4 CIRCULAR 2 7 8, U. S. DEPARTMENT OF AGRICULTURE 



etc.; if it is too high, it favors the development of decay, especially 

 in rooms where there is considerable variation in temperature. The 

 exact control of humidity is rather difficult, however, and is not often 

 attempted in commercial storage warehouses. 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 fortunately is about the opti- 

 mum for the particular commodity concerned. The same result has 

 been observed with other commercially important fruits and vege- 

 tables and probably is obtained in many of the larger, well-constructed 

 cold-storage warehouses of the country. 



For most fruits that are stored commercially the general statement 

 can be made that a relative humidity of 80 to 85 percent gives the 

 best results. Exceptions are discussed in the text. For leafy vege- 

 tables 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. Usually an increase in air circulation calls for an 

 increase in relative humidity. 



Various methods have been proposed or tested for the control of 

 humidity in cold-storage rooms, but at present there seems to be no 

 general agreement among operators of cold-storage plants as to which 

 is the best of these methods, nor has there been widespread use of any 

 of them. 



EVOLUTION OP 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 substances in 

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

 chemical composition which mature the commodity and may result in 

 serious deterioration or even complete break-down. The most im- 

 portant 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 decomposition 

 products and eventually carbon dioxide and water. During 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 refrigeration load 

 which must be considered in handling fruits, vegetables, and cut 

 flowers in cold-storage rooms or refrigerator cars. The approxi- 

 mate rate of evolution of heat by various commodities is given in 

 table 1. 



