6 CIRCULAR 2 78, U. S. DEPARTMENT OF AGRICULTURE 



Operators of certain large cold-storage houses have found that 

 when a room is filled with apples or celery, for example, the relative 

 humidity soon becomes constant at a fairly definite level, which for- 

 tunately is about the optimum for the particular commodity con- 

 cerned. The same result has been observed with other commercially 

 important fruits and vegetables and probably is obtained in many 

 of the larger well-constructed cold-storage warehouses of the country. 

 This is probably due to the fact that when a storage room is nearly 

 full and there is comparatively little air space left the pull on the 

 moisture in the produce is less, and it is easier to maintain the desired 

 humidity. In these cases, however, it must be assumed that the spread 

 between coil temperature and air temperature is kept narrow. 



For most fruits that are stored commercially the statement is true 

 that a relative humidity of 80 to 90 percent gives the best results. 

 Exceptions are discussed at various places in the text. For leafy 

 vegetables and root crops the relative humidity should be about 90 

 to 95 percent; for other vegetables, except as noted in the text, 85 to 

 90 percent. If it seems necessary to increase the relative humidity 

 in rooms used for common, or air-cooled, storage, this can best be done 

 by sprinkling the floor occasionally. Earth floors are more desirable 

 in air-cooled storages than floors of concrete because they are more 

 easily kept damp. An increase in air circulation calls for an increase 

 in relative humidity if wilting of the stored commodity is to be 

 avoided. Allen and Pentzer (2) found that doubling the rate of air 

 movement increased moisture loss by about one-third and was equiva- 

 lent to about a 5-percent drop in relative humidity. The drying effect 

 of increased rate of air movement is particularly marked if the hu- 

 midity of the air is lower than the moisture content of the commodity. 



Evolution of Heat by Commodity 



In any consideration of the storage of fresh fruits and vegetables, 

 cut flowers, etc., it should be remembered that these commodities are 

 alive and that by virtue of that fact they carry on within themselves 

 many of the processes characteristic of all living things. Unless the 

 relative humidity is high, they give off moisture to the surrounding 

 air and most of them, in time, become shriveled or wilted, even at 

 32° F. The enzymes, or ferments, they contain act on various sub- 

 stances in their tissues and gradually bring about changes in color, 

 texture, and chemical composition which ripen the commodity and 

 may result in serious deterioration or even complete break-down. 

 The most important of these changes are produced by respiration, 

 the process in which the oxygen of the air is combined with the carbon 

 of the plant tissues, occurring chiefly in sugars, to form various de- 

 composition products and eventually carbon dioxide and water. Dur- 

 ing this process energy is released in the form of heat, the amount 

 of which varies with the commodity and increases as the temperature 

 increases, up to about 100°. This heat is always a part of the re- 

 frigeration load which must be considered in handling fruits, vege- 

 tables, and cut flowers in cold-storage rooms or refrigerator cars. 

 The approximate rate of evolution of heat by various commodities is 

 given in table 1. 



