## 4 Discussion
In addition, the residual water from the filtration contained a quarter of the total protein of the sample, with the protein content of the residual water being significantly higher than that of the filtered meat mass. Thus, in the context of the present experiment, filtration makes the process of dry-matter separation more complex, the losses during filtration were high, and the potentially lower energy input during subsequent drying does not compensate for the lost protein. In addition, the filtration process prolongs the protein-extraction process, making it more economically costly. Nevertheless, in the current experiment, we only used one mesh size, and it may be possible that other mesh sizes result in better dry matter and protein yields. However, we believe that even if other mesh sizes are used, filtration still involves considerable losses of dry matter and proteins, along with high energy-consumption and challenges related to mesh clogging. The amount of protein in the residual water suggests that the solids in the suspension are of a very fine fraction and that a more efficient method of filtration is needed. It is possible to improve the efficiency of the decanting process and thereby increase the number of solids and protein that can be extracted from the crushed meat mass. To our knowledge, there are no similar valorization experiments in the Baltic Sea region, and therefore we cannot compare our results with other experimental trials. However, the BONUS Optimus project [32] investigated the efficiency of a Super Heat Steam Dryer System as well as grinding and winnowing to separate meat from the mussel mass; but all their experiments resulted in a very poor separation of shells and mussel meat (actual data were not reported). Outside of the Baltic Sea region [PERSON] et al. [25] reported very similar protein content in the processed mussel meat (58.7%) as obtained in the current study (54.8%). [PERSON] and [PERSON] [33] achieved a very high protein yield in their experiment (13.5% of meat wet weight opposing to 6.3% obtained in this study). However, their separation process was overly complex and thereby costs are expected to be very high. Moreover, the extraction of meat using their preprocessing method is difficult to conduct with the Baltic Sea small-sized mussels. The reason why we had lower protein levels was that the share of free proteins in the raw mussel homogenate was relatively low, and most of the proteins were present in the meat particles. In order to increase the proportion of soluble proteins, chemical and/or enzymatic digestion, optionally combined with ultrasonication or microwave digestion of the meat particles, is necessary [34]. Due to increased/faster digestibility, insulinotropic effect and flavor-enhancing properties, such protein hydrolysates, could be effectively used in specialized food and feed applications. Peptide-rich fractions have been shown to act as prebiotics and exert potential biological activities (e.g., antioxidant, antihypertensive properties), also making them valuable ingredients for cosmetic and pharmaceutical industries [11; 35]. In order to obtain high-quality protein hydrolysates, the optimal hydrolysis techniques and conditions for this particular raw material have yet to be determined. Nevertheless, the production of meat hydrolysates by enzyme technology is generally well established and scalable in practice. It is rewarding to develop new uses of separated mussel meat and the remaining residual material in order to better valorize the Baltic Sea mussel biomass. To date, employing residual meat as an additive for fishmeal and animal feed [36] and separated shell fractions with some protein as a source to produce poultry feed are a few of the most common industrial validizations [19; 37]. Even though the required industrial technologies are well established, other sustainable options such as emulsions for human consumption or as food-flavoring agents are not quite extended in an industrial context [38].