TRANSPORTATION PROBLEMS L 82 
is just as necessary to cool clean milk as it is to cool dirty milk. 
Unless it is done, the cleanest milk will soon contain as many bac- 
teria as the dirtiest milk. 
Straining and Filtering—The long-continued practice of 
straining the milk through a metal strainer or through cloth has 
in its favor the fact that it will remove the larger particles of dirt; 
but it does not remove the bacteria, for they will pass through 
any strainer. Sand filters have also been used by some dairy com- 
panies, and these are more efficient than simple straining. But 
these filters are not of very much value and they are not widely 
used. One centrifugal machine, known as a clarifier, has been 
widely advertised within recent years as a milk-purifier and is 
now in fairly common use. All of these means, while effective 
in removing the large particles of dirt, are practically of no value 
in removing the bacteria, which show as high numbers after such 
treatment as before. The consumer should be sold clean milk, 
not cleaned milk. 
IJ. TRANSPORTATION PROBLEMS 
Under this head will be included not only methods of treating 
milk during transportation, but also of preparing it for preserva- 
tion during the transportation or until it is consumed. Milk, 
as a rule, receives no preparation for transportation, except that 
of cooling and placing in cans that have at least been washed. 
Then, if rapidly shipped and kept cool, it should remain good until 
some time after it has reached the consumer. But the rapidity 
of bacterial growth, especially in hot weather, makes it difficult 
to transport milk, in good condition, for very long distances. 
Consequently, careful search has been made for some method of 
treating milk so as to preserve it. 
The Use of Preservatives.—It is easy to add to the milk 
various chemicals which will prevent the growth of bacteria, and 
consequently preserve the milk. Many such substances have 
been used. There are quite a number of preservatives on the 
