Bioactive Compounds from Citrus-Processing By-Products: A Narrative Review of Physical-Assisted and Conventional Extraction Technologies Plus Downstream Applications.

Citrus processing generates massive volumes of citrus-processing by-products rich in polyphenols, pectin, and essential oils, yet most of these materials are disposed of with low-value utilization, causing biomass loss and environmental pressure. Physical-assisted extraction techniques represent promising strategies for recovering high-value bioactive components from citrus-processing by-products. This narrative review assesses mainstream conventional and physical-assisted extraction routes (acid-assisted, ultrasound-assisted, microwave-assisted, and supercritical CO2 extraction) for phenolic compounds, pectin, and essential oils derived from citrus-processing by-products, summarizes optimal operational parameters, and compares their downstream food, biomaterial, and environmental remediation applications. Key findings demonstrate that ultrasound- and microwave-assisted extraction generally deliver higher extraction yields and better preservation of thermally sensitive bioactives relative to conventional reflux extraction; nevertheless, industrial-scale translation faces multiple bottlenecks, including high equipment investment, raw-material seasonal variability, incomplete solvent-recovery workflows, and scarce unified multi-dimensional evaluation benchmarks. Major limitations of existing research include the predominant focus on laboratory-scale yield optimization, with insufficient systematic quantitative comparisons covering energy consumption, product functional quality, life-cycle assessment (LCA), and techno-economic performance. Furthermore, thermal-driven oxidative degradation of d-limonene and polyphenols persists as a critical challenge hindering final product quality. Future perspectives highlight the need to establish standardized raw-material pretreatment protocols, combine chemical-based evaluation metrics and ISO 14040-compliant LCA frameworks to balance environmental benefits and economic profitability, and advance pilot-scale validation for hybrid coupled extraction processes. Key conclusions: although citrus-processing by-products possess enormous biorefinery potential, bridging laboratory-scale feasibility and industrial commercialization still requires joint progress in extraction-process optimization, safety validation, and circular-economy-oriented technical innovation.