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 
eurves represent the actual strains on the cans. These curves illus- 
trate again the necessity of sealing at relatively high temperatures. 
e 
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 with sweet potatoes in No. 3 tin cans. 
Te ture (°C.). Barometer readings. 
mperaiare 2) Process- 5 Vacuum 
ing period 5 ee] ices 
ase, Proc- After eae At Atiers | oUses 
pealits. essing. | cooling. utes). sealing. | cooling. cury). 
100 103 120 30. 30 30.17 13} 
TAGE SSA oi UI rd Oa Pe cee OE ass 116 74 120 30. 25 30.19 11 
121 133 120 29.77 29. 89 103. 
100 16 120 29. 98 29. 86 14 
po paralysis RO OCC ON oe SEE ind, 116 9 120 29. 87 30. 29 134 
9 
| 121 12 120 30. 29 30. 45 
The figures in Table 11 do not differ essentially 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 abowe 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 tem- 
; 
a ) 
j 
wt 
% 
: 
q 
j 
