170 CONTROL OF THE MILK-SUPPLY. 
they begin to develop with great rapidity. But if the milk is at once 
reduced to a low temperature, the bacteria that have found their 
way into the milk will not grow very rapidly. These facts are so 
simple as hardly to require statement; but, unfortunately, many a 
dairyman, although he may theoretically understand them, fails to 
appreciate their importance. The essential point to be" emphasized 
is the necessity of immediately cooling the milk to a temperature as 
low as 40 F. if possible. It 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 bacteria 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 niters 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. Centrifugal force is sometimes used for cleaning the milk, 
and is fully as efficient as sand filtering. 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. 
II. TRANSPORTATION PROBLEMS. 
Under this head will be included not only methods of treating 
milk during transportation, but also of preparing it for preservation 
during the transportation or until it is consumed. Milk, as a rule, 
receives no preparation for transportation, except that of cooling and 
placing in clean cans. Then, if rapidly shipped and kept cool, it 
should remain good until some time after it has reached the con- 
sumer. But the rapidity of bacteria 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. 
