TEMPERATURE CHANGES, ETC., DURING CANNING OPERATIONS. 49 
ing than is obtained when the reading is made following processing. In the 
light of the preceding consideration of the liberation of gases during processing, 
this result is to be expected. 
(2) The longer the processing period, the lower the vacuum obtained. 
HEAT PENETRATION, 
Figure 37 shows the heat-penetration curves for No. 2 cans of 
spinach starting at 70° and 80° C., processed for different periods at 
116° and cooled in water. The entire heating curve is also shown 
Tom toe. NO. .2, Cad, . ws (iui 
starting at 20° C. and + |_| 
processed at 116° un- pe 5B: i 
til the retort tempera- ae cs See 
VD SN Ge 
ture was reached. \ X 
Ss ‘NY 
PTINREEE 
Fei ii ell oe 
These curves are 
entirely similar to 
those obtained for 
sweet corn, though 
the rate of tempera- 
TEMPERATURE IN OCEGREES CENTIGRADE 
Fe Fe PP We 
A 
ar ro a 
ie a 20 390 o 50 co 70 B° 90 100 10 120 130 140 
By comparison of * 
these curves with the Fie. 87.—Experimental time-temperature curves for spin- 
entire curve, starting ach in No. 2 tin cans, starting at different uniform 
at 20° aie it will be temperatures, processed for different time periods at 
: 116° C., and cooled in water at 23° to 24° C. The arrows 
seen that after 60 indicate when the cans were taken from the retort and 
minutes of processing placed in water. A, Entire heat-penetration curve for can 
= starting at 20° C. and processed at 116° C. Curve for can 
the temperature at starting: B, At 70° C. and processed for 80 minutes; BD, at 
the center of the can 70° C, and processed for 45 minutes; b’, at 70° C. and 
: o: processed for 60 minutes; c, at 80° C. and processed for 
starting at 80° is only 45 minutes; ce’, at 80° C. and processed for 60 minutes. 
6 degrees above that 
of the can starting at 20°, and in the case of the can starting at 70° C. 
it is only 4 degrees higher. 
When the can is cooled in water the temperature at the center of 
the can continues to rise for one or two minutes only and then falls 
rapidly. It is obvious, therefore, that here also, as in the case of 
corn and sweet potatoes, the cooking of the material in the can may 
be closely controlled by cooling in water. 
TIME IN MINUTES 
PRESSURE AND VACUUM IN GLASS CONTAINERS. 
The studies upon pressure and vacuum as applied to foods in glass 
have not been completed. As is well known, the development of 
pressures in glass containers is accompanied by danger and _ loss, 
owing to liability to breakage, and this is avoided in present prac- 
tice by partially sealing at first and then completing the sealing oper- 
ation after the processing has been done. The escape of air, while 
nearly or entirely complete in most cases, may be slightly restricted 
