MARKET MILK 167 



to insure the quality of the milk, the low temperature must be 

 maintained until it is consumed. 



METHODS OF COOLING MILK (Ross) 



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 neces- 

 sary 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 unevenly, and 

 by the time the milk in the center of the can is cooled, that 

 part near the walls of the can may be frozen. The fat is not 

 evenly distributed in frozen milk; therefore it is not so good 

 as normal milk. 



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. (See Plate VI.) This arrange- 

 ment obviates lifting the cans to any great height and prevents 

 dirt from washing into the tank. The top of the walls of 

 the tank should be faced with strap iron to prevent the cans 

 cracking the cement as they are lifted in and out. Some outlet 

 should be provided in the bottom of the tank so that it can 

 be easily and thoroughly cleaned as often as may be neces- 

 sary. It is almost impossible to prevent milk from spilling 

 into a cooling tank of this sort, and unless this is cleaned out, 

 the tank soon becomes unfit for use from a sanitary standpoint. 

 Outlets should be made at the top of the tank in order to carry 

 off surplus water and to prevent the cans from being flooded. 



Another type of cooling tank is made of galvanized iron faced 



