36 



BULLETIN 956, IT. S. DEPARTMENT OF AGRICULTURE, 



minutes ana then ground in the food chopper and given identical 

 processing temperatures. No appreciable differences were observed. 

 Tests were also made with summer squash prepared as described 

 for the pumpkin. The time- temperature curves are practically the 

 same. These curves have been omitted to save space. 



SWEET POTATOES. 



The variety of sweet potato used in these tests was the Nancy Hall. 

 The potatoes were washed and steamed 30 minutes. After peeling 

 they were allowed to cool and were then ground in a food chopper 

 in order to get a uniform mash, commonly known as " pie stock." 



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Fig. 41. — Time-temperature relations for sweet potatoes when processed in Xo. 2 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 s 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. ; c', from 116° in water at 15° C. ; d', from 121° in water at 

 15° C. : a, from 100° in air at 20° to 24° C. ; h, from 109° in air at 20° to 24° C. ; c, from 

 116° in air at 25° to 251° C. ; d, from 121° in air at 25° to 251° C. 



Tests in glass were incomplete ; hence time-temperature curves for 

 these have been omitted. Figures 41 and 42 show the rise in tem- 

 perature and also the cooling in air and in water for both No. 2 and 

 No. 3 tin cans. 



It will be seen that the temperature changes at the center of the 

 can are very slow, in most cases slightly slower than in pumpkin 

 or sweet corn. The necessity of a considerably longer processing- 

 period for the No. 3 tin can than for the No. 2 is again emphasized. 

 Owing to the firmness of the pack and the absence of free liquid, 

 convection currents play no part in the temperature changes here. 

 Therefore, rotating the can in order to stir the material, as in an 



