TEMPERATURE CHANGES IX CANNING FRUITS AND VEGETABLES. 17 



It is seen from the curves that the rate of change of temperature 

 at the center of the can in 1 per cent and 2 per cent starch solutions 

 is not very different from that in distilled water, although there is a 

 • slight slowing down of the process. Curve (\ figure 10, shows that 

 the first part of the process of the 3 per cent solution is very rapid, 

 ' but when the center of the can reaches about 92° C. there is a very 

 marked slowing down of the rate of rise of temperature. Also in 

 curve />, representing the 4 per cent starch solution, the first part 

 of the process is very rapid, but when the temperature at the center 

 of the can reaches about 80° C. there is a marked slowing down of its 

 rate of rise. It stops almost entirely at about 83° and remains there 

 for 10 to 15 minutes; then it begins to rise again, and gradually ap- 

 proaches the temperature of the bath. In the 5 per cent solution the 

 process is slow from the beginning. It is clear that in the 1 per cent 

 and 2 per cent solutions and in the first part of the processes of the 

 3 per cent and 4 per cent solutions convection is occurring, which 

 explains the rapid rise in temperature at the center of the can. In 

 the 5 per cent starch solution and in the last part of the process of 

 the 3 per cent and 4 per cent solutions convection is not occurring to 

 any great extent, and the heat reaches the center of the can by con- 

 duction only. In the 4 per cent solution the resistance due to the 

 viscosity is not great enough to stop convection in the first part of 

 the process. The force tending to produce convection becomes less 

 and less as the temperature at the center of the solution approaches 

 the temperature of the bath. Hence, convection continues in the 

 solution until this force becomes so small that it fails to overcome 

 the resistance due to viscosity, when the process of convection stops. 

 Then the heat is conveyed only by conduction. This is probably Avhat 

 happened in the 3 per cent and 4 per cent starch solutions. Further 

 change in the starch may have been a factor, as no tests were made 

 to determine whether the starch solution had reached its maximum 

 viscosity in the preliminary treatment. The curves for cooling in 

 water would have been interesting, but unfortunately they were not 

 made. 



From these preliminary experiments it is concluded that the 

 factors affecting the rate of change of temperature at the center of 

 the can are the diameter of the container, the conductivity, thick- 

 ness and radiative power of its walls, the temperature, conductivity, 

 and mobility of its contents, and the temperature, conductivity, and 

 movement of the medium surrounding it. 



SINGLE-PERIOD PROCESSING. 



With the facts of the preliminary experiments in mind, work was 

 done with the various fruits and vegetables commonly canned, for 

 44900°— 21 3 



