44 THE BOOK OF BUTTER 



" The importance of cooling milk and of keeping it cold 

 is still further emphasized in Table X which sets forth 

 the results of keeping, at a high temperature, milk that 

 had a low initial bacteria count. In each case the milk 

 was held at a high temperature for a period of two hours, 

 and it will be noted that in every case there was a large 

 increase in germ-content." 



It would seem from the figures. given by Stocking, and 

 by Ross and Mclnerney, that there is no one thing to 

 prevent the growth of bacteria in milk more important 

 than cooling, and the maintenance of a low tempera- 

 ture. 



34. Methods of cooling milk.^ — " Milk becomes cool, 

 of course, when it gives up its heat to some substance 

 colder than itself, and in order to have a rapid exchange 

 of temperatures between two substances it is necessary 

 that they have approximately the same density. On 

 account of the great difference in density between air and 

 milk, the latter will cool very slowly in air even though the 

 temperature of the air is rather low. If milk is allowed to 

 cool by standing in a cold atmosphere, it will do so un- 

 evenly, and by the time the milk in the center of the can 

 is cooled, that part near the walls may be frozen." This 

 factor must not be overlooked. 



35. Tanks. — Ross and Mclnerney ' write : " On 

 farms milk is most often cooled by setting the cans con- 

 taining it in a tank of water. The most convenient and 

 in the long run the cheapest kind of tank for this purpose 

 is made of cement and sunk in the floor so that only about 

 twelve inches of the sides extend above it. This arrange- 

 ment obviates lifting the cans to any great height and 



> Ross, H. E., and Mclnerney, T. J., Cooling Milk, Cornell 

 Reading Course Lesson No. 102, 1915. 



