DAIRY METHODS 137 



A common method of cooling in large dairies is by causing it 

 to run over a metallic surface that is artificially cooled. This 

 cools and aerates the milk at the same time. 



Aeration. The practice of aerating at the time of cooling 

 the milk has become very general in the better dairies, and it is 

 undoubtedly of considerable value. It is accomplished by al- 

 lowing the milk to flow in a broad stream and a thin layer over 

 artificially cooled metallic surfaces. So far as concerns its 

 effect upon the bacteria of milk it is of no value at all, for it does 

 not decrease the numbers but, on the other hand, tends slightly 

 to increase them. It is easy to see that if the milk is for some 

 time exposed to a large surface of air, it will be constantly 

 absorbing bacteria. The number thus obtained, however, is 

 not great, and more probably come directly from contact with 

 the aerator itself. Both together do not furnish enough to over- 

 come the advantage to the milk from aeration. It is, however, 

 claimed by Marshall * that aeration increases the subsequent 

 rapidity of bacteria growth, since it removes the carbon dioxide 

 that is normally present in the milk which has a slight checking 

 action on bacteria growth. This has, however, been questioned 

 by others. The purpose of the aeration is both to cool the milk 

 by contact with cold surfaces and to remove some of the so- 

 called ''animal odors" that are commonly present. It should be 

 noticed that most of the "animal odors" are purely filth odors, 

 due mostly to manure. Milk that is drawn under perfectly clean 

 conditions, without chance for contamination or entrance of 

 manure, does not need aeration for the purpose of removing 

 odors. Such milk is best bottled at once without aeration and 

 cooled in the bottles. The need of aeration is directly propor- 

 tional to the amount of filth that enters the milk after it is 

 drawn. If aeration is adopted it should be carried on in a place 

 where the air is as pure as possible. To aerate in a room where 



1 Marshall. Ann. Rep. Mich. Bd. of Agri., p. 259, 1902. 

 Barthel. Rev. Gen. d'Lait, III., p. 294, 1904. 



