32 



PRECOOLING 



far in the Bureau investigations, this general rule has been 

 found to apply to all fruit products and thus this factor as- 

 sumes fundamental importance. 



The Ordinary Refrigerator Car. 



Close observation of carloads of fruits and vegetables has 

 shown that where the ice of the refrigerator car is depended 

 upon to cool the product, the rate of cooling is extremely slow, 

 when the carload as a whole is considered. As is well known 

 to all handlers and shippers of produce, the cooling of a pro- 

 duct after it is loaded in a refrigerator car under ordinary 

 conditions is dependent upon the circulation of the air within 

 the car induced by the presence of the ice in the bunkers at 

 either end. As the air passes through the bunker, it is cooled 

 by the ice, falls to the floor and flows toward the center of 

 the car. As the air passes among the packages, it absorbs 

 heat and rises to the top to be again circulated through the 

 ice bunkers. The circulation of the air throughout the car- 

 load is dependent upon the difference in weight or density of 

 the air column within the ice bunker and the current in the 

 warmer body of the load. This difl^erence is not suflflcient to 

 create a rapid circulation of the air. The movement of the 

 air currents within the car is sluggish; consequently, if the 

 produce is packed and loaded at a comparatively high tem- 

 perature, the rapidity with which the fruits or vegetables are 

 cooled is likewise slow. This is especially true with fruits 

 and vegetables which are tightly packed. While the outside 

 layers may be cooled without much difficulty, it requires a 

 considerable length of time to affect the interior of the pack- 

 ages. The bulk of the product may thus remain at a com- 

 paratively high temperature long enough to allow deteriora- 

 tion to begin or decay to develop. Once deterioration or de- 

 cay starts, these processes continue to develop even after the 

 temperature has been reduced; hence the importance of 

 prompt cooling as well as rapid cooling. 



In the case of citrus fruits shipped from California, un- 

 der ordinary refrigerator car conditions from three to five 

 days are required to cool the fruit to a temperature sufficient- 

 ly low to prevent the development of decay and deterioration. 



