TEMPEEATUBE CHANGES, ETC., DUEING CANNING 0PEEAT10NS. 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 experimental data are shown for the No. 

 3 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 c. c. 

 of water sealed at different temperatures and cooled. 



Temperatures (° C). 



Barometer readings. 



Vacuum 

 reading 



At sealing. 



After 

 cooling. 



At 

 sealing. 



After 

 cooling. 



(inches of 

 mer- 

 cury). 



50. . . 



16J 



18 

 19 

 20 



19i 



30.19 

 30.39 

 30.18 

 30.16 

 30.05 



30.43 



30.18 

 30.16 

 30.05 

 30.18 



8.62 



60 



10.12 



70 



11.75 



80 



13.87 



90 



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 



