40 



BULLETIN 1022, U. S. DEPARTMENT OF AGRICULTURE. 



the full maximum strains indicated in the curves would be exerted 

 on the cans from within, but the gradual release of pressure allows 

 some lessening from the maximum strain indicated. When the proc- 

 essing is done at 100° C. the maximum pressures indicated in the 

 curves represent the actual strains on the cans. These curves illus- 

 trate again the necessity of sealing at relatively high temperatures. 



VACUUM STUDIES. 



Table 11 presents the results of vacuum readings on the No. 3 cans 

 sealed at 70° and 80° C. used in the above tests when cooled to room 

 temperature. 



Table 11. — Vacuum tests witli sweet potatoes in No. 3 tin cans. 



Temperature (°C). 



Process- 

 ing period 

 (min- 

 utes). 



Barometer readings. 



Vacuum 

 (inches 

 of mer- 

 cury). 



Sealing. 



Proc- 

 essing. 



After 

 cooling. 



At 

 sealing. 



After 

 cooling. 



70 



f 100 

 \ 116 

 { 121 



f 100 

 < 116 

 I 121 



10| 



21 



131 



16 

 9 

 12 



120 

 120 

 120 



120 

 120 

 120 



30.30 

 30.25 

 29.77 



29.98 

 29.87 

 30.29 



30.17 

 30.19 

 29. 89 



29.86 

 30.29 

 30.45 



13i 

 11 



80 



10| 



14 

 13J 





9 



The figures in Table 11 do not differ essential^ from those ob- 

 tained with corn. They are below the theoretical, and there is the 

 decrease of vacuum with the higher processing temperatures. 



Concussion tests, as described under methods and apparatus, though 

 not altogether satisfactory in point of uniformity of results, show 

 that there is a relation between the vacuum and the susceptibility of 

 the can to bruising in handling. The figures seem to indicate that in 

 ordinary practice No. 2 cans may be safely sealed at as high as 85° 

 C. Above 90° they are quite readily dented, and they usually col- 

 lapse spontaneously when sealed between 95° and 100° C. Tests for 

 the No. 3 cans show that they are much more easily bruised than the 

 No. 2 cans where the vacuum is equal in the two cases. The results 

 seem to indicate that it is scarcely safe to seal No. 3 cans very much 

 above 80° C. Bruising occurs rather readily above 85° and the cans 

 may collapse spontaneously at 90° C. or above, depending upon their 

 strength. 



HEAT PENETRATION. 



The material used for the time-temperature studies in sweet pota- 

 toes was prepared in the same manner as that used in the foregoing 

 experiments on pressure and vacuum. The rates of temperature 

 changes in No. 2 cans of sweet potatoes, starting at different tern- 



