A Thermistor Based Method for Measurement of Thermal Conductivity and Thermal Diffusivity of Moist Food Materials at High Temperatures
van Gelder, Maarten F.
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CONDUCTIVITY AND THERMAL DIFFUSIVITY OF MOIST FOOD MATERIALS
AT HIGH TEMPERATURES
Maarten F. van Gelder
Kenneth C. Diehl, Jr., Chairman
Biological Systems Engineering
Thermistor probes were constructed in house and calibrated in three materials of known thermal conductivity and diffusivity, water, glycerol, and a heat transfer fluid, HTF 500. With few exceptions, the calibrated probe estimated thermal properties with an error of less than 5%, over the range of thermal properties spanned by the those of the calibration media. An alternate calibration using two media was also investigated. It was found to give better accuracy over a more limited range. Thermal contact in potato and lean beef was investigated through a comparative study that used a miniature line heat source probe as a reference method. The food materials were measured at 25, 50 and 100 Â°C. Good agreement was found between the measurements with the line heat source probe and the bead thermistor probe, indicating adequate thermal contact at the thermistor probe.
The effect of fluid viscosity and the magnitude of the temperature step on the occurrence of natural convection was studied for aqueous solutions of a thickening agent. During a sample time of 30 seconds, convection was absent in solutions with a viscosity of 25 cp or greater, when measured with a temperature step of 1.5 and 2.5 Â°C, and in solutions with a viscosity of 50 cp or greater, when measured with a temperature step of 5.0 Â°C. A Rayleigh number was defined to study the notion of a critical Rayleigh number at the onset of convection. This study found that when the Rayleigh number was below 43, convection could not be demonstrated. For a Rayleigh number of 84 and higher, convection was observed.
The performance at high temperatures in food materials was studied through tests in tomato concentrate and in a liquid food supplement. Tomato puree and tomato paste were sampled at 100, 130 and 150 Â°C. The thermal conductivity of tomato puree at 100, 130 and 150 Â°C was measured as 0.638, 0.645 and 0.647 W/mÂ°C respectively. The thermal diffusivity was 1.63, 1.64 and 1.62 10 -7 m 2 /s respectively. For tomato paste at 100, 130 and 150 Â°C, a thermal conductivity was obtained of 0.590, 0.597 and 0.534 W/mÂ°C respectively. The thermal diffusivity was 1.63, 1.84 and 2.36 10 -7 m 2 /s respectively. With some notable exceptions the results of this study agreed well with Choi and Okos (1983). A liquid food supplement was also studied at 95 and 150 Â°C. The thermal conductivity of the food supplement decreased with increasing solids content from 0.62 W/mÂ°C at a solids level of 15% to 0.41 W/mÂ°C at a solids level of 50%.
The results of this study indicate that the thermistor based method was suitable for measuring thermal conductivity and diffusivity of moist food materials at high temperatures. However, the type of thermistor used in the research, a glass encapsulated thermistor, was too fragile for routine work. In particular the high temperature use of the glass thermistor was impacted by its susceptibility to fracture.
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