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A Comprehensive Review on Food Applications of Terahertz Spectroscopy and Imaging Leili Afsah-Hejri , Parvaneh Hajeb, Parsa Ara, and Reza J

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A Comprehensive Review on Food Applications of Terahertz Spectroscopy and Imaging Leili Afsah-Hejri , Parvaneh Hajeb, Parsa Ara, and Reza J A Comprehensive Review on Food Applications of Terahertz Spectroscopy and Imaging Leili Afsah-Hejri , Parvaneh Hajeb, Parsa Ara, and Reza J. Ehsani Abstract: Food product safety is a public health concern. Most of the food safety analytical and detection methods are expensive, labor intensive, and time consuming. A safe, rapid, reliable, and nondestructive detection method is needed to assure consumers that food products are safe to consume. Terahertz (THz) radiation, which has properties of both microwave and infrared, can penetrate and interact with many commonly used materials. Owing to the technological developments in sources and detectors, THz spectroscopic imaging has transitioned from a laboratory-scale technique into a versatile imaging tool with many practical applications. In recent years, THz imaging has been shown to have great potential as an emerging nondestructive tool for food inspection. THz spectroscopy provides qualitative and quantitative information about food samples. The main applications of THz in food industries include detection of moisture, foreign bodies, inspection, and quality control. Other applications of THz technology in the food industry include detection of harmful compounds, antibiotics, and microorganisms. THz spectroscopy is a great tool for characterization of carbohydrates, amino acids, fatty acids, and vitamins. Despite its potential applications, THz technology has some limitations, such as limited penetration, scattering effect, limited sensitivity, and low limit of detection. THz technology is still expensive, and there is no available THz database library for food compounds. The scanning speed needs to be improved in the future generations of THz systems. Although many technological aspects need to be improved, THz technology has already been established in the food industry as a powerful tool with great detection and quantification ability. This paper reviews various applications of THz spectroscopy and imaging in the food industry. Keywords: food inspection, imaging, quality control, spectroscopy, terahertz Introduction can also be observed in the low frequency THz region. THz radi- The far-infrared or terahertz (THz) region refers to a very small ation is an excellent non-ionizing alternative to the use of X-rays gap between the microwave (MW) and infrared (IR) regions of the in generating high resolution images from the inner parts of a solid electromagnetic spectrum. THz waves with a frequency range of object. Many molecules exhibit unique dispersion or absorption 0.1to10THz(3.3cm−1 to 333.6 cm−1)haveuniqueproperties, in the THz range (Blanchard et al., 2007). Nonmetallic, nonpo- making them suitable for both fundamental research and industrial lar, and dry materials are transparent to THz waves while polar applications (Xie, Yang, & Ying, 2014). There have been great ad- molecules absorb THz waves due to their intermolecular activities vances in THz spectroscopy technology since its emergence in the (Mathanker, Weckler, & Wang, 2013). Water is a strong THz wave early 1990s. While they share some properties with MW and IR absorber, and the presence of water is a nuisance when performing radiation, THz waves have distinct properties. THz spectroscopy measurements in the THz region (Jin, Williams, Dai, & Zhang, provides an informative link between MW spectroscopy and IR 2017). Measurements made from THz images or spectroscopy can spectroscopy as well as reflects the interface of these techniques be performed in the two following forms: a) frequency-domain (Figure 1) (Kawano & Ishibashi, 2008; Mcintosh, Yang, Goldup, measurements that use continuous-wave (CW) THz sources and & Donnan, 2012). Due to the energy of photons in the THz detectors or b) time-domain measurements (THz-time-domain region, THz spectroscopy can be used to study the vibrational ac- spectroscopy (TDS)). tivities of molecules. Torsional and rotational modes of molecules THz applications include security screening (Garc´ıa-rial et al., 2018; Kaltenecker et al., 2018; (Liu, Zhong, Karpowicz, Chen, & Zhang, 2007), astronomical studies (Leisawitz et al., 2000), com- munication (Akyildiz, Miquel, & Han, 2014; Ergun¨ & Sonmez,¨ CRF3-2019-0116 Submitted 5/8/2019, Accepted 7/11/2019. Authors Afsah- Hejri and Ehsani are with Mechanical Engineering Dept., School of Engineering, Univ. 2015, nondestructive testing (Probst, Scheller, & Koch, 2011; of California, Merced, 5200 N. Lake Rd., Merced, CA 95343. Author Hajeb is with Rutz, Koch, Khare, & Moneke, 2006), medical imaging (Bow- Dept. of Environmental Science, Aarhus Univ., Frederiksborgvej 399, 4000 Roskilde, man et al., 2018; Chavez, Bowman, Wu, Bailey, & El-Shenawee, Denmark. Author Ara is with College of Letters and Sciences, Univ. of California, 2018; Dougherty et al., 2007; Fitzgerald et al., 2006; Sun et al., Santa Barbara, Santa Barbara, CA 93106. Direct inquiries to authors Afsah-Hejri and Ehsani (E-mail: [email protected]; [email protected]). 