TEMPERATURE CHANGES, ETC., DURING CANNING OPERATIONS. 17 
figures, to assist in the selection of the best sealing temperatures, and 
to obtain a base line for comparing various food materials, vacuum 
tests were made with No. 2 and No. 3 cans containing 550 and 950 
c. c. of water, respectively, sealed at different temperatures and cooled 
to room temperature. The éxperimental data are shown for the No. 
8 cans in Table 6. Figures for the No. 2 cans are omitted, as they do 
not vary essentially from those of the No. 3 cans except in having a 
slightly greater vacuum, due to greater resistance to external pressure. 
TABLE 6.—Vacuum readings obtained with No. 3 tin cans containing 950 ec. ec. 
of water sealed at different temperatures and cooled. 
| 
Temperatures (° C.). Barometer readings. Vacuum 
reading 
he ‘ | fe (inches of 
‘ ter | At ter mer- 
At sealing. cooling. | sealing. | cooling. | cury). 
| 
Sie. OS jet) 2 See Se ee ee oe eee 164 30.19 30. 43 8. 62 
AN «Sema = gn ot Bi, eee el pr 18 30. 39 30.18 10. 12 
(ICE Seis on ogi SS Eo eS eS See eee ee reno 19 30. 18 30. 16 ell. 75 
Lisl. soleil ie ee a ee ee 20 30. 16 30. 05 13587 
SNE NEP ee lr iN yn nl. Satta cic sibaion sod Woe einen 194 30. 05 30.18 19. 00 
Leading facts developed in comparisons of these findings with the 
theoretical curves show the following: 
(1) At the sealing temperatures of 70° C. and above the vacuums obtained are 
below the theoretical. This is to be expected, of course, inasmuch as the same 
factors which affect recorded pressures, discussed above, are operative here, 
although working in the other direction. That is, contraction takes place instead 
of expansion; the temperatures are lowered instead of increased; the can is 
distorted inward by external pressure instead of outward, etc. 
(2) The higher the sealing temperature the greater the variation from the 
theoretical vacuums. 
(3) In these tests the cans sealed at the lower temperatures show vacuums 
Slightly above their theoretical values. This was found, after examining 
the cans, to be due to permanent distortion of the cans during processing, 
for the vacuum readings were made on the cans employed in the above pressure 
tests which after cooling did not return to their normal shape. If the cans 
are sealed at these temperatures and cooled immediately to the desired temper- 
ature, the vacuum is slightly below the theoretical. 
RATE OF TEMPERATURE CHANGE WITHIN THE CAN. 
From the above statement it is obvious that in order to avoid undue 
strain upon cans, and for other reasons mentioned, the temperature of 
sealing should be considerably above room temperature. The ques- 
_ tion then immediately arises, How will this affect the length of time 
required to secure the proper temperature in the can? To shed light 
upon this matter and to obtain a base line for the comparison of the 
various food substances, tests were made upon the rate of temper- 
ature changes in distilled water. Figure 6 shows the results of these 
70344°—22——3 
