36 BULLETIN" 420, U. S. DEPARTMEN"T 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 beheved 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 coolmg 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 



