46 



BULLETIN 1022, U. S. DEPARTMENT OF AGRICULTURE. 



time-temperature changes which obtain under practical working con- 

 ditions. They emphasize again the fact brought out in the earlier 

 consideration of this subject that in substances where the processing 

 jDeriod is shorter than the time required for the center of the can to 

 reach the temperature of the retort, the entire heat penetration 

 curves, like those shown in figure 27, are not safe guides for deter- 

 mining the length of the processing period and that the entire cooling 

 curves as well do not illustrate working conditions. They show, fur- 

 ther, that the maximum temperatures actually attained vary less 

 with differences in initial temperature than might be expected from 



















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TIME IN MINUTES 



Fig. 34. — Experimental time-temperature curves for sweet potatoes in 

 the form of pie stock in No. 2 tin cans, starting at different uni- 

 form temperatures, exhausted for different time periods at 100° C, 

 processed for 1 hour at 116° C, and cooled in air at ordinary room 

 temperature. Curve for can starting- : A, At 26° C. and receiving- no 

 exhaust; B, at 26° C. and exhausted for 5 minutes; C, at 26° C. and 

 exhausted for 10 minutes ; D, at 26° C. and exhausted for 15 minutes ; 

 E, at 52° C. and receiving no exhaust ; F, at 52° C. and exhausted 

 for 5 minutes; G, at 52° C. and exhausted for 10 minutes; H, at 

 52° C. and exhausted for 15 minutes. 



the curves in figure 21. Although the entire curve must be given con- 

 sideration, certainly the most important part of this curve is when 

 it is at its maximum. The fact that the maximum temperatures vary 

 so very much less than the initial temperatures helps to explain why 

 often such uniform results are obtained where considerable variation 

 in initial temperatures exist. For instance, it will be seen, in the case 

 of the No. 2 cans, that in cans starting at initial temperatures differ- 

 ing as much as 30 degrees, the maximum temperatures reached may 

 not vary by more than 4 or 5 degrees, and in the case of the No. 3 

 can only slightly more. 



The actual temperature reached and the length of time the material 

 at the center of the can remained above any definite point may be 

 readily observed. 



