Nondestructive Sensing of Water Activity From Measurement of the Dielectric Properties at Microwave Frequencies

A rapid, nondestructive method for determining water activity of granular and particulate materials from measurement of the dielectric properties at a single microwave frequency is presented. Results are shown for almond kernels at 8 GHz and 25 °C. Situations in which bulk density is known and unknown were considered. For situations, when bulk density is known, direct correlations between the dielectric properties and water activity were identified. When the bulk density is unknown, correlations between the loss tangent (tan δ) and a density-independent calibration function (ψ), expressed in terms of the dielectric properties and water activity, were used. For purposes of illustration, analytical expressions of the water activity are derived for each situation for almond kernels for which the dielectric properties were measured at 8 GHz and 25 °C. In both situations, the dielectric constant, loss factor, loss tangent, and ψ increased exponentially with water activity. The standard error of calibration (SEC) was calculated to evaluate the effectiveness of each of these analytical expressions in predicting water activity from dielectric properties measurement. The SEC ranged from 0.02 to 0.04 for almond kernels with water activity ranging from 0.5 to 0.9 and moisture content ranging from 4.8 to 16.5%.