22 



BULLETIN 1022, TJ. 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 differences in the resistance of individual 

 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, clue primarily to the lesser resistance of the 

 larger can to external pressure, they are similar. 



Table 7. — Vacuum tests toith string beans. 



Temperatures (° C). 



Barometer reading. 



Vacuum 

 (inches 

 of mer- 

 cury). 



Sealing. 



Process- 

 ing. 



After 

 cooling. 



At 



sealing. 



After 

 cooling. 



70 



f 100 

 \ 116 

 I 121 



I 100 

 \ 116 

 [ 121 



201 

 19| 



181 



22J 

 18| 

 19 



29.89 

 29.86 

 29.86 



29.86 

 30.05 

 30.05 



29.86 

 29.86 

 30.05 



29.82 

 30.05 

 30. 05 



12J 

 llf 



80 



llf 



15§ 



13J 





13A 



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°, 70°, 80°, 90°, 

 and 100° C. were prepared for vacuum readings 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 emploj^ed 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 sho'fi's the effect of 



