52 



BULLETIN 956, U. S. DEPARTMENT OF AGRICULTURE. 



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creased viscosity interfered with convection and thus cut down the 

 rate at which heat penetrated to the center of the can. 



In the sweet potatoes the differences noted are extremely great. 

 During the first processing the free liquid surrounding the pieces of 



material was converted into 

 a starch jelly by the cook- 

 ing out of the starch dur- 

 ing the processing, so that 

 during the second process- 

 ing the heat passed to the 

 center only by conduction. 

 In the case of the soy 

 beans the same change of 

 rate of rise in temperature 

 is noted, though the ma- 

 terial that changed the vis- 

 cosity of the liquid filling 

 the interspaces between the 

 beans was protein, and not 

 pectin or starch. 



As heretofore noted, any 

 material which alters the 

 viscosity of the liquid fill- 

 ing the interspaces between the pieces of material will affect the rate 

 of change of temperature in the can. It may depend upon the going 

 into solution of pectins, starch, proteins, or any other mucilaginous 

 material. 



If sweet potatoes or 

 pumpkins are ground 

 in a food chopper and 

 packed closely in the 

 cans, there is no dif- 

 ference in the first 

 and second heating. 

 If the material at the 

 outset is of such a 

 nature that all con- 

 vection IS Stopped, Fig. 57. — Diagram showing the effect of the second proc- 

 theil the coinff into essing of different vegetables upon the rate of change 



solution of starch or 

 protein has little effect 

 upon the temperature 

 changes in the can. 



Any change in the material which affects the freedom of convec- 

 tion affects the rate of temperature change. It may be the solution 



r 



O /O 20 30 <?0 <50 GO 70 SO 90 /OO 

 T/ME /A/ M/MUTES 



Fig. 56.— Diagram showing the effect o£ the first 

 processing of different vegetables upon the rate 

 of change of temperature. These curves were 

 plotted from readings made at intervals of 1 

 minute. A, Soy beans ; B, pumpkin ; C, apple ; 

 D, sweet potatoes. 



8 



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 T/At 



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of temperature. Curves A and D were plotted from 

 readings made at intervals of 5 minutes ; curves B and 

 C were plotted from readings made at intervals of 1 

 minute. A, Soy beans ; B, pumpkin ; C, apple ; D, sweet 

 potatoes. 



