Valorisation of agri-food by-products into bioactive compounds: Green extraction, microbial biotransformation and circular biorefinery strategies

Agri-food processing generates large volumes of by-products that remain insufficiently exploited despite their content of phenolic compounds, peptides, polysaccharides, carotenoids, dietary fibres and other functional constituents. These side-streams represent strategic bioresources for developing functional food ingredients, natural additives, nutraceutical compounds and active packaging materials within circular food systems. This review critically examines the valorisation of agri-food by-products by integrating Scopus-based bibliometric mapping with a process-oriented evaluation of green extraction, microbial biotransformation and cascading biorefinery strategies. A PRISMA-inspired screening workflow was used to construct a bibliometric dataset of 2327 English-language articles and reviews published between 2000 and 2025. VOSviewer analysis revealed rapid growth in the field, especially after 2020, with 476 publications recorded in 2025. Country co-authorship analysis identified Spain, Brazil, Italy, India, China, Portugal and the United States as major contributors and collaboration nodes, while author keyword analysis showed strong thematic links among bioactive compound recovery, phenolics, antioxidants, green extraction, fermentation, encapsulation and biorefinery development. From a technological perspective, ultrasound-assisted extraction, pulsed electric fields, high-pressure processing, microwave-assisted extraction, supercritical and pressurised solvent extraction, enzyme-assisted extraction, membrane fractionation and microbial fermentation are evaluated in relation to recovery efficiency, bioactivity preservation, safety, scalability, sustainability and industrial readiness. The review highlights that although these strategies can improve resource efficiency and support the development of food-grade bioactive ingredients, their industrial adoption remains constrained by feedstock variability, safety and regulatory requirements, downstream purification, limited techno-economic evidence and insufficient product standardisation. Overall, this review provides a food-focused framework for transforming agri-food by-products into safe, functional and scalable circular food-system solutions.