## 5 Conclusions and Future Trends  
This review provided an overview of spectroscopic techniques used to monitor changes in muscle foods, (e.g., fish, meat, and derivative products), as induced by the application of thermal and non-thermal treatments. Quality retention and shelf life extension of muscle foods require both adequate processing/preservation methods and rapid assessment techniques. Product damages caused by the use of conventional thermal treatment methods have driven the development of novel thermal and non-thermal technologies. Microwave, radio frequency, ohmic, and infrared heating are among the most applied novel thermal techniques, while modified atmosphere packaging, high pressure processing, ultrasounds, cold plasma, and pulsed electric field are the most commonly known novel non-thermal techniques. Several studies have reported that these new technologies cause less damage to foods in terms of sensory and nutritional characteristics compared to traditional processing methods. On the other hand, the limitations related to the destructive nature of traditional measurements and the long time required to perform these measurements can be overcome by using spectroscopic techniques. Indeed, several spectroscopic techniques such as vibrational spectroscopy, fluorescence, and NMR have shown the potential to detect changes in quality resulting from processing. It is worth noting that the information obtained from spectroscopic techniques is not only associated with changes in the chemical composition but also to structural and other physical properties, enabling obtaining of spectra that are unique to the sample under investigation, acting as a fingerprint. Thus, in the light of the reviewed literature, it appears that spectroscopic techniques can provide alternatives to traditional sensory, microbiological, and physico-chemical assessment methods. Especially hyperspectral imaging (HSI) seems to be the most promising and investigated technique so far due to its well-known features, namely the capability to provide spectral and spatial information of the scanned samples and the possibility to be applied inline of production when the samples are moved on conveyor belts, for example. Moreover, the distribution map, or the chemical images that can be generated after the application of multivariate classification or regression models to the hyperspectral data, facilitate the visualization of the different measured parameters over the entire scanned samples. That is why the number of studies dealing with the application of spectroscopic techniques has been increasing in recent years, in the area of muscle foods or other food products. However, these new technologies (processing and assessment methods) are not without drawbacks and only a few applications can be found at the industrial scale. The challenges that can be met might be summed up in three levels; the muscle food level, processing technology level, and assessment or analytical method level. From the muscle food matrix point of view, the heterogeneity of muscle foods in terms of size, shape, and chemical composition remains one of the main challenges. This makes it challenging to both apply processing techniques and measure their impact on the treated samples. This problem can be reduced by using HSI or multispectral imaging. Although some investigations have been carried out aiming at applying HSI with visible and near infrared, very limited research has examined HSI in fluorescence or Raman mode. Therefore, it is our opinion that the possibility of coupling HSI with other spectroscopic techniques should be investigated more in future research. From the processing point of view, there is a major issue related to the consumer perception of novel technologies due to concerns about food safety [10]. Public health issues have become even more politicized in the present time with the current COVID-19 crisis ranging around the world and affecting every aspect of life, including food choices and nutrition habits.