6 



CIRCULAR 



U. S. DEPARTMENT OF AGRICULTURE 



Table 1. — Approximate rate of evolution of heat by certain fresh fruits and vegetables 

 when stored at the temperatures indicated — Continued 



Commodity 



Tem- 

 pera- 

 ture 

 (°F.) 



British ther- 

 mal units of 

 heat per ton of 

 fruits or vege- 

 tables per 24 

 hours 



Litera- 

 ture 

 refer- 

 ences 



String beans 



Sweet corn 



Sweetpotatoes 



Tomatoes (mature green) 



Turnips 



1,694 



2,706 



10, 802 



2,640 



3,806 



8,118 



880- 1, 320 



, 600- 8, 800 







132 



2,574 



66 



572 



682 



(*) 



(18) 



( 2 ) 



2 Unpublished work on the respiration of vegetables by R. C. "Wright and T. M. Whiteman. 



It will be noted that lettuce, Bartlett pears, peaches, and cherries 

 have a much higher respiration rate than potatoes, apples, and 

 onions. This means that the first group requires considerably more 

 refrigeration than the second to keep them at a specified temperature. 

 Less pronounced differences occur between other commodities in the 

 list and are important to a lesser degree in determining the amount 

 of refrigeration necessary to cool them and keep them in sound, 

 usable condition. 



It is difficult to determine the heat to be removed in cooling fruits 

 and vegetables to cold-storage temperatures. This depends mainly 

 on the following factors: The specific heat of the product, the rate 

 at which it produces heat (by respiration), and its initial and final 

 temperature. If the product could be cooled to the storage tempera- 

 ture instantaneously, the heat to be removed would be only the num- 

 ber of British thermal units (B. t. u.) or calories obtained by multiply- 

 ing the specific heat of the product by the difference between the 

 initial and the final temperature, and this result by the weight of the 

 product in pounds or kilograms. This is usually called the sensible 

 heat. The cooling process, however, requires time, and during this 

 interval additional heat is produced by the respiration of the stored 

 fruit or vegetable. 



In order to determine the amount of this additional heat it is 

 necessary to know the rate of heat production at any temperature 

 and the length of time the product is in each temperature range. For 

 example, if the respiration rate (or rate of heat production) for a 

 given commodity is twice as great at 70° F. as at 50°, the number 

 of hours this commodity is at each of these temperatures must 

 be known before the total heat produced can be calculated. When 

 fruits and vegetables cool, the rate at which they produce heat de- 

 creases, and the total heat produced depends not only upon the time 

 required for cooling but also upon how long the commodity stays 

 in each temperature range. 



Table 2 shows the approximate amounts of sensible heat and of 

 heat produced by respiration which must be removed from seven 

 varieties of four kinds of fruit in cooling them from various tempera- 



