## 4 Development of Optical Sensing Techniques for Red Meat Quality Prediction
### Spectroscopic Techniques
#### 4.1.2 Raman Spectroscopy
The Stokes band is observed when a molecule, upon interacting with excitation radiation, undergoes a transition to a higher vibrational level [150]. On the other hand, the anti-Stokes band can be observed if the molecule was initially in an excited vibrational state before being impacted by the excitation radiation [143]. A Raman spectrometer typically consists of a laser as a light source, a monochromator, a sample holder, and a detector [151]. The wavelength of the laser used is a crucial parameter in Raman spectroscopy [152]. There are a great variety of lasers with specific wavelengths that can be employed as excitation sources, such as krypton (530.9 and 647.1 m), helium-neon (632.8 nm), neodymium (1064 nm and 532 nm), argon (488.0 and 514.5 nm) and diode lasers (630 and 780 nm) [146]. Raman spectroscopy faced limitations for many years, primarily due to the low efficiency of normal Raman scattering and the high costs associated with the necessary spectrometer components that were not suitable for on-site analysis [153]. The overcoming of these limitations was made possible through the development and enhancement of Raman spectroscopy equipment, ultimately opening up extensive research opportunities in various fields [153]. Nowadays, alongside lab-based systems, portable and low-cost hand-held Raman spectrometers are available [154], but their performance is restricted and makes it hard to meet the actual demands [155; 156; 157; 158; 159]. While not as widely applied as Vis-NIR spectroscopy, over the years, a number of studies providing insights into various attributes, such as the tenderness, colour, and other sensory attributes, of beef and pork muscles have been conducted (Table 3). \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Source** & **Animal** & **Muscle** & **Number of** & **Wavelength** & **Statistical** & **Measured** & **Performance** \\  & & & **Samples** & **Range (nm)** & **Method** & **Attribute** & **Performance** \\ \hline  
[PERSON] et al. [163] and [PERSON] et al. [155], analysing beef meat, using the same reference method but different validation techniques (cross-validation and external validation, respectively), achieved similar R\({}^{2}\) values for beef tenderness. Among the studies on beef instrumental tenderness, [PERSON] et al. [160] recorded the highest performance, homogenising the meat samples, providing a more uniform and consistent sample for the Raman measurements. [PERSON] et al. [161], applying homogenisation as well, focused on beef _M. longissimus thoracis_ and reported performance values for the overall sensory attributes. Unlike previous authors, [PERSON] et al. [162] used a texture analyser as a reference method, resulting in high R\({}^{2}\)p values for the tenderness, firmness, chewiness and hardness. Moreover, the results of [PERSON] et al. [164] demonstrated that Raman spectra have the potential to predict a wide range of trained sensory panel scores with varying levels of accuracy. In terms of beef colour, [PERSON] et al. [165] studied beef _M. longissimus dorsi_, using uninformative variable elimination-partial least squares (UVE-PLS) analysis, reporting high performance values. Finally, in their research on pork meat, [PERSON] et al. [166] showed the good predictive accuracy of sensory data using Raman spectra, whereas [PERSON] et al. [159] showed the limited ability of Raman spectra to predict shear force data in lamb meat. Like with NIRS, Raman also has well-known advantages such as being a rapid and non-invasive technique, requiring only a small amount or no sample pre-treatment at all. The evidence from the literature in Tables 2 and 3 seems to indicate that the use of Raman spectroscopy may result in better predictive performance values for the sensory traits of meat compared with NIRS. Water is a weak Raman scatterer, so Raman spectroscopy presents itself as a suitable tool for meat analysis as water is the main component of meat and this is an advantage over NIR spectroscopy, which potentiality is limited by the intense absorption of water.