Speaker
Description
Introduction
Several methods have demonstrated the use of terahertz waves to estimate the mass diffusion coefficient under transient conditions. Monitoring transient water content is recognized as a key parameter in many drying processes (e.g., wood, paper, etc.). Some techniques allow water content to be measured locally – for example, in geology, where time-domain reflectometry (TDR) is commonly employed 1. This technique has been successfully used to monitor the progression of a wetting front in dry soils gradually moistened by irrigation.
For food product, accurate control of water content remains essential to ensure product quality and to extend their shelf life. In this study, terahertz waves are employed to locally measure the water content in a cereal-based product
Terahertz (THz) radiation, by contrast, is non-ionizing and thus safe for use in food and biological systems. It refers to electromagnetic waves with frequencies ranging from approximately 0.15 to 5 THz.Moreover, THz radiation is strongly absorbed by water and reflected by metals—properties that make it particularly suitable for investigating moisture distribution in complex matrices.
Experimental device
The experimental setup consists of a commercial Terahertz scanner distributed by Terasense (San Jose, CA, USA). The Terahertz Imaging Scanner system comprises two main components: a linear Terahertz imaging camera and a 100 GHz Terahertz generator. Both components are optimized and synchronized. THz radiation is efficiently delivered from the generator to the camera sensor (256 pixels, up to 5000 lines/sec). The camera pixel size provides an image resolution of 1.5 mm2. The sample to be analyzed was transported along a conveyor belt (Figure 1), with the Terahertz source positioned above the belt and the line camera mounted below.
Method
A freshly prepared muffin was cut into slices of equal thickness and placed on a plastic sheet, which was then conveyed between a Terahertz source and a line scanner to measure the attenuation associated with both the solid matrix and the contained water. Subsequently, the slices were dried in an oven to remove moisture, and a second scan was performed to obtain the attenuation corresponding solely to the solid matrix. The ratio of the two transmittance images was then used to generate a 2D map of the water content in each slice. Finally, by assembling all slices, a 3D visualization of the water distribution within the muffin was reconstructed.
Results
Each slice shown in Figure 3 represents the local water content integrated over the thickness of the slice. Thus, the information obtained per slice allows the water content in different regions of the muffin to be estimated with a resolution of 6 mm. By integrating the data from all slices, the average water content across the entire thickness of the muffin is obtained, providing volumetric information per voxel. Using a principle analogous to X-ray tomography, the 3D volume is reconstructed from an average of the information collected by the detector.
Conclusions
This results provides detailed insights into the spatial distribution of water within a porous food matrix, complementing conventional techniques that typically quantify only global moisture content. While other methods, such as Near-Infrared (NIR) imaging, can also offer local water content information [21], Terahertz imaging provides additional advantages: it is rapid and non-ionizing, making it particularly suitable for industrial applications in food quality control and process monitoring.
References :
1. Walczak A., Szypłowska A., Janik G., Pęczkowski G.: Dynamics of volumetric moisture in sand caused by injection irrigation physical model, Water, Switzerland, 13 (11), art. no. 1603 (2021).
2. Ledieu J., De Ridder P., De Clerck P., Dautrebande S.: A method of measuring soil moisture by time-domain reflectometry Journal of Hydrology Volume 88, Issues 3–4, Pages 319-328, (1986).
