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 
 tvith 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 can 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. 
