16 



BULLETlK 956. U. S. DEPARTMENT OF AGRICULTURE. 



In the cooling off, however, there is an increase in the viscosity as 

 the temperature falls. Also, as the temperature at the center of the 

 can falls the temperature gradient between the center and the mar- 

 gin becomes flatter : hence, the force tending to cause convection be- 

 comes smaller and smaller, until finally the resistance due to viscosity 

 is great enough to stop all convection, and the process becomes one of 

 pure conduction, which is very much slower than convection. A 

 pronounced flattening of the curve for 60 per cent sirup at about 30° 

 C. is significant. The difference in viscosity at high and low tempera- 

 tures makes the cooling curve much different from the upward curve. 

 It appears from other tests that sugar solutions of 1 to 4 per cent 

 when added to materials which are canned have very little effect upon 

 the temperature change in the can, but concentration as high as 30 

 to 60 per cent will have considerable effect. 



v 70 



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— ZX^- 



■\—~~ — ~^~— - 











Ju — - 















U ' 



\A 





I 

 i 







t v 



v\ 













t ^ 















t zz 















t / 













' 



t/ 





1 











30 fO SO 60 70 



Fig. 10. — Time-temperature relations for 1 per cent, 2 per cent, 3 per cent, 4 per cent, 

 and 5 per cent starch solutions when processed in No. 3 tin cans at 100° C. Curves 

 A to D were plotted from temperature readings made at intervals of 1 minute and 

 curve E from readings at intervals of 5 minutes. Rise in temperature : A, For 1 per 

 cent solution ; B, for 2 per cent solution ; C, ,for 3 per cent solution ; D, for 4 per cent 

 solution ; E, for 5 per cent solution. 



STARCH SOLUTIONS. 



To study further the effect of viscosity upon the rate of change of 

 temperature in the can 1, 2, 3, 4, and 5 per cent starch solutions were 

 tested in Xo. 3 tin cans in the boiling-water bath. No cooling tests 

 were made. Carefully dried starch was weighed out and enough 

 water added to make 1, 2, 3, 4, and 5 per cent solutions, respectively. 

 The starch was gelatinized or brought into colloidal solution by 

 heating on a steam bath for one hour, with constant stirring, and at the 

 end of this time enough water was added to each lot to equal that lost 

 by evaporation. Each lot appeared as a homogeneous grayish semi- 

 transparent solution or paste. The lots were then cooled to 20° C, 

 put into cans, and the thermometer-holding device soldered to the 

 cans, as described under ''Distilled water/' Figure 10 shows the 

 curves for these tests, Each curve represents the average of three 

 tests. 



