TEMPERATURE CHANGES, ETC., DURING CANNING OPERATIONS. 39 



during this latter half of the processing period indicates that some 

 influence other than increase of internal temperature is in operation. 

 That this is the case is shown in the vacuum figures given later. 



The maximum pressures attained vary in both directions from the 

 theoretical, some being higher and others lower. The variable factors 

 of sudden bulging and gradual distention of the cans, liberation of 

 gases, etc., contribute to this result and account for it. The swell- 

 ing of the starch can 

 hardly account for 

 any increase in pres- 

 sure, as the material 

 was well cooked be- 

 fore it was put into 

 the cans. 



The processing pe- 

 riod used in these 

 tests is longer for the 

 higher temperatures 

 than is commonly em- 

 ployed, but the maxi- 

 mum pressures for 

 any period of 120 

 minutes or less will 

 be shown on these 

 curves, for when the 

 steam in the retort is 

 cut off the pressures 

 fall almost immedi- 

 ately. 



As pointed out in 

 considering the re- 

 sults on string beans, 

 the actual strain on 

 cans at any time dur- 

 ing the process may 

 be found by subtract- 

 ing the retort pres- 

 sure from the pressure 

 The strain on the can 































































































































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Pig. 26. — Experimental time-pressure curves for sweet pota- 

 toes in the form of pie stock in No. 3 tin cans, 

 sealed at different uniform temperatures and processed 

 for 2 hours at 100°, 116°, and 121° C. Curve for can 

 sealed: A, At 25° C. and processed at 100° C. ; a, at 

 25° C. and processed at 116° C. ; a', at 25° C. and proc- 

 essed at 121° C. ; B, at 70° C. and processed at 100° C. ; 

 b, at 70° C. and processed at 116° C. ; b', at 70° C. and 

 processed at 121° C. ; C, at 80° C. and processed at 

 100° C. ; c, at 80° C. and processed at 116° C. ; c\ at 

 80° C. and processed at 121° C. 



indicated for that time in the curve. 



is greatly increased when the retort 

 pressure is released, but the greatest strain is felt when the pres- 

 sure in the retort reaches zero. This amounts to only about 3 to 

 5 pounds less than the maximum pressure indicated in the curves, 

 depending upon the temperature of sealing, the processing tempera- 

 ture, the bulging of cans, the rate of release of retort pressure, etc. 

 If the retort pressure should be completely released instantaneously 



