TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 35 



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The differences in the containers are the same as in the case of 



the sweet corn. The temperature rises fastest in the pint glass jar, 



the No. 2 tin can next, 



then the quart glass jar, 



and most slowly in the No. 



3 tin can. The retarding 



influence of the glass ceases 



to be a factor here. This 



is because the conductivity 



of either the tin or the 



glass is greater than the 



conductivity of the ma- 

 terial. Hence, the rate of 



temperature change at the 



center of the can follows 



the order of the diameters 



of the containers. This is 



true for sweet corn and 



pumpkin and also for sweet 



potatoes, as will be seen 



later. Sometimes pumpkin 



is concentrated before 



being canned. Evap- 

 orating the pulp to half 



its original volume would probably have only a small effect upon the 



rate of change of temperature. There is very little convection in the 



material as thus pre- 

 pared, so it is al- 

 most certain that 

 evaporation to one- 

 half would make it 

 only slightly slower. 

 Some experiments 

 were made to de- 

 termine what effect 

 cooking before fill- 

 ing into the can 

 would have upon 

 the temperature 

 changes in the can. 

 Cans were filled 

 with raw material 

 ground in the food 

 chopper and with 

 material which had 

 been steamed 30 



Fig. 39. — Time-temperature relations for pumpkin 

 when processed in pint glass jars at 100°, 109°, 

 116°, and 121° C. These curves were plotted 

 from temperature readings made at intervals of 

 5 minutes. Eise in temperature when processed : 

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

 D, at 121° C. 



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Fig. 40. — Time-temperature relations for pumpkin when 

 processed in quart glass jars at 100°, 109°, 11G°, and 

 121° C. These curves were plotted from temperature 

 readings made at intervals of 5 minutes. Rise in tem- 

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

 C, at 116° C. ; D, at 121° C. 



