90 Dairy Bacteriology. 



are anxious to escape the exactions that must be met lay 

 all who attempt to handle milk in the best possible 

 method. Dirty milk drugged with these chemicals is fre- 

 quently able to falsely pass as an extra good product. 



88. Physical agents. The physical methods of milk 

 preservation as a rule are much more feasible than the 

 chemical, as they do not injure the nutritive value of the 

 fluid to such an extent. The efficiency of physical meth- 

 ods of preserving milk is dependent upon the elimination 

 or destruction of bacterial life, or at least an inhibition 

 of their powers of growth. Some methods, such as the 

 use of electricity, or of compressed air, or different gases 

 have been suggested, but a .practical treatment of milk 

 with these agents has not as yet been devised. Different 

 methods of nitration have been tried. Gravel or sand 

 niters have been recommended for purifying milk- supplies. 

 In these the suspended foreign matter, and a part of the 

 bacteria can undoubtedly be removed, but the incon- 

 venience of sterilizing such a filter would seem to be 

 considerable. Filters of this character have, however, 

 been satisfactorily used by the Copenhagen Dairy Co., for 

 a number of years. Germ-proof filters, such as the Pasteur 

 filter that are so efficient in water purification, are not 

 applicable to milk as they clog so quickly. Recently the 

 use of cellulose as a filtering substance has been proposed, 

 and according to Backhaus and Comlien, 1 this is satis- 

 factory from a bacteriological as well as a mechanical 

 stand-point. 



89. Freezing-. Chilling the milk has always been 

 practiced as a means of enhancing the keeping quality, 

 but recently the attempt has been made to transport milk 

 for long distances in a frozen condition, in which state 

 bacterial growth is entirely suspended. Milk can be pre- 



1 Ber. Landw. Inst. Univ. Konigsberg, 2: 12, 1897. 



