation of the 

 machine is not as great as when it is desired to separate the 

 milk from the cream. Careful testing shows that this treatment 

 certainly does remove a large part of the dirt from the milk. 

 One who examines the mass of sediment that collects in the 

 drum of such machines cannot doubt that a large amount of dirt 

 is removed. Further, an examination of the milk itself and an 

 estimation of the milk which it holds shows that there is a 

 noticeable reduction. Curiously enough, however, such a proc- 

 ess does not reduce the number of bacteria. It certainly would 

 be expected that, if it removes dirt, it would at the same time 

 reduce the bacteria. On the contrary, the number of bacteria 

 usually appears to be increased by passing through the centrif- 

 ugal machine, and is practically never reduced. The reasons 



iWeigmann. Milchztg., p. 289, 1901. 

 Weil. Milchztg.. p. 739, 1901. 

 Vieth and Martiny. Zeit. f. Fleisch u. Milch Hyg., XI., p. 326, 1901. 



