By Sangeetha Karunanithi*, Dakshayani R**, and Divya Panneerselvam***

THZ radiation is an excellent non-ionizing alternative to the use of X-rays in generating high resolution images from the inner parts of a solid object. The terahertz region (THZ) is a very narrow, but very significant part of the electromagnetic waves ranging from 0.3 to 10 THZ waves ranging from 0.3 to 10 THZ (wavelength from 1 mm to 30 μm) situated between the microwave (MW) and the infrared (IR) region. THZ spectroscopy provides an informative link between MW spectroscopy and IR spectroscopy as well as reflects the interface of these techniques. Due to the energy of photons in the THZ region, THZ spectroscopy can be used to study the vibrational activities of molecules. Torsional and rotational modes of molecules can also be observed in the low frequency THZ region. Many molecules exhibit unique dispersion or absorption in the THZ range. Nonmetallic, nonpolar and dry materials are transparent to THZ waves while polar molecules absorb THZ waves due to their intermolecular activities. Water is a strong THZ wave absorber, and the presence of water is a nuisance when performing measurements in the THZ region. The wavelength (100 μm−1 mm) of the THZ region can be resolved spatially, which is sufficient for imaging applications; that is, specific absorption peaks can be used for both identification and mapping of chemicals with the spatial distribution of individual chemical components in a heterogeneous mixture. Moreover, the THZ wave can determine the differences in density and dielectric constant. This is because the THZ wave can penetrate a wide range of dielectric materials, such as clothing, paper, cardboard, wood, plastic and ceramics. These advantages can be successfully applied to the nondestructive inspection of chemicals. Absorption spectra in the THZ range provide rich information for both intermolecular and intramolecular interactions, including inter-ring interactions of disaccharides. Absorption spectra in the THZ range are very sensitive to differences in crystal structure and have been applied to the study of drug polymorphs. THZ has exclusive properties of both electrical and optical waves,which make it especiallyusefulinavariety of applications suchas biomedical imaging including medical diagnosis, pharmaceutical analysis and security enhancement, packaged goods inspection, food and water contamination detection.

Measurements made from THZ images or spectroscopy can be performed in the two following forms: a) frequency-domain measurements that use continuous-wave (CW) THZ sources and detectors or b) time-domain measurements (THZ-time-domain spectroscopy (TDS)). THZ time domain spectroscopy (THZ-TDS) is the most common THZ spectroscopy method. THZ pulses are mainly generated using femtosecond laser pulses, coherent transition radiation or free electron laser (FEL). THZ pulses generated by femtosecond laser pulses are useful for measuring the optical properties in wave number (frequency) ranges below 100 cm-1 (3 THZ), and give a better signal-to-noise ratio than the far infrared.Terahertz time-domain spectroscopy (THZ-TDS) is a spectroscopic technique in which the properties of matter are probed with short pulses of terahertz radiation. The generation and detection scheme is sensitive to the sample’s effect on both the amplitude and the phase of the terahertz radiation. By measuring in the time-domain, the technique can provide more information than conventional Fourier-transform spectroscopy, which is only sensitive to the amplitude.

Components of THZ Systems

The terahertz sources, components and detectors are the main modules of the application of THZ technologies. Several new THZ source techniques have been brought forth during the past few decades. These components including mirrors, lenses and polarizers manipulate the specific radiation. Modern innovations in THZ mirror technology include semiconductor, hybrid mirrors and the tunable mirrors are based on photonic crystals (PCs). Terahertz lenses are typically made of plastics such as polyethylene or TPX. Recently,  significant innovations allow the rapid production of a large number of lenses which include Fresnel zone plates, plasmonic resonances, variable focal length lenses, 3D printed diffractive lenses and even THZ lenses made of paper. The polarizers are one of the significant components in THZ imaging, data transmission, and spectroscopy. Recently, reconfigurable polarizers and carbon nanotube fiber polarizers are used. THZ detectors that can measure the THZ radiation upon reception, play a significant role in many areas including astrophysics, biological, chemistry and explosions detection, imaging, astronomy application.

Generation and Detection of THZ Waves

THZ systems can now be developed using a variety of sources, such as Gunn diodes and super-lattice electronic, which operate at the low frequency end of the THZ band or photonic sources such as lasers and photoconductive dipole antennae which are more widespread and can generate radiation across a broadband frequency range. Other THZ generation methods include optical rectification and mixing of light from two laser sources whose wavelength difference is in the THZ region. Optical systems using THZ waves generated by femtosecond laser pulse. Generally, THZ waves are generatedby irradiation of semiconductor with femtosecond (fs) laserpulses. For example a mode-lockedTi:sapphire laser, with a repetition rate at 82 MHz, a pulsewith around

