46 BULLETIN 1022, U. S. DEPARTMENT OF AGRICULTURE. 
time-temperature changes which obtain under practical working con- 
ditions. They emphasize again the fact brought out in the earlier 
consideration of this subject that in substances where the processing 
period is shorter than the time required for the center of the can to 
- reach the temperature of the retort, the entire heat penetration 
curves, like those shown in figure 27, are not safe guides for deter-- 
mining the length of the processing period and that the entire cooling 
curves as well do not illustrate working conditions. They show, fur- 
ther, that the maximum temperatures actually attained vary less 
with differences in initial temperature than might be expected from 
° 
120 
8, 
3, 
8, 
TEMPERATURE IN DEGREES CENTIGRADE 
uy 
So 
285 16 ) rT} 20D 50 Oo 7o 60 90 i TT 
TIME IN MINUTES. 
Fic. 34.—Experimental time-temperature curves for sweet potatoes in 
the form of pie stock in No. 2 tin cans, starting at different uni- 
form temperatures, exhausted for different time periods at 100° C., 
processed for 1 hour at 116° C., and cooled in air at ordinary room 
temperature. Curve for can starting: A, At 26° C. and receiving no 
exhaust; B, at 26° C. and exhausted for 5 minutes; C, at 26° C. and 
exhausted for 10 minutes; D, at 26° C. and exhausted for 15 minutes; 
FE, at 52° C. and receiving no exhaust; F, at 52° C. and exhausted 
for 5 minutes; G, at 52° C. and exhausted for 10 minutes; H, at 
52° C. and exhausted for 15 minutes. 2 
the curves in figure 21. Although the entire curve must be given con- 
sideration, certainly the most important part of this curve is when 
itis at its maximum. The fact that the maximum temperatures vary 
so very much less than the initial temperatures helps to explain why 
often such uniform results are obtained where considerable variation 
in initial temperatures exist. For instance, it will be seen, in the case 
of the No. 2 cans, that in cans starting at initial temperatures differ- 
ing as much as 30 degrees, the maximum temperatures reached may 
not vary by more than 4 or 5 degrees, and in the case of the No. 3 
can only slightly more. j 
The actual temperature reached and the length of time the material 
at the center of the can remained above any definite point may be 
readily observed. 
