42 



BULLETIN 956, U. S. DEPARTMENT OF AGEICULTtJEE. 



only one of the leafy vegetables that has been included, in these tests. 

 The rate of change of temperature at the center of the can in the 

 leafy vegetables depends upon the nature of the packing. If the 

 cabbage in this case had been cut coarser and a little more water had 

 been added, the rate of change would have been very much faster. 

 On the other hand, if the material had been sliced finer and packed 

 into the can a little closer, the rate would have been slower. Any 

 alteration of the packing which would affect convection would affect 

 the rate of change of temperature at the center of the can. In the 

 leafy vegetables almost any results may be obtained between that of 



string beans and 

 sweet corn. As 

 usually packed, how- 

 ever, the changes are 

 perhaps quite slow. 

 This, again, shows 

 how important it is 

 to know how rapidly 

 the temperature in 

 the can approaches 

 that of the retort. 



FACTORS AFFECTING 

 THE CHANGE OF TEM- 

 PERATURE AT THE 

 CENTER OF THE CAN. 



In considering the 

 factors affecting the 

 rate of change of 

 temperature in the 

 can the laws of heat 

 transmission, espe- 

 cially the rapidity of 

 convection and the slowness of conduction and radiation, should 

 be held in mind. From all the preceding experiments the follow- 

 ing facts seem clear. 



The first important factor is the size and nature of the container. 

 If the rate at which the material in the can transmits heat is slower 

 than the conductivity of the walls of the container, then the nature 

 of the container ceases to be an important factor and the diameter 

 of the container is the chief factor. If the rate at which the material 

 carries heat to the center of the can is faster than the conductivity 

 of the walls of the container, then the nature of the container is 

 important and variation in its conductivity affects the temperature 

 changes at the center of the can. Thus in string beans there is a 

 difference in the rate of change of temperature in the tin and glass 



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Fig. 49. — Time-temperature relations for cabbage when 

 processed in No. 2 tin cans at 100°, 109°, 116°, and 

 121° C. These curves were plotted from readings made 

 at intervals of 5 minutes. Else in temperature when 

 processed : A, At 100° C. ; B, at 109° C. ; C, at 116° C. ; 

 D, at 121° C. 