100 fs and its center wavelength around 800 nmwas used as the pump source. The emitter and detector werea dipole-type photoconductive antenna on a low-temperaturegrown GaAs (LT-GaAs) substrate. Theantennas were excited and triggered by laser pulses with anaverage power of 10 mW. The bias voltage applied to theemitter photoconductive antenna was 20 Vp-p modulated at53 kHz. THZ radiation from the emitter was collectedand focused onto the sample using a parabolic mirror. THZ radiation transmitted through the sample was collectedand focused with another parabolic mirror onto the photoconductive antenna. Hyper-spherical Sisubstrate lenses were used for both the emitter and detectorantennas to reduce the total reflection in the GaAs and increase the beam collection efficiency.The photoconductive current signal was detected with a lock-in amplifier and its time-domain signal was obtainedby scanning the optical delay of the probe pulse. By Fouriertransformation of the time-domain signal, the amplitudespectra were obtained. To reduce the absorption due to watervapor in the ambient air, the THZ beam path was filled withdry air.

Significance of THZ-TDS in Food Science

THZ-TDS wave is widely used in many applications in food science to control the quality of food. THZ waves are sensitive to water and their transmission is scarcely affected by microstructures in the target because of long wavelength. Therefore, the state of water in food or other materials can be observed clearly using THZ waves. By the THZ imaging method, the water in the leaf and the internal structure of shrimp can be observed clearly. THZ waves also applicable in food safety aspects. THZ waves can also selectively detect the materials like DNA, protein and polysaccharides by seeing absorption bands in the THZ image. The THZ waves used for testing of microbes, viruses and toxins, including prion, in food. Every chemical has a different absorption wave form poses discriminatory images used to detect the chemical profile of the materials. In addition, THZ waves can be applied to evaluation of cheese quality and the freshness of fruits, vegetables and eggs.

Moisture content determination

Due to the high absorbance of THZ radiation by water, the most obvious application of THZ spectroscopy would seem to be for quantification of moisture content in foods. This is especially important in controlling drying processes where low moisture content can alter the sensory qualities and shelf life of products.THZ range for evaluation of moisture content of whole and crushed wheat will be 0.1 to 4. The crushed samples were housed in a PTFE (Polytetrafluoroethylene) sample holder during measurement. Since this polymer is transparent to THZ radiation and the system was purged with nitrogen gas to avoid absorption by water vapor. Scattering of THZ light due to the shape and orientation of grains led to the appearance of apparent peaks in the absorbance spectrum. Such facts were not present in the spectra of crushed wheat and it was possible to identify water specific absorption peaks in crushed samples varying in moisture content from 12 to 18%. Subtracting the THZ spectrum of dry crushed wheat from that of wetted samples facilitated the development of a calibration model for prediction of moisture. The moisture content of food wafers has also been examined using THZ-TDS. Dehydrated wafers were brought to various moisture contents (1 to 30%) by storage in a humid environment. Samples were then encased in sealed cells during for THZ transmission measurement in order to avoid dehydration by nitrogen purging of the THZ system.

Foreign body detection

Detection of metallic contamination is rather straightforward in food processing; however, detection of non-metallic contamination, such as glass or plastic, is more challenging. The implementation of imaging systems with high spatial resolution would allow identification of the

precise location of foreign bodies, thus minimizing rejects. However, existing methods such as X-ray imaging do not perform well when the densities of the foreign body and the food product are similar. THZ imaging provides both phase and amplitude information, offers improved characterization of foreign bodies. Due to its high fat and low moisture content, chocolate is relatively transparent to THZ energy. When foreign objects such as glass or plastic are placed in chocolate, they alter the scattering profile of a transmitted THZ wave and are thus detectable. Foreign bodies (glass, stone and metal) were concealed within the interior of a bar of chocolate and THZ images of the contaminated sample were obtained using a raster scanning THZ-TDS system operating in transmission mode. It was possible to identify foreign bodies in the chocolate sample, both in the presence and absence of its plastic foil packaging.

Residue detection

The presence of residues in foods is becoming a growing concern among consumers. Consequently, there has been interest in developing rapid non-destructive techniques for residue detection in foods and THZ spectroscopy is being evaluated for this purpose. THZ-TDS as a non-invasive tool for the detection of pesticides in food powders like sticky rice, sweet potato, and lotus root. They demonstrated that four pesticides such as imidacloprid, carbendazim, tricylazole, and buprofezin had specific absorbance peaks in the THZ range between (0.5 to 1.6 THZ) while the spectra of the food powders themselves were similar to each other exhibiting broad features. This indicated that the pesticide samples could be easily distinguished both from each other and from the food powder matrices.  However, it was noted that the physical properties of food matrix of sticky rice powder can alter the signal due to scattering and absorption. In order to verify this method, the measurement of a wider range of physical conditions and food matrices is required.

