22 BULLETIN 1022, U. S. DEPARTMENT OF AGRICULTURE. 
100° C. there is no retort pressure to counterbalance that of the can, 
and consequently the pressures indicated represent actual strains on 
the cans. 
Sharp breaks and occasional irregularities in the form of the 
curves are due to sudden bulging and gradual distortion of the cans. 
‘Small variations due to Pee in the resistance of indivi vidual 
cans are to be noted. 
VACUUM STUDIES. 
Vacuum tests were made upon the cans used in the pressure tests 
recorded here by cooling the cans to about room temperature and 
then reading the vacuum with the mercury manometer, as previously 
described. Table 7 shows the results obtained for the No. 2 cans. 
Figures for the No. 3 cans are omitted, as, aside from showing a 
slightly smaller vacuum, due primarily to the lesser resistance of the 
larger can to external pressure, they are similar. 
TABLE 7.—Vacuum tests with string beans. 
Temperatures (° ©.). ; Barometer reading. 
Vacuum 
(inches 
; Process- | After At After of mer- 
Sealing. ing. | cooling. | sealing. | cooling. | CUrY)- 
100 203 29. 89 29. 86 12% 
( (| Semcon ieh aes eee ar Cia che eee gine Ohana Sei teeta 116 192 29. 86 29. 86 113 
121 182 29. 86 30.05 112 
100 223 29.86 | 29.82 153. 
RIN siti terete on A Mae ce ts rele ak a Fe cee oe 116 183 30. 05 30. 05 133 
121 19 30. 05 30. 05 1335 
These figures show that experimental vacuums in string beans are 
always below the theoretical and that the higher the processing tem-_ 
perature employed the lower the vacuum obtained. This may 
amount to 2 inches or more. This may be due in part to the pressure 
of air included in the material which is not expelled during the pre- 
liminary blanching and in part to individual differences in cans, but 
it is probably due primarily to the liberation of gases during the 
processing. 
Cans of string beans in lots sealed at 20°, 50°, 60°, (02, 8022502. 
and 100° C. were prepared for vacuum “enfin and for concussion 
tests, but the percentage of leaky cans was so great that the results 
were not considered of sufficient value to present here. 
HEAT PENETRATION. 
From the foregoing discussion of pressures developed in the can 
it is obvious that sealing temperatures well above-room temperatures 
should be employed when canning in tin. It is important, therefore, 
to have a full knowledge as to the temperature changes occurring when 
the initial temperatures are different. Figure 10 shows the effect of 
