42 



BULLETIN 1022, U. S. DEPARTMENT OF AGRICULTURE. 



potatoes, also when cooled in water, there is a continued rise of 

 temperature at the center of the can for an indefinite period. 

 Accurate information for this purpose must be obtained by pro- 

 cessing for different periods and cooling in air and water, for 

 which purpose the formulas of Thompson and of Bigelow are 

 scarcely applicable. 



The cooling curves are of interest in that they illustrate clearly 

 the desirability of cooling cans of sweet potatoes in water. Owing 

 to the high percentage of sugar in the sweet potato, caramelization 

 is very pronounced when the processing temperatures are long con- 

 tinued, and it. is desirable, therefore, that the processing period be 

 carefully controlled. Air cooling, as shown by the curves, results 

 in a slow fall of temperature, and the cooking is long continued. 

 It must be rememered, however, that in slow cooling the sterilizing 

 effect is continued for a longer period. How promptly the tempera- 

 ture falls when the can is cooled in water is illustrated by the other 

 cooling curves. 



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D ^S 



































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2 

 P TO 



















































































Fig. 28. — Experimental time-temperature curves for sweet potatoes in the form of pie 

 stock in No. 2 tin cans, starting at a uniform temperature of 70° C, processed for 

 different time periods at 116° C, and cooled in air at ordinary room temperature. 

 The arrows indicate when the cans were removed from the retort. Curve for can 

 processed: A, For 30 minutes; B, for 45 minutes; G, for 60 minutes; D, for 75 

 minutes; E, for 90 minutes. 



Special attention is directed to the form of the curves for water 

 cooling during the first 15 minutes. In the one case the curve is 

 quite regular, while in the other there is a sudden drop in the curve, 

 which later becomes more regular and- eventually coincides with the 

 first. The only difference in the conditions under which these tests 

 were performed was that in the first case the can had a very small 

 head space, while in the other a head space of about three-eighths of 

 an inch was allowed. A similar condition was pointed out in the 

 discussion on the cooling curves for corn where, also, the consistency 

 was shown to be a factor. Owing to the heavy consistency and the 

 necessity of filling the can almost completely, the cooling curves of 

 sweet potatoes are more nearly the exact opposite of the heating 

 curves than in any other substance tested. 



In figure 28 are shown the experimental time-temperature curves 

 for No. 2 cans of sweet potatoes, starting from a uniform initial tern- 



