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2016 Joint International Conference on Social Science and Environmental Science (SSES 2016) and International Conference on Food Science and Engineering (ICFSE 2016) ISBN: 978-1-60595-390-8

Application of Terahertz Spectroscopy and Imaging Techniques in Food Adulteration Ying-Ying LI1,a, Chang-Cheng SHI2,b, Hua-Bin WANG2,c*, Zhong-Dong LIU1,d* 1

College of Food Science and Technology, Henan University of Technology, Zhengzhou, China 2

Research Center for Terahertz Technology, Chongqing Key Laboratory of Multi-scale Manufacturing Technology, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing, China a

[email protected], [email protected], [email protected], d [email protected] *Corresponding author

Keywords: THz Spectroscopy, Featured Absorption, Imaging, Food Adulteration.

Abstract. Terahertz (THz) waves lie between millimeter and far-infrared bands, the frequency of which ranges from 100 GHz to 10 THz. Some food materials show featured absorption for THz wave radiation, which can be utilized to characterize different food materials. Compared with other spectral analysis methods, THz spectroscopy has low photon energy and no ionizing radiation. The rapid development of ultrafast laser technology and semiconductor material has enable the further progress of THz techniques in food adulteration. In this review, the principles and applications of THz technology employed in the detection of food adulteration are discussed and the challenges of THz technology in the field of food adulteration are also summarized. Introduction During the past few years, food safety and quality assurance have attracted more and more attention. Food adulteration has become one of the most serious food safety issues in China. Food adulteration distorts the food market and may threaten people's health. The 2008 Chinese milk scandal was caused by the milk powder adulterated with a toxic chemical called melamine, which caused severe infant illnesses [1]. There are various food adulteration detection methods, such as chromatography, mass spectrometry, electrophoresis, polymerase chain reaction and enzyme linked immunosorbent assay. Although those methods have a high accuracy, the cost is high and the analysis process is complicated and time consuming. Therefore, food safety and quality assurance are in great need of an accurate, reliable, rapid and nondestructive detection technology. Terahertz (THz) wave is an electromagnetic spectrum lies between millimeter and far-infrared with frequency ranging from 100 GHz to 10 THz (Fig. 1). Prior to the mid-1980s, due to the lack of high efficiency of the THz emission source and the sensitive detector, THz band was called “THz gap” [2]. However, the development of ultrafast laser technology and progress of semiconductor materials, has increased the efficiency and accuracy of THz technology. THz technology has been widely applied in the field of environmental science, physics, aerospace, security, biomedical, pharmaceutical and materials science [3-9]. Due to its unique properties, THz wave drew the attentions of researchers. THz radiation could transmit through nonpolar substance, and image opaque object. THz photon energy is relatively low, which means that there is no ionizing radiation produced and no tissue damaged when it interacts with biological samples. THz band contains a massive amount of spectral information, which is caused by the rotational and vibrational transitions of biological molecules. These properties enabled more and more researchers involved in 491

its application of food adulteration detection. In this review, the principles of THz technology is explicitly explained, the applications of THz for food adulteration detection in various food materials are extensively reviewed, the challenge and future perspective of THz technology applied in real food adulteration are also discussed.

Terahertz

Electronics Microwave

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10 6

Millimeter wave

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Photonics

Infrared

X-ray

 -ray

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Visible light

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Frequency/Hz Figure 1. Electromagnetic Spectrum. Terahertz Time Domain Spectroscopy and Imaging Techniques Terahertz Time Domain Spectroscopy

Figure 2. Schematic Diagram of Typical THz Pulse System. (a) Transmission Mode of THz System; (b) Reflection Mode of THz System. Terahertz time domain spectroscopy (THz-TDS) system is mainly composed of ultrafast pulse laser, time delay, THz emitter and detector [10]. Typical THz pulse systems are generally divided into transmission and reflection modes, as shown in Fig.2 (a) and (b) [11]. The principle of 492

