70 BULLETIN 98, U. S. DEPARTMENT OF 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 5J°, while that in the can 
wrapped in wet burlap was about 8^°, and the unjacketed can showed 
a rise in temperature of 28 J°. 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 
