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

Authors: Trabelsi, Samir

Submitted to: Journal of Microwave Power and Electromagnetic Energy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2022
Publication Date: 11/8/2022
Citation: Trabelsi, S. 2022. Use of measurement of dielectric properties at microwave frequencies for real-time monitoring of water activity of almonds. Journal of Microwave Power and Electromagnetic Energy. https://doi.org/10.1080/08327823.2022.2142755.
DOI: https://doi.org/10.1080/08327823.2022.2142755

Interpretive Summary: Water activity is a critical parameter often used to determine the shelf life and condition of safe storage of foods and agriculture products. It is mostly used to predict growth of bacteria, yeast, and mold. Conventional methods are tedious, time consuming, and not practical for use in dynamic situations where the water activity is continuously changing. In addition, sampling a few grams for water activity determination is not always representative of the entire sample, especially when large quantities are involved. In this paper, a method based on measurement of the dielectric properties at a single microwave frequency is proposed for real-time monitoring of water activity. As an example, samples of almond kernels were placed in a high-humidity environment in side a chamber and changes in their water activity were tracked through measurement of their dielectric properties at 10 Gigahertz with a free-space transmission technique. Results indicate that the water activity can be predicted from dielectric properties measurement with a standard error of calibration ranging from 0.01 to 0.05. The method can be implemented in microwave sensors for nondestructive, real-time determination of water activity.

Technical Abstract: A dielectric-based method for real-time tracking of changes of water activity of 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 kept in a chamber with high humidity and their dielectric properties were measured at 10 GHz and room temperature (23 oC) every few hours. The dielectric properties increased linearly with time and exponentially with water activity. A three-parameter exponential growth provided equations correlating the dielectric constant (e'), the dielectric loss factor, (e”), and the loss tangent (e”/e’) with water activity with high coefficient of determination (r2). From these equations, water activity calibration equations were obtained. Performance of these equations in predicting water activity in dynamic situation were assessed by calculating the standard error of calibration (SEC). Values of SEC ranged from 0.01 to 0.05.