A Microwave Dielectric Method for Rapid and Nondestructive Determination of Moisture Content and Water Activity of Almonds

摘要

A method for rapid and nondestructive determination of moisture content and water activity of almond kernels was developed. The method relies on measurement of the dielectric constant and dielectric loss factor at a single microwave frequency. A free-space transmission technique was usedfor accurate measurement of the dielectric properties. Samples of Bute Padre almond kernels with moisture content ranging from 4.8% to 16.5%> and water activity ranging from 0.50 to 0.93 were loaded into a Styrofoam sample holder and placed between two horn-lens antennas connected to a vector network analyzer. The dielectric properties were calculated from measurement of the attenuation and phase shift at 8 GHz and 25 "C. The dielectric properties increased linearlywith moisture content while they showed an exponential increase with water activity. Linear and exponential regressions provided the equations correlating the dielectric properties with moisture content and water activity with coefficients of determination, r2, greater than 0.96. Analytical expressions for moisture content and water activity in terms of the dielectric properties measured at 8 GHz and 25 °C are provided.