transmission system is: femtosecond pulse laser through a beam splitter is divided into two light beams, one is pump light, and the other is probe light. The pump light is converged to the optical waveguide antenna to generate THz wave. THz wave through the lens is focused the sample surface, the THz signal passed the lens to converge to the photoconductive antenna detector; On the other hand, the probe light pass through the delay system and focus on photoconductive antenna detector, and then the THz wave becomes discrete signal. Using a lock-in amplifier, the signal is locked in a certain frequency, amplified and obtained. The spectral information of THz wave can be obtained by Fourier transform, and the optical parameters such as the refractive index, extinction coefficient, absorption coefficient and dielectric constant of the sample can be obtained by processing the spectral data. The working principle of reflection system and transmission system is basically the same, the difference lies in the reception of reflection system is composed of THz wave reflection on the surface of the sample. This method can be used for the absorption of relatively strong material to measure, when the reflectivity of the sample is relatively high, the measurement is more accurate. THz Imaging Technology THz imaging mainly includes THz pulsed imaging and THz continuous imaging. THz pulsed imaging is based on the THz-TDS technology and combined with the image processing and scanning control technology. The imaging modes include transmission and reflection imaging. At the same time, it could record the THz wave intensity and phase information, and obtain the THz image by proper processing and analysis. THz continuous imaging is different from THz pulsed imaging. THz continuous imaging is usually intensity imaging, the structure of this system is relatively simple, only need pump light and detection part, and the imaging speed is faster. However, THz continuous imaging system needs to provide a higher intensity of terahertz source than THz pulsed imaging system. In the experimental process, THz imaging technology is usually used in THz pulsed imaging technology. THz pulsed imaging is similar with other imaging techniques, which obtains the characteristic information of the sample by transmission or reflection of samples, and then imaging. The difference is that THz pulsed imaging of each pixel is covered by the time waveform of the entire THz pulse. The spectral information of the pixel can be obtained by Fourier transform. Therefore, THz pulsed imaging not only has the ability to identify the shape of the object, but also can distinguish the internal parts of the body of the material composition. Application of THz Technology in Food Adulteration Application of THz Technology in Edible Oil Adulteration Edible oil is integral to daily diet, and oil adulteration is a popular problem. One of the examples is olive oil, there are multiple research detecting olive oil adulteration [12-15]. THz technology has also been extensively investigated for edible oil adulteration detection. Hu et al. [16] investigated the THz spectra of 5 kinds of vegetable oils and two kinds of animal fats, and then obtained the absorption spectra and refractive spectra of each sample in the 0.2-1.6THz frequency range. Vegetable oils were detected at room temperature, and animal fats were detected at different temperatures. The results showed that the absorption coefficient and refractive index of each five kinds of vegetable oils were different, the absorption coefficient increases with the increase of frequency in 0.2-1.2THz, but the refractive index was opposite, with the increase of the frequency, the refractive index value was slightly lower, and its value was 493

between 1.46-1.66. The absorption coefficient of animal fat increased with the increase of frequency in the range of 0.2-1.2THz, but the refractive index changed with the change of frequency, and increased with the increase of temperature, and its value was between 1.4-1.52. Li et al. [17] detected the 30 edible oil samples by THz-TDS technology, the absorption spectrum of the samples was obtained. And the 4 kinds of edible oil components were analyzed by the support vector regression (SVR), and the prediction model was established. The results showed that the correlation coefficients of the model were more than 0.9, and the THz-TDS technology could be used for the rapid analysis of vegetable oil composition. Xu et al. [18] detected two kinds of waste oils and vegetable oils by THz technique, and the time domain, frequency domain, refraction and absorption spectra of the samples were obtained. Among them, the waste oils were repeatedly fried soybean oil and peanut oil, vegetable oils were without the use of soybean oil and peanut oil. The results showed that the refraction spectrum and absorption spectrum of the trench oil and vegetable oil in the 0.2-1.5THz wave band were obviously different. This showed that THz technology could be used to identify the high temperature of vegetable oil and the unused vegetable oil. Lian et al. [19] investigated the spectrum of soybean oil and cooked soybean oil, two kinds of oils were obtained in the time domain and frequency domain of 0-3.0THz, and the refraction and absorption spectra in the range of 0.2-1.5THz were obtained. Results showed that there was a significantly different refractive index and absorption coefficient of soybean oil and cooked soybean oil. Among them, in terms of refractive index characteristics, cooked soybean oil average refraction rate was 1.7, soya bean oil in the average refractive index was 1.6, and the change of mature soybean oil was more obvious than that of soybean oil. In terms of absorption properties, there was an absorption peak at 1.33 THz and 1.47 THz, while soybean oil had no absorption peak. This provided a basis for the detection of waste oil by THz-TDS technology. Bao et al. [20] detected edible oil and waste cooking oil by THz-TDS technique, and the absorption spectrum and refractive spectrum of the two kinds of oils in the 0.16-0.96THz range were obtained. The results showed that the absorption spectrum and refractive spectrum of the waste oil and edible oil at different frequencies were different, which could be used as a THz-TDS technology to identify the index of cooking oil and edible oil. Due to the oil molecular vibrational and rotational frequencies were in THz band and THz wave interaction may produce a resonance reaction. Therefore, Tian et al. [21] detected detect the regular oil (new procurement), waste oil frying and cooking oil (recycling) by THz-TDS technique and got the 0.3-1.6THz band samples in time domain and frequency domain spectra. By analyzing the optical parameters such as the delay time, refractive index, absorption coefficient and absorption peak, the characteristic information of THz spectrum of waste oil was obtained, which provided the experimental basis for the detection and identification of waste oil. Shan et al. [22] identified common edible oil and waste oil by THz-TDS technology, and the absorption spectrum of 0.16-1.30THz range was obtained, and the waste oil and edible oil were classified according to statistical clustering analysis method. Random selection of 2 kinds of waste oils as validation of oils, the remaining oil would be clustered, and then using probabilistic neural network method to determine the validation of the samples, and then successfully distinguish the 2 kinds of waste oils. As the terahertz system is vulnerable to water interference in the air and THz radiation source power is low, these researches have been stuck in the laboratory stage. With the development of THz technology, the THz system has started commercial operation, the United States, Europe and Japan and other countries there have been a number of enterprises started production of commercial THz time domain spectrometer. For our country, Shanghai key laboratory of modern optical system 494

