168 MANUAL OF MILK PRODUCTS 



with iron at the top and the bottom. (See Plate VI.) Such 

 a tank is not so serviceable as one made of cement, but it is 

 more durable than a wooden one and is easier to keep clean. 

 A galvanized iron tank large enough for cooling four or five 

 cans of milk may be bought for eight to ten dollars. 



In size the tank should be large enough to hold the required 

 number of cans and to allow about three inches between each 

 can and about four inches between the cans and the walls of 

 the tank. The larger the tank, the greater is the amount of ice 

 needed to cool the water around the cans ; therefore the tank 

 should be no larger than necessary. It must of course be deep 

 enough to allow the water to rise around the necks of the cans. 

 Refrigerating material. 



The refrigerating material most commonly used in cooling 

 milk in tanks is cold water or ice water. It is generally neces- 

 sary to use ice, since few wells or springs furnish water sufficiently 

 cold to cool milk to the proper temperatures. The amount of 

 ice necessary can best be determined by experiments, because 

 it varies with the amount of milk to be cooled, the temperature 

 of the surrounding atmosphere, and the temperature of the 

 water in which the ice is placed. 

 Effect of stirring milk during cooling in tanks. 



The cooling process, in order to be thorough, requires more 

 than setting the can of milk in a tank of water; the milk 

 must be stirred frequently. If the milk is not stirred, that 

 which is near the walls of the can will become cold, while that 

 in the center of the can will, for a long time, maintain a high 

 temperature favorable to the growth of bacteria. This is 

 shown by Ross and Mclnerney in the table on page 169. 



According to this data, at the end of twenty minutes the 

 difference in temperature due to stirring the milk varied from 

 3° to 17° F., and the average difference in temperature between 

 the milk stirred and not stirred was 9.7° F. This average drop 

 in temperature, 9.7° F.,in twenty minutes, due to stirring, means 



