TEMPERATURE CHANGES IN CANNING FRUITS AND VEGETABLES. 43 



due to the differences in the conductivity of the walls of the con- 

 tainer, but in sweet corn the rate of change of temperature follows 

 more nearly the order of the diameter of the container, the glass not 

 being an important factoi'. The diameter of the container is of very 

 much less importance in material where there is a free liquid in 

 which convection carries the heat rapidly to the center of the can. 

 Thus the difference in the rate of change of temperature in No. 2 

 and in No. 3 tin cans is so small in string beans that only slight differ- 

 ences in the processing periods 'are necessary. 



Variations in the composition of the material are of importance 

 when such variations affect convection. If the material is of such 

 a nature that no convection occurs, its composition may vary widely 

 without greatly affecting the temperature changes in the can. Thus 



/30 

 /SO 



//o 

 /oo 



90 



foso 



. \p 



■— - — I c~ 



^' " ^-^""^ ^g_ 



-^ ^■'^ _^ ; =^ ' 



so ^30 fO ■SO 60 70 SO 



Fig. 50. — Timc-temporatiirc rolatloiis for cabbage when processed in No. 3 tin cans at 

 100°, 109°, 116°, and 121° C. These cnrvos were plotted from readings made at inter- 

 vals of 5 miuntos. Risi; in temperature wlicn processed : A, At 100° C. ; B, at 10!)° C. ; 

 C, at 116° C. ; D, at 121° C. 



sweet potatoes and pumpkin, though of very different chemical 

 composition, have similar time-temperature curves. The going into 

 solution of starch may change the viscosity of the material and 

 hence affect the temperature changes in the can, but changes in the 

 physical nature of the starch have very little effect if the character 

 of the pack at the outset is such that no convection can occur. In 

 some cases there may be the cooking out of soluble proteins, pec- 

 tins, or other viscous materials which would interfere with con- 

 vection. Variations of materials, in this respect must be considered 

 in processing. Usually in processing where there is a breaking 

 up of the material the rate of change of temperature becomes slower, 

 but in the tonuito the opposite is true, because the tomato contains 

 little starch, pectin, or other niiuilaginous material. The liquid be- 

 comes free, tlius allowing convection. 



