A microwave dielectric method for rapid and nondestructive determination of moisture content and water activity of almonds

Authors: Trabelsi, Samir

Submitted to: ASABE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 6/22/2019
Publication Date: 7/7/2019
Citation: Trabelsi, S. 2019. A microwave dielectric method for rapid and nondestructive determination of moisture content and water activity of almonds. ASABE Annual International Meeting. ASABE Paper No. 1901908, 2019.

Interpretive Summary: Moisture content and water activity of almonds are critical parameters for determining the quality of the nuts, conditions for safe storage, shelf life, and market value. While moisture content is a measure of total water content in a product, water activity is a measure of the water available in a product to support biological action such as bacterial or fungal growth. Conventional methods for determining moisture content and water activity are destructive for the product sample and also time consuming. Therefore, research has been conducted to find out whether water content and water activity might be successfully sensed in almonds by a new technique. Microwave dielectric-based methods and sensors can be used for rapid and nondestructive determination of moisture content in grain, nuts and other products. This is because there are direct correlations between the dielectric properties and physical properties including moisture content and bulk density. The dielectric properties of a material are intrinsic electrical properties that determine the interaction of electric fields with the material. They are dependent on frequency, temperature, moisture content, and composition. For granular and particulate materials, they are also dependent on the bulk density of those materials. It is this dependence between dielectric properties and physical properties that constitutes the core of rapid sensing methods with applications in different industries including food, agricultural, pharmaceutical, and mining. In this research, the dielectric properties of almond nuts of various moisture contents were measured over a range of moisture content at three different bulk densities at 25 C. The moisture content and water activity were also determined by traditional methods. Then moisture content and water activity were calculated from the dielectric properties at each point, and inter-relationships were analyzed. Results of this research have revealed a nondestructive method for rapid determination of moisture content and water activity of almond kernels from measurement of the dielectric properties at a single microwave frequency. The method relies on linear correlations between the dielectric properties and moisture content and exponential growth-type correlations between the dielectric properties and water activity. Among the dielectric properties used for moisture and water activity determination, the loss tangent is most useful because it can be used without knowledge of the bulk density. Further research and development of practical instruments for sensing moisture content and water activity by using the techniques developed in this research will provide useful tools and information for management of storage and handling conditions for almonds and other agricultural products that will be important in economy of production and preservation of high quality for the consumer.

Technical Abstract: 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 used for 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 oC. The dielectric properties increased linearly with 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 oC are provided.