Date Published: March 13, 2017
Publisher: Public Library of Science
Author(s): Mfrekemfon Samuel Obot, Changcheng Li, Ting Fang, Jinquan Chen, Xiao-Dong Wang.
Thermal properties are necessary for the design and control of processes and storage facilities of food materials. This study proposes the measurement of thermal properties using easily constructed probes with specific heat capacity calculated, as opposed to the use of Differential Scanning Calorimeter (DSC) or other. These probes were constructed and used to measure thermal properties of white radish in the temperature range of 80–20°C and moisture content of 91–6.1% wb. Results showed thermal properties were within the range of 0.71–0.111 Wm-1 C-1 for thermal conductivity, 1.869×10−7–0.72×10−8 m2s-1 for thermal diffusivity and 4.316–1.977 kJ kg-1C-1for specific heat capacity. These results agree with reports for similar products studied using DSC and commercially available line heat source probes. Empirical models were developed for each property through linear multiple regressions. The data generated would be useful in modeling and control of its processing and equipment design.
Thermal properties are those physical properties of a material that are significant in heat transfer problems. Some of these properties include: dimensional characteristics, density, fluid viscosity, unit surface conductance latent heat, specific heat, thermal conductivity, thermal emissivity, thermal diffusivity, coefficient of thermal expansion etc. The dependence of these properties on the temperature of the materials’ ambience and their moisture content is necessary in the modeling, simulation and equipment design of various food processing operations . Process thermal controls can only be put to use by the precise knowledge of the thermal properties of the food. The primary thermal properties are specific heat capacity (at constant pressure), thermal conductivity and thermal diffusivity. The specific heat energy measures the amount of energy that is required to raise the temperature of a body by 1°C; thermal conductivity is an intrinsic property which measures the ability of a substance to conduct heat, while thermal diffusivity is the ability of a material to conduct thermal energy relative to its ability to store energy. To calculate heat transfer in a food, its thermal properties, geometry and thermal processing condition are used as the major parameters .
The moisture content at fresh weight was 91% wet basis. The results of the thermal conductivity probe calibration were 0.630 (±0.012) Wm-1°C-1 for 5% agar gel, 0.0653 (±0.001) Wm-1°C-1 for powdered milk respectively. These were compared to the values published by  which were 0.628 Wm-1°C-1 and 0.0650 Wm-1°C-1 for 5% agar gel and powdered milk respectively at 30°C.
The thermal properties of white radish were measured using easily constructed line heat source probe and the thermal diffusivity probe in this study and the effects of temperature and moisture on white radish were evaluated within the range of 20–80°C and 91–6.1% wb respectively. The thermal conductivity was between 0.71 and 0.111 W m-1°C-1, the thermal diffusivity was between 1.869×10−7 m2s-1 and 0.72×10−8 m2s-1 and the specific heat was between 4.316 and 1.977 kJ kg-1°C-1.Our results were in agreement with earlier studies in terms of range of values and in the trend of results. The results of this study were fitted to appropriate thermal property models in each case. They all had good fits showing the reliability of the measured data. The specific heat capacity was calculated from the values measured with the thermal conductivity and diffusivity probes. Both thermal diffusivity and specific heat were in the range reported in the literature for materials with similar properties. The regression models from this study may be used to control storage and processing of white radish.