TEMPERATURE CHANGES IX CAX^IXG FRUITS AND VEGETABLES. 21 



The temperature changes obtained in these tests are easily under- 

 stood when the composition of the beans and the character of the 

 pack are held in mind. The string beans contain only a small pro- 

 portion of starch or other colloidal material which would readily 

 go into solution or gelatinize, so the free liquid in the can is thus 

 maintained throughout the processing period. This medium allows 

 of convection, which rapidly distributes the heat throughout the can. 



The surface tension between the liquid and the insoluble particles 

 of material tends to obstruct convection currents, but since the pieces 

 of material in this case are comparatively large the effect of the 

 force of surface tension is correspondingly small. However, if the 

 material is finely divided, as when it is ground in a food chopper, 

 the surface tension is increased to such an extent as to cut clown very 

 greatly the rate of change of temperature. 



Experiments were also made to determine whether the fullness 

 of the pack has any effect upon the rate of change of temperature 

 in the center of the can. In cans packed extra full and in cans 

 lightly packed the differences were found to be so small as to be 

 almost negligible. However, when the beans are thoroughly cooked 

 they may be packed in the can so tightly as to make the interspaces 

 filled with liquid more or less discontinuous, in which case there is a 

 very marked slowing down of the temperature changes at the center 

 of the can. 



PEAS. 



The variety of garden peas used in these tests was the Early 

 Alaska. The peas were gathered from the field as used and brought 

 into the laboratory and shelled by hand. In some of the tests the 

 peas had somewhat passed the prime stage for canning. No attempt 

 was made to grade them. They were blanched five minutes in the 

 boiling-water bath and cooled to room temperature. The tin cans 

 were filled to within one-fourth of an inch, and the glass jars to 

 within half an inch of the top. Then enough 2 per cent brine was 

 added to cover the peas. Each kind of container was processed at 

 100°, 109°, 116°, and 121° C. They were each cooled in air, and the 

 tin cans were also cooled in water. Figure 15 shows the time- 

 temperature record of a No. 2 can during the processing period at 

 the various temperatures and also the cooling in air and in water. 

 Figures 16, IT, and 18 show the temperature curves for the process- 

 ing period in the No. 3 tin can and in the pint and quart glass jars, 

 respectively. 



It is observed from these curves that the temperature rises very 

 rapidly. The No. 2 tin can approaches the temperature of the retort, 

 or bath, in about 12 minutes, the No. 3 can in about 15 minutes, the 

 pint glass jar in 30 minutes, and the quart jar in about 35 minutes. 