and Shanghai analysis test association cooperation developed a terahertz instrument which detected drainage oil. Terahertz electromagnetic wave can produce resonance with organic matter in oil. Most of the waste oil contains animal fat, or the production of peroxide in the process. The structure of animal fat is more complex than that of plant oil, and both of them are different from the frequency of THz electromagnetic wave. According to this principle, through the data comparison, we can find out the potential waste oil, and the detection accuracy rate has reached 90%. Application of THz Technology in Flour Adulteration Flour and flour products are the main food of people's daily life, and its quality directly affects people's health. Some unscrupulous manufacturers in order to increase the weight of flour, the flour mixed with a large number of talc powder. However, adding talc powder to the flour can cause great harm to the health of consumers, and it is very important to test whether or not contain talc powder in flour. Zhang et al. [23] detected the mixed samples of wheat flour and talc powder by the technology of THz-TDS, and obtained the absorption spectrum and refractive spectrum in the 0.2-3.0THz band. The study found that the talc mass fraction and the sample absorption coefficient and refractive index were positively correlated in the sample. Therefore, to detect the content of talc in wheat flour could be quantified by using THz technology. Zhao et al. [24] tested five proportions of flour and talc powder samples by THz-TDS technology, and the time domain, frequency domain, absorption spectrum and refractive spectrum of the samples were obtained. The results showed that two samples in the range of 0.2-1.6THz absorption and refractive index had obvious differences, and conclusion consistent with Zhang et al. Although Zhang and Zhao et al. detected the mixed samples of talc by terahertz spectroscopy, but they did not point out the unique fingerprint information of talcum powder. Fu et al. [25] detected wheat flour, talcum powder and two tested samples by THz-TDS technology, and obtained the absorption spectrum and refractive spectrum in the range of 0.2-1.5THz. The results indicated that talc specific absorption peak at the wavelength of THz did exist and it could be used for material identification. At the same time, using partial least squares (PLS) for the quantitative analysis of talc content in wheat flour. The results showed that the high correlation of talc content in wheat flour and terahertz absorption coefficient, and the correlation coefficient was 0.9939, the root mean square error was 1.48%, and the detection limit was higher than 2%. The study also showed that THz technology could identify the talc. Qin et al. [26] tested benzoyl peroxide in flour by THz-TDS technology, and obtained the time domain spectrum, frequency spectrum, absorption spectrum and refractive spectrum in the range of 0.2-1.5THz. The results showed that the absorption coefficient and refractive index of brightener containing flour and flour whitening agent were significantly different. It could demonstrate that THz technique could effectively detect benzoyl peroxide whitening agent containing flour. Application of Terahertz Technology in Harmful Food Additive Food additives are used to improve food quality, prolong the shelf life of food, facilitate the processing of food and increase the nutritional composition of a class of chemical synthesis or natural substances. However, unscrupulous traders in order to seek profits, put "non-edible substances" treated as food additives in our diet; it seriously endangered people's health. In recent years, THz technology has shown great application value in the detection of harmful 495

