Cascade biorefinery strategies for the integrated valorization of orange peel and cocoa processing residues

The sustainable valorization of abundant agro-industrial residues is a key strategy for improving resource efficiency and advancing circular biorefinery models. Orange peel waste (OPW) and cocoa processing residues (CPW) are generated in millions of tons annually worldwide and represent two of the most abundant and compositionally rich agro-industrial biomasses that remain largely underutilized. This review provides a comprehensive analysis of recent advances in integrated valorization strategies, with particular emphasis on hydrothermal carbonization (HTC), a resource-efficient technology capable of processing wet biomass without energy-intensive drying, together with mechanochemical activation and integrated extraction-conversion approaches. We discuss the chemical composition of these residues, the selective recovery of high-value bioactive compounds, and their conversion into platform molecules (furans, carboxylic acids, phenols, and γ-valerolactone) and functional carbon materials. Catalytic upgrading pathways are also discussed, highlighting the potential of hydrochar-derived and other biomass-derived carbon materials as efficient supports for metal catalysts in the synthesis of bio-based chemicals. Finally, we highlight functional applications of the derived materials and summarize available techno-economic and environmental assessments, which suggest potential sustainability advantages of these valorization routes under specific process assumptions. Overall, the integrated utilization of OPW and CPW illustrates the potential of cascade biorefinery strategies to maximize biomass utilization, improve resource efficiency, and support more sustainable management of agro-industrial residues.