Temperature-Dependent Dielectric and Thermal Properties of Whey Protein Gel and Mashed Potato

Temperature-dependent dielectric properties (dielectric constant and dielectric loss factor) and thermal properties (thermal conductivity and specific heat capacity) of whey protein gel and mashed potato were measured from -20°C to 100°C. A dielectric properties measurement system and a multipoint temperature calibration protocol were developed. The system consists of an impedance analyzer, a high-temperature coaxial cable, a high-temperature coaxial probe, a micro-climatic chamber, and a metal sample holder. Calibrations at two temperatures (25°C and 85°C) were sufficient to accurately measure the dielectric properties of foods from frozen to hot temperatures. Dielectric constant and dielectric loss factor both rapidly increased from -20°C to 0°C. Thereafter, dielectric constant linearly decreased from 0°C to 100°C, while dielectric loss factor decreased first and then linearly increased. The thermal conductivity values of whey protein gel and mashed potato decreased with increasing temperature in the frozen range and did not change considerably after thawing. The latent heat of fusion values of whey protein gel and mashed potato were 219.1 and 186.8 kJ·kg^-1, respectively. The temperature-dependent material properties can be used in microwave heat transfer models for improving heating performance of foods in domestic microwave ovens.