THE MILK CONTRACTOR 293 



of loss from evaporation in passing over the cooler; and that ordinary 

 cardboard caps may be used to seal the bottles. The disadvantages are 

 several. To the cost of cooling the hot milk is added that of cooling the 

 hot glass bottles and the cases that hold them. One quart bottle weighs 

 about as much as the milk it contains and it has to be cooled from 145F. 

 to 40F. or lower. Ayres, Bowers and Johnson have recently published 

 the results of their investigations on cooling hot bottled pasteurized milk 

 with forced air. As there is very little difference in the relative rate of 

 cooling milk and water they found it advisable to use water in their 

 experiments but distributed bottles of milk throughout the crates for 

 temperature readings and for bacteriological studies. They found that: 

 1. A bottle of hot milk will cool about one-third faster in circulated than 

 in still air and that this is also true of milk in cans but that since in cool- 

 ing by this method the time required is chiefly dependent on the size of 

 the container, quart bottles are about as large sized vessels as could be 

 used in commercial practice. Cooling hot-bottled milk by natural cir- 

 culation of air is too slow for practical work. 2. In their experiments, 

 which were conducted on a basis of 30 crates stacked in six piles, each five 

 crates high, it was found that when cold air was forced up through the 

 crates, there was too much variation in the temperature of the same sized 

 bottles in different positions in the stack and also in that of quart and 

 pint bottles in the same position to admit of successfully cooling in this 

 way in commercial work. 3. Better results were obtained by reversing 

 the direction of the cooling air first up and then down through the crates 

 every 15 min. during the cooling period but this way was not wholly 

 successful. 4. Satisfactory results were obtained by forcing the air 

 from the top downward. This cools the milk in the top of the bottles 

 first and the cool milk being of greater density settles to the bottom 

 and in so doing forces the warmer lighter -milk into its place. The con- 

 vectional currents that are then established result in there being a dif- 

 ference of only a few degrees in temperature in the different bottles in 

 the stack and in a considerably greater rate of cooling. When air at 40F. 

 was forced down through the crates at about 2,500 ft. a minute the bottles 

 were cooled from 140F. to 50F. in 2 hr.; when the air was at 30F. 

 and the rate 1,700 ft. a minute the bottles were cooled through the same 

 range of temperature in 1 hr. and 30 min.; when it was at 20F. 

 and was forced down through at 1,700 ft. a minute the bottles were cooled 

 from 140F. to 50F. in 1 hr. and 20 min. 5. The cost of cooling 

 by forced air circulation when the outside air is at 40F. or lower, is 

 materially less than that of the usual methods of refrigeration. 6. Bac- 

 teriological studies indicate that if the milk is cooled from 145F. to 50F. 

 within 5 hrs. after pasteurization, no more bacterial increase takes place 

 than when the milk is cooled immediately to the same temperature. 

 In fact, in these experiments on a 30-crate basis, there was marked re- 



