24 BULLETIN 744, U. S. DEPARTMENT OF AGRICULTURE. 



In the northern section of the United States milk is frequently 

 shipped to the city during the winter when the temperature of the 

 air is below zero. Since milk freezes at a temperature between 

 31° and 29° F., it is important that some provision be made to pre- 

 vent freezing in transit. A test, therefore, was conducted to deter- 

 mine the efficiency of the same cans used in the previous tests for 

 preventing freezing. Because of the limited space in the refrigerator 

 but two cans could be handled at a time. It was not possible to 

 hold, therefore, all cans at exactly the same temperature during the 

 test because of the difficult}^ in keeping the refrigerator at exactly 

 the same temperature from day to day. The temperature of the re- 

 frigerator ranged from ^° to 4^° F., the average temperature being 

 about 2-| degrees above zero. The initial temperature of the milk, 

 when the cans were placed in the refrigerator, was 11° F., and the cans 

 were held until its temperature had reached 30° F. The drop in 

 temperature is graphically shown in figure 18. The time required for 

 the milk in the different types of cans to drop from 50° to 30° F. 

 was as follows: 



Hours. 



Can No. 1 20 



Can No. 2 21 



Can No. 4 181 



Hours. 

 Can No. 5 16£ 



Can No. 6 7 



Can No. 3, the ice-compartment can, was not used as it was not 

 suitable for the experiment. Comparing the time necessary for the 

 temperature of the milk to drop from 50° to 30° F. with that of the 

 ordinary can (No. 6) we find: 



Can No. 1 required 2.9 times as long. 

 Can No. 2 required 3.0 times as long. 



Can No. 4 required 2.6 times as long. 

 Can No. 5 required 2.3 times as long. 



The showing above indicates the efficiency of felt jackets or in- 

 sulated cans for shipping milk in zero weather. 



TRANSPORTING MILK AT LOW TEMPERATURES. 



A large percentage of city milk supplies is delivered to the rail- 

 road station by the producers or is brought to the station by wagons 

 or trucks which collect milk from several farms. A certain quantity 

 of milk is hauled directly to the city by means of teams or motor 

 trucks. Unless the milk is especially protected its temperature will 

 rise several degrees in the journey from the farm to the station, and 

 if afterwards in the haul of from a few hours to all day to the city 

 it is exposed to high temperatures its temperature when it reaches 

 the market is usually not low enough to prevent the rapid multiplica- 

 tion of bacteria. Much milk is transported to the city in ordinary 

 baggage cars or in uniced milk cars. To insure its reaching the 

 city in the best condition milk should be cooled on the farm to 50° 

 F. or below and should be carefully protected during shipment to 



