36 



PRECOOLING 



outside. There is no practicable way of circulating the cold 

 air through the package itself. The same is true of the 

 method of loading in the car. There are a number of ham- 

 pers which cannot be reached by the cold blast. The cooling 

 of these is dependent, therefore, upon the equalization of the 

 temperature of the entire load after the circulation of the 

 cold air has ceased. 



In the cooling of a carload of lettuce or other vegetables, 

 the limitations of the work are defined by the temperature 

 which the produce most exposed to the blast will withstand 

 without danger of freezing; and therefore, the time necessary 

 to cool a carload will depend upon the volume of air circulat- 

 ed and the temperature of the circulating air. It follows 

 that if the circulating air can be held at a low temperature 

 without danger of freezing the product, the cooling can be 

 accomplished more quickly than where air at a higher tem- 

 perature must be circulated. These limitations are being 

 carefully studied by the Bureau workers. In the case of 

 some products — notably fruits which have been wrapped — 

 it has been found that air several degrees below the freezing 

 point of the produce may be circulated for a considerable 

 length of time without danger of freezing. 



So far, in the vegetable work, the temperature of the air 

 current has not been allowed to fall much below the freez- 

 ing point of the produce being cooled. It is barely possible, 

 therefore, that after more experience has been obtained, it 

 will be found that colder air may be safely used, and in that 

 case the length of time required to cool a carload may be 

 materially reduced. 



There are limitations in the process itself. For example: 

 If a very large volume of extremely cold air is circulated, the 

 most exposed packages and the outer layers in the packages 

 will be cooled to the danger point of freezing more quickly 

 than where air at a higher temperature is circulated. The 

 limits of operation are the temperatures of the crates most 

 exposed to the blast, and as soon as these are in danger of 

 freezing, the work must cease. It may easily follow that the 

 danger point may be reached in these exposed packages be- 

 fore the greater bulk of the carload has been materially af- 



