## 4 Development of Optical Sensing Techniques for Red Meat Quality Prediction
### Spectroscopic Techniques  
#### 4.1.1 Near-Infrared (NIR) Spectroscopy  
Infrared spectroscopy utilises the interactions between infrared (IR) radiation and matter to analyse samples [107]. The IR region is segmented into the near-infrared (780-2500 nm), mid-infrared (2500-25,000 nm), and far-infrared (25,000-1,000,000 nm) wavelength regions [108]. In practice, both the NIR and MIR regions are employed in food analysis using various commercial instruments [109]. The utilisation of the Fourier transform, a mathematical process that allows the combination of wavelengths in a single measurement when utilised alongside an interferometer, enhances the speed of analysis, rendering these approaches highly advantageous [110]. Near-infrared spectroscopy (NIRS) has been effectively used to quantify the main components of meat and meat products, including the moisture, fat, and protein [111]. NIRS operates based on the principle of the light absorption exhibited by different substances in the visible to near-infrared (Vis-NIR) wavelength range within the electromagnetic spectrum [112]. The measurements of NIRS are reliant on the characteristic wave-absorption frequencies exhibited by chemical bonds present in functional groups, such as C-H (carbon-hydrogen), O-H (oxygen-hydrogen), N-H (nitrogen-hydrogen), and C=O (carbon-oxygen) [113]. These absorption frequencies are directly associated with the chemical composition and structural properties of the substances under investigation [114]. As mentioned above, the typical range of wavelengths for NIR is from 780 to 2500 nm, while the spectral range encompassing Vis-NIR wavelengths is from 350 to 2500 nm, effectively encompassing both the visible range (350-780 nm) and the NIR range (780-2500 nm) [115]. NIR spectrometers typically comprise several components, including a light source, a wavelength selector, a sample selector, an optical detector, and a data processing/analysing system [116]. NIR spectrometers utilise a light source to emit radiation in the NIR wavelength range and the common sources are tungsten-halogen lamps, deuterium lamps, or light-emitting diodes (LEDs) [116]. A beam splitter system functions as the wavelength selector, separating the incident light into different wavelengths, and this can be achieved using filters, diffraction gratings, or interferometers [117]. The sample detector captures the NIR radiation transmitted through or reflected off the sample and this can be a photodiode or an array of detectors, depending on the specific instrument design [118]. The optical detector measures the intensity of the NIR radiation that passes through the wavelengthselector and different types of detectors, such as photodiodes or photomultiplier tubes (PMTs), may be used [107]. Finally, the collected data from the sample and optical detectors are processed and analysed to obtain meaningful information and this can be conducted using the appropriate chemometric techniques, such as partial least squares (PLS), principle component analysis (PCA), and support vector machine (SVM), depending on the complexity and requirements of the analysis [119]. It is crucial to select and optimise these components based on the intended usage of the instrument to ensure effective and consistent performance [107]. Factors such as the sensitivity, resolution, wavelength range, and signal-to-noise ratio should be considered while choosing the components for an NIR spectrometer [118]. Depending on the instrument type and analysis requirements, NIR systems typically operate in one of four modes: transmission, reflectance, diffuse reflectance, or transflectance [120]. For analysing meat and meat products, the reflectance mode is commonly used [120]. NIR spectrometers can also be categorised as either benchtop or portable [111]. The introduction of portable NIR spectrometers has fuelled an increasing trend towards testing their capabilities in abattoirs and meat processing units, eliminating the need for sampling and lab transport [121; 122].