Inspection of packed goods

Common packaging materials made from cardboard and polymers are transparent to THZ radiation. This makes THZ spectroscopy and imaging attractive tools for quality validation of packaged products. Although it is possible to apply some hyperspectral imaging techniques to packaged goods such analysis is typically limited to thin layer polymer packaging and imaging in the visible range, which is possible to detect foreign bodies in packaged chocolate bars. Indeed, for most if not all of the applications listed in the preceding sections, it is possible to carry out the measurement while a product is cased in glass, plastic or paper. Another potential area of application is in the detection of production defects in packaging (e.g. holes or tears).

Beverage characterization

It is important to characterize the dielectric properties of liquid foods in order to optimize certain processes such as microwave heating. In addition, dielectric properties can be related to compositional information. THZ-TDS system, operating in reflection from 0.1 to 1.0 THZ frequency range for measurement of the dielectric properties of liquids in aqueous sugar and alcohol solutions. The developed system was employed for simultaneous determination of the sugar and alcohol content of commercial alcoholic beverages independent of other properties such as color, organic matter content, carbonation and flavor.

Detection of microorganisms

Rapid and accurate detection of microorganisms is vital for food safety control. Currently time-consuming culturing methods, molecular methods, and mass spectroscopy techniques are being used for bacterial detection. Conventional methods are based on biochemical and phenotypic tests and usually take days or weeks in the case of slow-growing microorganisms. Molecular methods are also complex but are fast and have LOD of several genome equivalents. Mass spectroscopy techniques are reagent-free, very fast and simple with an LOD of between 105 and 106 CFU/mL. None of these methods can detect microorganisms inside a package or differentiate between dead and living bacteria. THZ spectroscopy is a reagent-free, simple and fast technique that can see-through the packaging material and is able to differentiate between living and dead organisms. Bioparticles like yeasts and bacterial cells have a relatively low absorption coefficient and allowing THZ radiation to propagate through them. Cell components namely proteins and genetic material have distinct absorption coefficients and contribute to the THZ signature of bacterial spores or cells.

Detection of vitamins

THZ spectroscopy has been used to measure the low-frequency vibrations of vitamin B2 (riboflavin) and some of its related compounds. It was found that both the intensity and position of the THZ spectrum of vitamin B2 were temperature-dependent. Used for high precision quantum chemical calculations for measuring the vibrational mode of the compounds. Later,THZ-TDS for detection of intracellular metabolites such as vitamins. Riboflavin appeared to be more absorptive to THZ waves and presented sharp and strong peaks. Riboflavin and astaxanthin were then detected inside the bacterial cells with shifts in position of the absorption peaks.

Challenges and limitations

A major barrier to the adoption of Terahertz imaging systems at present is the high cost of detectors and sources. Although new developments have drastically decreased these costs and they are currently too high to render the technology economically beneficial for many applications. Related to this is the low acquisition speed typical of most systems. Although it has high-speed systems are feasible, many of these are based on sampling a sub-region of the THZ waveform. The development of such systems is generally application specific, since the sampled sub-region would need to match the desired response. Relatively high signal to noise ratio is also still problematic for certain regions of the THZ spectrum, although this may be ameliorated by acquisition of multiple scans and averaging (although this increases the acquisition time). These challenges are equally relevant for the adoption of THZ in the food industry. Fusing complementary spectroscopic techniques to optimize process monitoring capabilities could be regarded as the next major challenge in the implementation of PAT strategies for food quality control. One of the limitations of THZ spectroscopy for moisture content detection is that it is not suitable for high moisture products of thickness greater than 1 mm. This is due to the high absorption of THZ radiation by water.  Reflection THZ imaging is another option; however, the appearance of standing waves due to differences in the optical path length can produce image artefacts. Another challenge facing practitioners of THZ spectroscopy is the effect of physical variations in a sample (e.g. particle size) on the refractive index of a material. This is particularly pertinent in the case of quality monitoring of fresh produce which shows high variability in this respect. Scattering effects may adversely affect measurements of THZ absorption in certain materials.

Future outlook

The ability of THZ waves to pass through a wide variety of packaging materials combined with their ability to characterize the molecular structure of many biological substances makes it an attractive process analytical tool for enhanced monitoring of food production. Although the potential of THZ-TDS spectroscopy and imaging has been demonstrated for a number of issues in food quality control, it is clear that much remains to be investigated in this area. The development of THZ spectral library databases is something that is still in its infancy. Important work has already commenced on this as evidenced by the applications described in the previous section. Although plenty more remains to be done, with the increased availability of turnkey THZ systems. It is likely that the establishment of a library for the wide group of compounds related to food safety will be accomplished in the future. This, combined with lowering costs for sources and detectors, and faster systems should lead to the increased adoption of THZ spectroscopy and imaging for food process monitoring and control.

Ph.D. – Agricultural and Food Engineering, Ph.D – Food Science and Technology,

SRF- Department of Dairy Engineering, ICAR – National Dairy Research Institute, Bangalore, Karnatak-560030

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