## 4 Use of Spectroscopic Techniques for Monitoring Changes in Muscle Foods during Processing
### Fish and Other Seafoods
#### 4.1.1 Thermal Treatments  
Evaluation and understanding of quality changes induced in fish and other seafoods during the application of thermal treatments are necessary in order to better monitor and optimize the heating process [32; 153; 154]. In addition to the reference and official methods used for evaluating heat treatments (discussed in Section 3.1 of this review), many studies have investigated the potential of several spectroscopic techniques, such as Raman, infrared-based techniques, fluorescence, and NMR. Practical examples of these techniques will be given in this section. Raman Spectroscopy: This technique can give structural information about proteins, lipids, and water of muscle foods during processing or storage [155]. Raman spectroscopy is especially useful when studying changes in secondary structure of proteins. For instance, several studies have been conducted using Raman spectroscopy to investigate changes occurring in the secondary structure of surimi protein during the application of different thermal treatments [156; 157; 158]. More specifically, Raman spectra can provide useful information about changes taking place in \(\alpha\)-helix and \(\beta\)-sheet structures of protein as affected by thermal treatments. Indeed, it has been frequently reported \begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Spectroscopic Technique** & **Wavelength Limits** & **Type of Transition** & **Advantages** & **Limitations** \\ \hline Fluorescence & 250–750 nm & Bonding electrons in molecules & Rapid, high accuracy, sensitivity, relatively low cost & Limited to samples containing fluorophores, sample surface technique, inner filter effect \\ \hline Near-infrared & 750–2500 nm & Overtones and combinations of fundamental bands & Less sample preparation requirement, high sensitivity to physical structure and presence of water & Requires reliable reference methods, low specificity, overlapped and complex spectra \\ \hline Hyperspectral imaging & 400–1000 nm & - & Providing spatial information (pixel-to-pixel signal) & Huge amount of data and data processing, costs \\ \hline Mid-infrared & 2500–25,000 nm & Fundamental stretching, bending, and rotating & High sensitivity to chemical compositions, distinct absorption peaks & Water interference; limited suitability of moist samples. Low light penetration \\ \hline Raman & 750–1064 nm (excitation), 2500–200,000 nm & Vibrational transitions & Provides structural and qualitative information, low sensitivity to water & Interference from biological fluorescence background signals, small part of the sample is irradiated (laser spot), low sensitivity, and complex instrumentation \\ \hline Nuclear magnetic resonance & 1–1000 m & Nuclei orientation into a magnetic field & Accuracy, determination of precise structures, minimal sample preparation, spatial information (magnetic resonance imaging: MRI) & Expensive equipment, low sensitivity, overlapping signal, especially when analyzing complex mixtures \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The pros and cons and other characteristics of the main spectroscopic techniques. The table is adopted from [131; 151]. that the content of \(\alpha\)-helix decreases, while the content of \(\beta\)-sheet increases with increasing temperature [54; 154; 157]. Visible and Infrared-Based Techniques: Visible (Vis), NIR, and MIR are more frequently applied techniques for monitoring thermal treatments in fish and other seafood. Vis-NIR spectroscopy has been tested in several studies for assessing endpoint and surface temperatures in fish and other seafood [159; 160; 161]. Two similar studies reported that the spectral range between 400 and 550 nm was especially relevant for predicting thermal loads in heat-treated surimi [162; 160]. In these studies, the authors observed a decrease in absorption with increasing cooking time and temperature, which was attributed to increased scattering caused by protein denaturation.