50 BULLETIN 956, U. S. DEPARTMENT OF AGRlCULTtJEE. 



i^o. 2 tin can, 2 hours and 25 minutes; and No. 3 tin can, 5 hours. 

 The temperature fell to about 20° C. over night. 



The time-temperature relations were entirely different in the 

 processing on the second day. The temperature change was very 

 much slower, as during the first processing the soluble proteins 

 cooked out into the liquid to such an extent as to form a colloidal 

 jelly. The change in consistency of the liquor was such that all con- 

 vection was prevented in the second processing, and the heat passed 

 in only by conduction. The temperature curves for the second heat- 

 ing were almost exactly the same as those of sweet corn. As in sweet 

 corn, the temperature did not reach 100° C. at any time. The third 

 processing was entirely similar to the second. 



The very slow rate of change of temperature in the second and 

 third periods of processing increased very greatly the possibility of 

 an incomplete sterilization. Therefore, a period longer than 1 hour 

 for the second and third processing would decrease the possibility of 

 spoilage. 



SWEET POTATOES. 



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

 The potatoes were washed, steamed for 30 minutes, peeled, and al- 

 lowed to cool. They were then ground in the food chopper in order 

 to get a uniform mash commonly known as " pie stock." This was 

 placed in the cans and processed for 1 hour on each of three succes- 

 sive days, as in the preceding tests. Figure 55 shows the time- 

 temperature records for No. 2 and No. 3 tin cans and for pint and 

 quart glass jars. 



The rate of rise in temperature was very slow, in no case reaching 

 100° C. during the processing. The maximum temperature attained, 

 between 80° and 90° C. in the different containers, was reached at a 

 considerable time after removal from the processing bath. The high- 

 est temperature was reached in the pint glass jar, the No. 2 tin can 

 being next, then the quart glass jar, and the No. 3 tin can the lowest. 

 The cooling was as might be expected from material of this sort. The 

 temperature remained above 80° C. long enough to destroy most 

 vegetative forms of bacteria. The results of the second and third 

 period of processing were entirely similar to the first. 



FACTORS INFLUENCING THE RATE OF CHANGE OF TEMPERATURE. 



All those factors discussed under the heading " Single-period 

 processing " apply to the first period of the intermittent process. 



In addition to these factors the first period of the processing has 

 an effect upon the material as treated during the second and third 

 periods. As already observed in soy beans, there is often a change in 

 the material during the first processing period which greatly affects 



