22 



BULLETIN 956, IT. S. DEPAETMENT OE AGRICrrLTtTEE. 



The temperature changes are like those in string beans except that 

 the time required to pass through the last degree is in most cases very 

 much longer in the peas than in the string beans. Some viscous 

 colloidal material cooks out into the free liquid during the processing 

 period. The liquid seems to reach such viscosity as to stop convection 

 currents at this point. The viscosity does not have to be very great 

 in order to stop convection Avhen the difference in temperatures at the 

 center of the can and at the margin is one degree or le&s. 



The difference in cooling in air and water is very marked, as is 

 shown by the curve in figure 15. The curves for cooling in water 

 show a marked slowing down of the fall of temperature when it 



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Pig. 15. — Time-temperature relations for peas in 2 per cent brine when processed in 

 No. 2 tin cans at 100°, 109°, 116°, and 121° C. and also when cooled in water and in 

 air. The curves representing the rise in temperature during processing and the fall 

 in temperature during cooling in water were plotted from readings made at intervals 

 of i minute and 1 minute ; those for cooling in air, from readings at intervals of 

 5 to 10 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 20° C. ; V, from 109° in water at 17° C. ; c' , from 116° in water at 20° C. ; a', from 

 121° in water at 19° C. ; a, from 100° in air at 26° to 30° C. ; b, from 109° in air at 

 24° to 28° C. ; e, from 116° in air at 24° to 28° C. ; d, from 121° in air at 24° to 28° C. 



reaches about 45° C, in those curves cooling from 121°, 116°, and 

 109° C. The one processed at 100° does not follow the same course 

 as the other three. During the processing at the higher temperatures 

 some soluble colloidal material was cooked out, Mdiich formed a solu- 

 tion so viscous as to stop convection at this point in the cooling. This 

 does not occur in every case, and the rather mature condition of the 

 samples used in these cases perhaps explains its occurrence here. 



As in string beans, there need be little difference in the length of 

 the processing period for No. 2 and No. 3 tin cans, as far as the rate of 

 change of temperature at the center is concerned. The time for the 

 processing of glass jars, however, should be longer. 



