TEMPERATURE CHANGES, ETC., DURING CANNING OPERATIONS. 21 



from the distillecl-water curves are noted, however, in the case of 

 those curves for cans processed at 116° and 121° C, in which an 

 equilibrium is not attained throughout the entire period of the tests. 

 Furthermore, these differences are increased with the higher process- 

 ing temperatures, the final portion of the curves for cans processed 

 at 121° C. being steeper than those for cans processed at 116° 

 C. That -this has no correlation with the rate of heat penetration is 

 shown by the fact that the increase in pressure is continued long after 

 the equilibrium of temperature has been reached. Experiments show 

 that a temperature equilibrium is reached in string beans in about 

 15 minutes in the No. 2 and in 18 to 22 minutes in the No. 3 can. In 

 one case during the present tests the pressure continued to' increase 

 for four hours after the processing was begun, at which time the 

 experiment was terminated. 



This seems to indicate that at the higher temperature there is a 

 decomposition of the material in the can, with a consequent libera- 

 tion of gases. It was thought that the setting free of hydrogen by 

 the action of the food material on the metal of the container, as 

 pointed out by Bigelpw (1) might explain this result, but compara- 

 tive tests, using both plain and enameled cans under these conditions, 

 failed to give evidence in support of this view. 



These curves also illustrate how strains are greatly lessened by 

 sealing at as high temperatures as practicable, especially when proc- 

 essing is to be done at the higher retort temperatures. It will be 

 seen that even when cans are sealed at temperatures as high as 

 80° C. the pressures obtained are well above the theoretical values. 



The processing period in these tests, except in the single instance 

 noted, was 1 hour. This may not conform to regular practice, but 

 the maximum pressures for any period less than this will be shown on 

 these curves, for when the steam in the retort is cut off the pressure 

 in the can begins to fall almost immediately. 



The actual strain on the can at any time during the process may be 

 found by subtracting the retort pressure from the pressure indicated 

 for that time in the curve. When the retort pressure is released, 

 however, the strain on the can is greatly increased. If a complete 

 release of pressure in the retort should be effected instantaneously 

 the full pressure indicated would be felt, but the gradual decline in 

 retort pressure in practice favors the can somewhat. The greatest 

 strain occurs when the pressure in the retort reaches zero, but at this 

 point the strain is, however, only 3 to 5 pounds, or (in some cases 

 more than this) less than the maximum pressure indicated in the 

 curve. The extent of this variation from indicated pressures is de- 

 pendent upon the temperature of sealing, the processing temperature, 

 the nature of the material, the amount of bulging of the can, and the 

 rate of release of pressure in the retort. When processing is done at 



