TEMPERATURE CHANGES, ETC., DURING CANNING OPERATIONS. 51 



first and then becomes slower after the first few minutes. An 

 equilibrium of pressure apparently is never reached, since in ex- 

 periments where processing was continued for several hours the 

 pressure continued to rise as long as the retort temperature was 

 maintained. 



(5) The continued increase in pressure long after an equilibrium 

 of temperature is reached has been explained as due to the decom- 

 position of the food material with the consequent liberation of 

 gases. The setting free of hydrogen as a result of the action of the 

 acids of the material upon the metal of the can would give this 

 result, and doubtless it does with some acid fruits, but experiments 

 with vegetables seem to indicate that this is not the sole cause of the 

 increase in pressure. 



(6) In the heat exhausting of cans the vacuum may not be propor- 

 tional to the average temperature of the material at the time of seal- 

 ing, but is determined largely by the temperature of the head space. 

 Thus, a short exhaust results in a comparatively high vacuum if the 

 sealing is done immediately. On the other hand, a long exhaust may be 

 very ineffective if the sealing is delayed so that the head space cools. 



(7) The vacuum developed in tin cans is generally below the 

 theoretical, the causes contributing to the variation from theoretical 

 values being the distortion of the can, the swelling of colloidal sub- 

 stances, and the liberation of gases during processing. Lower vacuums 

 are obtained where long processing periods are used and the higher 

 retort temperatures are employed. 



(8) The resistance of the can to internal pressure is very much 

 greater than its resistance to external pressure ; hence, the vacuum and 

 the pressure ca'n not safely be made numerically equal when process- 

 ing much above 100° C. In order to reduce the strain due to in- 

 ternal pressure during processing, the sealing temperature is made as 

 high as is possible without danger of collapse of the can in handling 

 when subsequently cooled to normal temperature. The strain upon 

 the can during processing is found by subtracting the pressure in the 

 retort from that in the can. When the pressure in the retort is re- 

 leased the strain upon the can is increased by an amount somewhat 

 less than the pressure in the retort, owing to the cooling which occurs 

 during the release and to the further distortion of the can. The great- 

 est strain upon the can occurs at the time the pressure in the retort 

 reaches zero. The strain due to internal pressure is greater the lower 

 the sealing temperature and the higher the processing temperature. 



(9) The experimental work herein reported indicates that for most 

 vegetables the optimum temperature for the sealing of No. 2 cans is 

 80° to 85°, and for No. 3 cans 75° to 80° C. This would be different, 

 obviously, for fruits and other substances having high acidity and 

 where the processing temperatures are low. 



