TeRecent advances in Terahertz (THz) technology have enabled various applications in food inspection and quality control of powders and food. THz technologies do have some limitations in food inspection application, one of which is that THz is strongly absorbed by water. While this actually offers an advantage when using THz for quality control of powders, this makes the transillumination of some food products with water content such as meat impossible. The second limitation is imposed by metal layers in food packages: as soon as the thickness of such a metal layer is more than a few microns, THz waves cannot pass the barrier.
Despite these limitations there are still many scenarios in which food products can benefit from a THz inspection system. One example is using THz imaging to reveal potentially hazardous inclusions in chocolate bars. While metallic contaminations are easily detected by conventional quality control systems using X-ray, dielectric contaminations are often hard to find due to similar densities or lack of contrast. When the physical contrast between the material and the contamination is slight existing approaches such as ultrasonic or X-ray scans may fail, so that certain types of inclusions, e.g. plastic and wood splinters, etc. remain undetected. However, such contaminations can still present serious health risks for consumers.
In contrast to X-ray systems, THz scans do not only consider the amplitude but also phase information. Due to the fact that the dielectric contrast mechanisms are much more pronounced at THz frequencies, hidden inclusions can be reliably revealed in THz images. In the figure an image is shown of a chocolate bar contaminated with a buried glass splinter, a stone and a piece of metal, is depicted. It can be seen that the contaminants can be clearly detected.
Another promising application of THz technology lies in the area of quality control of packaged goods. Terahertz radiation can be used to detect production faults in flexible plastic packages. Relying on the large difference between the absorption coefficients of plastic and water (for water-filled channel defects) and on the refraction index difference between plastic and air (for air-filled channel defects), the technique consists of focusing and scanning a terahertz beam on the sealed area of the package, followed by detection of the transmitted signal.
Compared with traditional methods, such as visual and ultrasound inspection, this technique can be applied to optically opaque packages and does not require immersion in a matching liquid. The detection limit (the minimum size of a detectable defect) depends on the conveying speed. The results show that THz system has the potential for application in an actual production line for real-time inspection. In the near future, THz system may be used for real-time inspection in actual production line.
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