## 4 Vibrational Spectroscopic Techniques for Non-Invasive Food Authentication
### NIR Spectroscopy for Non-Invasive Food Authentication  
NIR spectroscopy has found widespread application in both compositional assessments and classification tasks within the food and agricultural sectors, largely owing to its capability to evaluate large sample volumes with minimal or no preparation requirements. Initially, it served as a supplementary tool alongside other optical instruments, but by the 1980s, dedicated NIR systems emerged for the chemical analysis of products. Advances in both hardware and software significantly expanded its capabilities. Early NIR spectrometers were dispersive, measuring wavelengths one at a time, which required repositioning the grating for each measurement. This method was slow, and the detector was often a major source of noise. To overcome these limitations, FTIR spectroscopy was introduced. FTIR systems use a prism or a moving grating to separate frequencies from the NIR source, allowing the detector to simultaneously capture energy passing through the sample at multiple frequencies. This approach speeds up the analysis and produces a unique signal that encodes all infrared frequencies. Depending on their optical architecture, near-infrared spectrometers are generally divided into three principal categories: (i) stepwise scanning systems that acquire spectral information sequentially via monochromators or optical filters; (ii) interferometric designs, such as Fourier transform-based configurations, which capture comprehensive spectral data simultaneously through interferogram processing; and (iii) array-based or multichannel systems, in which distinct detectors concurrently monitor absorption at different wavelengths [59; 60]. Advancements in instrumentation have been accompanied by significant progress in analytical methods and software, particularly those incorporating mathematical and chemometric techniques, which have greatly improved data storage and interpretation efficiency. These developments have made NIR spectroscopy an increasingly reliable, efficient, and standardized method for quality control. As a result, NIR spectroscopy is now widely adopted across a variety of industries, including food and pharmaceuticals, as well as the petrochemical and chemical sectors. In NIR spectroscopy, different spectral modes can be applied to assess both the external and internal characteristics of samples. These modes include reflectance (specular and diffuse), transmission, interactance, and transflectance, each suited for different types of samples. Reflectance or interactance is typically used for solid samples, while transmission is ideal for liquids, and transflectance is applied to thin or transparent materials. The choice of mode depends on the specific properties of the sample, which need to be understood in detail. In reflectance mode, the light that is reflected or scattered from the sample's surface is measured. Specular reflection occurs on smooth surfaces, while rough surfaces result in diffuse reflection. Specular reflectance offers limited information about the sample's composition, whereas diffuse reflectance provides more detailed insights into the chemical and physical properties of the sample. This latter mode has gained significant attention in the agro-food sector, especially for food analysis, due to its ability to offer valuable data. In recent years, diffuse reflectance NIR techniques have been widely used in food adulteration detection. Studies have successfully identified meat muscles [61], detected adulteration in beef hamburgers [62], and identified adulteration in crabmeat [63; 64]. Furthermore, high classification accuracy has been achieved in distinguishing between different types of meat, and diffuse reflectance NIR (1100-2500 nm) achieved 95% accuracy in detecting horsemeat adulteration in beef [65]. This technique has also been effectively used to detect melamine adulteration in milk powder and soybean meal.