Development of a multi-temperature calibration method for measuring dielectric properties of food

In the most commonly used, open-ended coaxial probe method, dielectric properties of food products are measured after calibrating the instrument at 25 °C using air (open circuit), short (short circuit) and deionized water. Measurement accuracy may be compromised when dielectric properties are measured at temperatures other than 25 °C. The main objective of this study was to systematically perform calibration at multiple temperatures, quantify measurement errors and develop a method for multitemperature calibration to measure dielectric properties of materials over a wide temperature range. The dielectric properties of deionized water were measured from 1 to 90 °C after calibrating the dielectric measurement system using air, short and deionized water at six different temperatures (1, 25, 40, 55, 70, and 85 °C). The temperature-dependent dielectric properties of water calibrated at six temperatures were then compared with the reported values at two typical microwave frequencies of 915 and 2450 MHz. The results showed that the 25 °C calibration is valid for dielectric constant measurement, but not valid for dielectric loss factor measurement at temperatures far from the calibration temperature. Multitemperature calibration is helpful for reducing errors and improving the accuracy of the temperature-dependent dielectric properties measurement, especially for low-loss materials. Calibrations at two temperatures (25 and 85 °C) within the range studied were found to be suitable for the temperature-dependent dielectric properties measurement. The dielectric properties of lasagna components (ricotta cheese, pasta, and meat sauce) were measured using this multitemperature calibration method.