2017; Truong, Fitzgerald, Fan, & Wallace, 2018), imaging of dental caries (Karagoz, Altan, & Kamburoglu, 2015; Ripoche, Reynard, C 2019 Institute of Food Technologists® r doi: 10.1111/1541-4337.12490 Vol.18,2019 ComprehensiveReviewsinFoodScienceandFoodSafety 1563 Food applications of THz spectroscopy and imaging . Electronics Photonics -6 Longer Shorter 103 10-2 10-5 10-8 10-10 10-12 Wavelength (in meters) 0.5x10 Common name of wave Radio waves Microwave Infrared Visible Ultraviolet X-ray Gamma-rays Frequency (wave per second) 4 Lower 10 108 1012 1015 1016 1018 1020 Higher Sources 3x10-3 to 3x10-5 THz gap 100 GHz to 10 THz Thickness Process control measurement Inspection and quality control Moisture content measurement Non- Hazardous destructive Harmful compound material and testing and adulteration drug detection detection Safety and Food industries People and security Defects and foreign baggage body detection screening Food component Terahertz characterization spectroscopy Microorganisms Drought and imaging and antibiotic monitoring detection Seed inspection and Agriculture Communication quality control Soil inspection and pesticide detection Medical and Telecommunication biomedical Operation Pharmaceutical Tissue Dentistry characterization and and cosmetics cancer diagnosis Figure 1–Location of THz region in the Electromagnetic Spectrum and some applications of THz spectroscopy and imaging. Narcisse, & Roberts, 2018), pharmaceuticals (Markl, Dong, Li, & 2018; Wang, Sun, & Pu, 2017; Yan, Liu, Qu, & Liu, 2018), envi- Zeitler, 2018; Pei et al., 2018; Sibik & Zeitler, 2015; Warnecke ronmental monitoring (Bigourd et al., 2006), and biosensing and et al., 2019, agricultural products (Liu et al., 2018; Liu, Liu, Hu, label-free monitoring of various biological processes (Yang et al., Yang, & Zheng, 2016; Luo, Zhu, Xu, & Cui, 2019; Nie, Qu, 2016). Full body THz scanners are installed in many airport check- Lin, Dong, & He, 2017; Ogawa et al., 2006; Qin, Li, Chen, & points for detection of concealed objects and explosives. THz Chen, 2017), food inspection and quality control (Ge, Jiang, Lian, waves are considered as data carriers for wireless communications. & Zhang, 2016; Huang et al., 2015, Shin, Choi, & Ok, 2018; Ok, The insulation of the space shuttle is nondestructively inspected Park, Kim, Chun, & Choi, 2014; Ok, Park, Lim, Jang, & Choi, using THz imaging (Redo-Sanchez, Laman, Schulkin, & Tongue, r 1564 ComprehensiveReviewsinFoodScienceandFoodSafety Vol.18,2019 C 2019 Institute of Food Technologists® Food applications of THz spectroscopy and imaging . 2013). Some defects are not optically detectable but are clearly sition and data analysis based on the combination of chemometric visible in THz images. THz systems are being used as noncontact methods with THz imaging for identification and discrimination corrosion detection and thickness measurement devices in pipe purposes. Spectral preprocessing methods, such as smoothing and and cable production lines. THz imaging can differentiate between standard normal variate, are used to remove irrelevant information the normal and cancerous organs and tissues. Cancerous tissue ac- from the optical parameters and increase the signal-to-noise (SNR) cumulates more water and consequently absorbs THz waves more ratio. Multivariate analysis techniques can be used for qualitative than the normal tissue (Sun et al., 2017). THz imaging is also an classification. excellent alternative to X-ray for detection of dental caries. One of The majority of chemical compounds show a very specific the main applications of THz systems has been in the pharmaceuti- frequency-dependent absorption spectrum in the THz region (Hu cal industry for thickness determination, coating examination, and & Nuss, 1995), and chemicals in food can be detected and quan- chemical characterization of tablets (Pei et al., 2018). THz imag- tified based on their THz fingerprint. The detection of toxins, ing is a good tool for monitoring drought stress and water content antibiotics, additives, and harmful compounds are some of the of plants. THz systems are used for detection of pesticides, soil important applications of THz technology in the food industry. contaminants, and transgenic and insect damaged seeds (Figure 1). Nonpolar and nonmetallic substances, such as plastic, cardboard, Previous studies reviewed some early applications of THz tech- and common packaging materials, are transparent to THz radiation nology in agriculture and food (Gowen, O’Sullivan, & O’Donnell, and show very weak interaction with THz
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