## 4 Use of Spectroscopic Techniques for Monitoring Changes in Muscle Foods during Processing
### Fish and Other Seafoods
#### 4.1.1 Thermal Treatments
In more detail, it was found that the peak intensity of the band located around 1652 cm\({}^{-1}\), related to the \(\alpha\)-helical structures, increases with increasing heating temperatures, whereas an inverse behavior was observed for \(\beta\)-sheet structures (around 1618 cm\({}^{-1}\)), in agreement with the previously reviewed results obtained by Raman spectroscopy. Fluorescence Spectroscopy: The use of fluorescence spectroscopy for monitoring thermal treatments has been widely reported on several food products [167; 168; 169; 170; 171], while limited research has been carried out on fish and other seafood. Moreover, in most of these studies, fluorescence spectroscopy was used as a supplementary technique that confirmed the results obtained by using other measurement techniques [49; 172; 173]. One of the chemical changes that occur in muscle foods during cooking or other thermal processing treatments is the formation of Schiff bases. These compounds result from the crosslinking reaction between amino groups of proteins and aldehydes from lipid oxidation, and can be measured by fluorescence spectroscopy [49; 172]. In a study conducted by [PERSON] and al. [49], a much higher impact of roasting and frying of sturgeon (_Aciensen gueldenstaedtii_) on fluorescence intensity was observed than that of billing or steaming. In addition, depending on cooking methods, different shifts in the maximum emission wavelength were observed, suggesting the accumulation of Schiff bases with various chemical structures. In a similar study, the length of the roasting time of the same fish species was found to exert a significant effect on the fluorescence spectra and, consequently, on the formation of Schiff bases [172]. Other studies investigated the impact of different cooking methods on the formation of Maillard reaction products, as assessed by fluorescence spectroscopy. For example, the formation of these products was measured in hairtail (_Thichiurus lepturus_) fillets prepared by three cooking methods, namely boiling, baking, and frying. Higher fluorescence levels were observed in the baked and fried fillets compared to that in the boiled ones. One of the main conclusions of this study was that the measurement of fluorescent compounds can be used to evaluate the progress of Maillard reaction in cooked foods. In addition to the use of fluorescence spectroscopy for studying Schiff bases and Maillard reaction products, in other studies, good correlations were found between fluorescence data and traditional physico-chemical measurements. Recently, measurements based on fluorescence spectroscopy and microscopy resulted in interesting findings regarding the assessment of changes in some quality parameters (e.g., changes in fish texture, formation of oxidation products) occurring in fish during the application of different thermal treatments [174; 175; 176]. It is well-known that fluorescence from proteins is due to emission from the aromatic amino acids, especially tryptophan. Decreases in fluorescence intensity and red shift of the maximum emission are reported in several studies in the literature to be the result of thermal protein denaturation and tryptophan exposure to increasing solvent polarity [54; 177]. Thus, the fluorescence properties of tryptophan have been used to provide information about the extent of thermal treatments and protein denaturation occurring during various processing. For example, it was found that the degree of fluorescence intensity in surimi gel was dependent on the duration of microwave heating [177]. Another interesting field of application of fluorescence spectroscopy, as other spectroscopic techniques, is the possibility to exploit the spectra for classification purposes, with the basic underlying idea that the spectrum for a specific sample exhibit specific features compared to the spectrum of other samples [178]. In this context, our recent results showed that the fluorescence data were able to classify cod fillets subjected to different cooking temperatures [176].