food additives. Yoneyama et al. [27] detected several kinds of food additives by THz-TDS technology, using a biosensor in this study, and extract the optical parameters of additive solution. Experimental results showed that the vibration characteristics of harmful food additives not only showed in the solid particles, but also showed in solution. Therefore, the system could detect harmful food additives. Li et al. [28] detected Melamine, pure milk powder and milk powder mixed with melamine by THz-TDS system, and the time domain spectrum, frequency domain spectrum and 0.5-2.0THz spectrum of the sample were obtained. Melamine and milk powder ratio of a large number of 1:7, a small amount of 1:1 and trace of 1:20. Results found Melamine had characteristic absorption peaks that could be identified in milk powder, among them, the absorption peak of 1.273THz was the major differential peak, and the absorption peak of 1.536THz and 0.819THz were the auxiliary differential peak, but the experiment was only a preliminary qualitative analysis. Using artificial neural network to establish a neural network model for the detection of melamine in milk powder and data analysis, and it was proved that the artificial neural network for the use of melamine milk powder of THz TDS detection was feasible. Zhou et al. [29] detected melamine and melamine by THz-TDS system, and the absorption spectra and refractive spectra of two samples at room temperature were obtained. Among them, the results showed that melamine had obvious characteristic absorption peaks at 1.98THz and 2.24THz. According to the density functional theory, the absorption spectrum of THz was mainly caused by the collective vibration mode of the intermolecular hydrogen bond. Therefore, The THz spectra could be used for quantitative analysis of melamine content in milk powder, but the results different from Li et al. The reason may be effective in different frequency ranges and different experimental conditions. Zhu et al. [30] detected the tony red I by THz-TDS technology, and obtained the absorption spectrum and refraction spectrum in the range of 0.5-2.0THz, the terahertz absorption spectrum and the vibration absorption spectrum of density functional theory calculations supported each other. It proved that THz-TDS technology could detect the structure and vibration tony red I molecules. Chen et al. [31] detected the clenbuterol hydrochloride by THz-TDS technology, and obtained the absorption spectrum and refraction spectrum in the range of 0.2-2.6THz. Absorption of clenbuterol molecule and clenbuterol hydrochloride molecule characteristics of THz band was simulated by density functional theory, the calculated vibrational frequencies are crystal. The results showed that the THz spectral method based on solid simulation could explain the source of absorption of clenbuterol hydrochloride. The THz-TDS technique could identify clenbuterol, and it provided a new experimental method for the identification of clenbuterol. Xia et al. [32] detected rongalite, benzoyl peroxide and their mixtures by THz-TDS technique, the absorption spectrum and refractive spectrum in the range of 0.2-1.5 THz were obtained. The results showed that the whitening agent in THz region existed obvious characteristic absorption peak, and it could as THz fingerprint for identifying substances; in addition, the higher the content of sodium formaldehyde sulfoxylate in mixture, the smaller absorption coefficient was, and the bigger refractive index was. At the same time combined with partial least squares (PLS) method for the quantitative analysis of brightening agent content of formaldehyde. The results showed that the high correlation of brightening agent content in formaldehyde and terahertz absorption coefficient, and the correlation coefficient was 0.9998, the root mean square error (RMSE) was 1.21% and the detection limit was better than 2.0%. It proved that THz-TDS technique could effectively detect the whitening agent mixed with formaldehyde. 496

