36 BULLETIN 420, U. S. DEPARTMENT OF AGRICULTURE. 
2. A bottle of hot milk will cool about one-third faster in circulated 
air than in still air at the same temperature. This is also true of hot 
milk in 10-gallon cans. The time required to cool through a given 
range, other things being equal, depends upon the size of the con- 
taining vessel, and it is believed that in commercial practice quart 
bottles are about the largest-sized containers which should be used 
with this method. Cooling by natural circulation is too slow for 
satisfactory application on a commercial scale. 
3. The cooling experiments were operated on a basis of 30 crates, 
stacked in 6 piles, each 5 crates high. When cold air was forced up 
through the crates, there was a wide variation in the temperature 
in the same sized bottles in different positions in the stack, as well 
as in quart and in pint bottles in the same position. This variation 
was too great for satisfactory operation on a commercial scale. 
4. When the direction of the cooling air was reversed every 15 
minutes during the Cooling period, first up then down through the 
crates, the variation in the temperature of the bottles was reduced, 
but the cooling was not entirely satisfactory. 
5. When air was forced down through the stacks of crates the 
cooling process was much more effective. The maximum difference 
in temperature -in any of the bottles was about 4.5° F. (2.5° C), 
and the average difference only approximately 2.5° F. (1.4° C.) 
when the bottles were all at the same initial temperature at the 
beginning of the cooling period. There was a difference of only 
2° F. (1.1° C.) between the average and the maximum variation 
in temperature of different bottles, showing that the cooling was 
practically uniform. With air at 40° F. (4.4° C.) forced down through 
the crates at the rate of approximately 2,500 feet a minute, the bottles 
were cooled from about 140° F. (60° C.) to 50° F. (10° C.) in about 
two hours. With air at 30° F. (— 1.1° C), and at the rate of about 
1,700 feet a minute, the bottles were cooled through the same range 
of temperature in approximately 1 hour and 30 minutes, and when 
it was at 20° F. (-6.6° C.) and forced down through the crates at 
the last-mentioned rate, the bottles were cooled from about 140° F. 
(60° C.) to about 50° F. (10° C.) in about 1 hour and 20 minutes. 
6. There was a wide variation in temperature between the top 
and the bottom of the same bottle during the cooling period when air 
was forced up through the crates; this variation was practically 
eliminated when air was forced downward through them. 
7. Cost of cooling by forced-air circulation, when the outside air 
temperature is 40° F. (4.4° C.) or lower, is materially less than that 
of the usual methods of refrigeration. 
8. Bacteriological studies indicate that if milk is cooled from 
145° F. (62.8° C.) to 50° F. (10° C.) within five hours after pasteuriz- 
ing, no more bacterial increase will take place during the slow cooling 
