Nondestructive sensing of water activity from measurement of the dielectric properties

Authors: Trabelsi, Samir

Submitted to: IEEE Sensors Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/30/2020
Publication Date: 12/4/2020
Citation: Trabelsi, S. 2020. Nondestructive sensing of water activity from measurement of the dielectric properties. IEEE Sensors Journal. https://ieeexplore.ieee.org/document/9281317.
DOI: https://doi.org/10.1109/LSENS.2020.3042749

Interpretive Summary: Water activity is the most critical parameter that determine the safety and shelf life of water-containing materials including agricultural products, foods, and pharmaceuticals. Conventional techniques for water activity determination are destructive, time consuming, and not practical for in-process measurements when large amounts are involved. Therefore, there is a need for the development of novel methods and sensors for in-process nondestructive and rapid determination of water activity of materials. In this paper, an original dielectric method is proposed for determining water activity in granular and particulate materials from measurement of their dielectric properties at microwave frequencies. Because of the effect of changes in bulk density on dielectric properties, situations where the bulk density is known and situations where bulk density is unknown were considered. For purpose of illustration, analytical expressions for water activity from measurement of the dielectric properties at a single microwave frequency were established for almonds kernels. Dielectric properties of almond kernels were measured in free space at 8 Gigahertz and room temperature (25 degree Celsius). To evaluate the effectiveness of these water activity calibration equations, the standard error of calibration (SEC) was calculated for each equation. 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.%. Similar approach can be used to determine water activity calibration equations for other granular and particulate materials. Once these calibration equations are established they can be embedded in sensing systems for in-process nondestructive determination of water activity in those materials.

Technical Abstract: A rapid and 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 oC. Both situations where the bulk density is known and unknown were considered. For situations where 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 d) and a density-independent calibration function (y) expressed in terms of the dielectric properties and water activity were used. For purpose 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 oC. In all situations, it was found that the dielectric properties, the loss tangent, and ' (y) 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.%.