70 BULLETIN 98, U. S. DEPARTMENT OP AGRICULTURE. 



If, however, milk is thoroughly cooled on the farm and placed in 

 large cans properly jacketed it will arrive at the city plant in good 

 condition under ordinary weather conditions providing the time 

 required for transportation is not over four hours. The transfer of 

 heat through milk is principally by convection, and when in large 

 volumes the transfer is very slow unless the milk is agitated. 



The time taken in the transportation of milk from one point to 

 another, together with the facilities available for holding it at low 

 temperatures, determine, to a great extent, the initial temperature 

 to which it should be cooled. For short distances, or short preserva- 

 tion of a few hours only, it is believed that a temperature of less than 

 50° F. should be maintained. Some lactic acid bacteria will multiply 

 even at this temperature and will cause a souring of the milk, but the 

 increase is slow and for a few hours no serious results will occur. At 

 temperatures below 50° F., however, the rate of bacterial growth is 

 materially decreased. 



If, on the other hand, milk is to be shipped long distances, the 

 initial temperature must be lower, assuming that no provision is 

 made for maintaining temperatures during transportation. For com- 

 paratively long-distance shipments, where the milk is in transit for 

 several hours, it is necessary to cool it down near the freezing point. 

 The point to which milk should be cooled, therefore, depends on the 

 time taken in transportation and must be determined for each par- 

 ticular case. 



In order to maintain a low temperature as long as possible, the cans 

 should be well jacketed. The curves in figure 33 (p. 71) show the result 

 of jacketing the cans. The cans were set in an open truck with no 

 covering to shield them from the direct rays of the sun. Long- 

 stemmed thermometers were inserted through holes drilled in the 

 covers of the cans. Thermometer readings were taken every 15 

 minutes and the results plotted in the form of curves. The milk 

 was hauled a distance of 13 miles through the country and the average 

 air temperature during the trip was 82.65°. It will be noted by refer- 

 ring to the curves that the total rise in temperature of the milk con- 

 tained in the hair-quilt-jacketed can was 5£°, while that in the can 

 wrapped in wet burlap was about 8£°, and the unjacketed can showed 

 a rise in temperature of 28£°. It is obvious from the curves that it 

 pays to jacket the cans in order to maintain a low temperature during 

 transportation. 



There are at the present time two types of refrigerator cars de- 

 signed especially for the transportation of milk. One is an ordi- 

 narily constructed car of the baggage type, in which the milk cans 

 are set and crushed ice packed around them. These cars are only 

 good for comparatively short hauls, as they are poorly insulated or 

 in most cases not insulated at all. The water from the melted ice 



