Use of dielectric properties measurements at microwave frequencies for real-time monitoring of water activity in almonds

A dielectric-based method for real-time tracking of changes in water activity in almond kernels is proposed. The method is based on measurement of the dielectric properties at a single microwave frequency. Samples of almond kernels (Butte/Padre and Nonpareil) were placed in a chamber with high humidity and their dielectric properties were measured at 10 GHz and room temperature (22 °C) every few hours. The dielectric properties increased linearly with time and exponentially with water activity. A three-parameter exponential growth equation provided relationships correlating the dielectric constant (ε’), the dielectric loss factor, (ε”), and the loss tangent (ε”/ε’) with water activity with high coefficients of determination (r2). From these equations, water activity calibration equations were obtained. Performance of these equations in predicting water activity under dynamic situations were assessed by calculating the standard error of calibration (SEC). Values of SEC ranged from 0.01 to 0.05.