TEMPERATURE CHANGES IX CANNING FRUITS AND VEGETABLES. 37 



agitating cooker, would have much less effect with sweet potatoes 

 than the same treatment with sw r eet corn. 



COMPARISON OF RAW AND COOKED MATERIAL. 



Tests with sweet potatoes ground in a food chopper and packed 

 into the can raw and with material cooked 30 minutes, ground, and 

 packed into the can were made, the processing being conducted under 

 identical conditions. There were no appreciable differences in the 

 rate of change of temperature in the can. It is apparent that the 

 gelatinization of the starch has little effect if the nature of the ma- 

 terial at the outset is such that convection is prevented. 



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Fig. 42. — Time-temperature relations for sweet potatoes when processed in No. 3 tin 

 cans at 100°, 109°, 116°, and 121° C. and also when cooled in air and in water. These 

 curves were plotted from temperature readings made at intervals of 5 minutes. Rise 

 in temperature when processed : A, At 100° C. ; B, at 109° C. ; C, at 116° C. ; D, at 

 121° C. Fall in temperature when cooled : a', From 100° in water at 15° C. ; V , from 

 109° in water at 15° C. ; &j tfron/ 116° in water at 19° C. ; d' ' , from 121° in water at 

 19° C. ; a, from 100° in air at 18° to 22° C. ; b, from 109° in air at 20° to 24° C. ; c, from 

 116° in air at 25° to 25J° C. : d, from 121° in air at 25° to 25J° C. 



PROCESSING FOR DIFFERENT LENGTHS OF TIME AT 11G C. 



Figure 43 shows the result of a series of tests of No. 3 tin cans 

 processed at 116° C. for 50, 60, TO, 80, 90, and 100 minutes and then 

 put immediately into the air. The temperature of the air varied 

 somewhat, so that the results are not exactly uniform. Two facts are 

 brought out by this series of tests: (1) The temperature continues 

 to rise for 20 to 30 minutes after the cans are put into the air; (2) 

 at this processing temperature 90 to 100 minutes are required to carry 

 the temperature at the center of the can to 100° C, or above. This 

 is simply a " cut and try " method of finding the length of time neces- 

 sary to sterilize any particular pack of canned material. If the cans 

 had been left in the retort or put into water, the results would have 