Application of Terahertz Technology in Nutrition Adulteration and Foreign Body Detection in Foods The use of THz spectroscopy and imaging technology research on nutrition adulteration and foreign body detection in foods, it can be better for food composition and organization quality analysis and monitoring, and may make the technology breakthrough in the field of food quality detection. In the aspect of nutrition adulteration detection, nutrition supplement and lack of main nutrients the body loss, such as vitamins, minerals, fiber, lactic acid bacteria, protein, chondroitin etc. These components are the elements of the human body, and keep the body healthy, if the adulteration in nutritional products, it will cause serious harm to human health. The terahertz technology also shows great potential in the field of food adulteration. Zhang et al. [33] detected the donkey hide glue, antler glue and tortoise shell by THz-TDS technology, and obtained their absorption spectrum in the range of 0.2-2.6THz. Using BP neural network method for the three samples of THz absorption spectrum of training and recognition, found that BP neural network could realize the recognition and identification of donkey hide glue, antler glue and tortoise shell, and the recognition rate was 100%. In the aspect of foreign body detection in food, Jordens et al. [34] detected the contaminants which hided in chocolate (glass, stone and metal) by THz transmission imaging, due to chocolate were a food with high fat and low moisture content, chocolate for terahertz waves was relatively transparent. When chocolate contained a foreign body, the transmission of THz wave would change scattering characteristics, and then detected the foreign body. No matter whether the chocolate plastic packaging had or not, it could be detected by this method. The Challenge of Terahertz Technology in the Detection of Food Adulteration With the continuous improvement of human living standard, it is eager to have an effective, rapid and undamaged technology for applying in the detection of food adulteration. THz technology, as a new nondestructive testing technology, has unique advantages. However, a few challenges have to be solved when it is used in food adulteration detection: (1) Water has strong absorption of THz radiation, there still are limitations for beverage adulteration. How to break through the absorption of water has become the focus of the current THz researches. (2) Lack of more efficient THz production and sensitive detector, it is necessary to further develop novel THz sources and detectors. (3) The theoretical mechanisms for the interaction between THz radiation and matter have not been clearly explained, and the THz spectral library of various substances in food has just been established, and a large number of studies have to be conducted to collect data. (4) THz technology in food adulteration detection is only studied in the laboratory stage of operation, such as the need in sealed condition flushed with nitrogen in order to avoid water effect; system is large, it is not convenient to portable detection. How to use terahertz technology in practice needs to be further studied.

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Summary In this study, the applications of THz technology in the field of food adulteration detection have been summarized. So far, there are many difficulties when the THz technology is applied to the studies of food adulteration detection, but this technology has very attractive potential in the field of nondestructive detection. The application of THz technology in the field of food adulteration detection has unique advantages such as non-destructive, easy to operate, samples without pretreatment and good experimental reproducibility and so on. With the continuous progress of THz technology, in the near future, THz technology will play a pivotal role in the field of food adulteration detection. Acknowledgement This research was supported by the National Key Research and Development Program (2016YFC0101002), National Natural Science Foundation of China (31470090 and 11504372), Chongqing Science and Technology Commission (cstc2014jcyjA10002, cstc2015jcyjA10057 and YJ500061LH1) and Chinese Academy of Sciences (R52A500Z10). References [1]Ghazi-Tehrani A. K. and Pontell H. N. Corporate crime and state legitimacy: the 2008 Chinese melamine milk scandal, Crime, Law and Social Change. 63 (2015) 247-267. [2]C. Sirtori. Bridge for the terahertz gap, Applied Physics. 417 (2002) 132. [3]Yin M., Tang S.F. and Tong M.M. The application of terahertz spectroscopy to liquid petrochemicals detection: A review, Applied Spectroscopy Reviews. 51 (2016) 379-396. [4]P. Mukherjee, B. Gupta. Terahertz (THz) frequency sources and antennas-a brief review, International Journal of Infrared and Millimeter Waves. 29 (2008) 1091-1102. [5]I. Amenabar, F. Lopez and A. Mendikute. In introductory review to THz non-destructive testing of composite mater, Journal of Infrared, Millimeter, and Terahertz Waves. 34 (2012) 152-169. [6]A.A. Gowen, B.K. Tiwaria, P.J. Cullen, K. McDonnell and C.P.O’Donnell. Applications of thermal imaging in food quality and safety assessment, Trends in Food Science and Technolog. 21 (2010) 190-200. [7]Wang Y.Y., T. Notake, Tang M., K. Nawata, H. Ito and H. Minamide.. Terahertz-wave water concentration and distribution measurement in thin biotissue based on a novel sample preparation, Physic in Medicine and Biology. 56 (2011) 4517-4527. [8]Hou D.B., Li X., Jinhui Cai, Ma Y.H., Kang X.S., Huang P.J. and Zhang G.X. Terahertz spectroscopic investigation of human gastric normal and tumor tissues, Institute of Physics and Engineering in Medicine. (2014) 5423-5440. [9]P.F. Taday, I.V. Bradley, D.D. Arnone. Terahertz Pulse Spectroscopy of Biological Materials: L-Glutamic Acid, Journal of Biological Physics. 29 (2003) 109-115. [10]Xu J.Z., Zhang X.C. Terahertz science and technology and application. Beijing: Publishing House of Peking University, 2007. [11]A.A. Gowen, C. O’Sullivan and C.P. O’Donnell.. Terahertz time domain spectroscopy and imaging: Emerging techniques for food process monitoring and quality control, Trends in Food Science and Technology. 25 (2012) 40-46. 498

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