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



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 

 temperatures. 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 decreases, 

 and the total heat produced depends not only upon the time required 

 for cooling but also upon how long the commodity stays in each tem- 

 perature range. 



Table 2 shows the approximate amounts of sensible heat and of 

 heat produced by respiration which must be removed from eight 

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

 tures to a temperature of 35° F. These figures are based on experi- 

 mental determinations of the rate of respiration at various tempera- 

 tures ; some of the data are from the tables given by Magness and his 

 associates (^, Ifi), and the remainder from data reported by Haller 

 et al. (SO) . The figures for Bartlett pears are based on the maximum 

 values given by Magness and Ballard (44). The figures given in 

 this table have been obtained by assuming that the heat of respiration 

 is produced by oxidation of a hexose sugar and can be calculated from 

 the rate of production of carbon dioxide, which has been determined 

 experimentally; very few calorimetric measurements of heat produc- 

 tion by fruits and vegetables have been made, and this assumption 

 seems to be the best available basis for calculating heat production at 

 any given temperature. Recent investigations (26) indicate that 

 calculations made on this assumption give values that are within 10 

 percent of those obtained calorimetrically. 



The assumption has also been made, although it is believed to be 

 only approximately correct for fruits and vegetables, that the rate 

 of temperature drop at any given time during cooling is proportional 

 to the difference between room temperature and fruit temperature 

 at that time. With this assumption as a basis, the temperature and 

 thus the rate of respiration at any time during the cooling period, as 

 well as the total heat produced during the whole cooling period, have 

 been calculated. 3 



3 The authors wish to acknowledge the valuable assistance given by W. V. Hukill, of the 

 Bureau of Agricultural Chemistry and Engineering, in making the calculations and in pre- 

 paring this statement on the production of heat by fruit and vegetables. 



