Valorisation of Freshwater Fish By-Products: A Review on Extraction Technologies, Stability, Functional Food Applications and Prospects.

The valorisation of freshwater fish by-products represents an emerging opportunity to reduce waste and enhance resource efficiency in the aquatic processing sector. These by-products, including skin, bones, scales, viscera and heads, are rich in nutritionally valuable components such as proteins, collagen, gelatine, peptides and fish oils with bioactive potential. This review provides an overview of the composition and functional properties of these compounds and discusses current extraction technologies to maximise their recovery. Conventional methods, such as acid or alkaline extraction, remain widely used, but many compromise product quality and generate chemical waste. In contrast, enzymatic and assisted extraction methods, including ultrasound-, microwave- and hydrothermal-assisted processes, have demonstrated improved yield, hydrolysis efficiency and bioactivity, with lower environmental impact. Nonetheless, challenges in cost, scalability and process optimisation remain. The current review also highlights factors influencing the stability and storage of extracted compounds, including temperature, pH, oxidation and enzymatic degradation, which can affect bioactivity and shelf life. Stabilisation approaches such as freeze-drying, encapsulation and active packaging have shown promise in preserving the structural integrity and viability of freshwater fish by-products, enabling their transformation into high-value ingredients for food, nutraceutical and biomedical applications, and supporting a more sustainable fisheries sector.