Coffee residues-to-biohydrogen: chemical fingerprint, pretreatment trade-offs and process-integrated design rules

Hydrogen production from coffee residues remains highly variable and poorly transferable across studies, primarily due to intrinsic chemical constraints rather than microbial or operational limitations. This review demonstrates that the complex chemical fingerprint of coffee by-products—including lignocellulosic carbohydrates, lipids, phenolic compounds, alkaloids, and Maillard reaction products—imposes coupled restrictions on substrate accessibility, microbial tolerance, redox balance, and hydrogenase activity. A critical analysis of recent literature (2022–present) shows that although pretreatment enhances solubilization, it frequently co-mobilizes inhibitory compounds and introduces system-level penalties associated with energy demand, chemical consumption, and salinity. As a result, the dominant process limitation shifts from structural recalcitrance to hydrolysate fermentability and process stability. The findings reveal that hydrogen yield is governed by the balance between solubilization, inhibition, and electron flux distribution, rather than by isolated improvements in pretreatment or fermentation conditions. In this context, biohydrogen production from coffee residues should be interpreted as a process integration challenge, in which the performance of each stage depends on its impact on the overall system. Integrated configurations—such as dark fermentation coupled with volatile fatty acid recovery and anaerobic digestion—emerge as more robust and scalable strategies compared to single-stage optimization approaches, as they require pretreatment to be designed based on system-level performance rather than maximizing hydrogen yield alone. Future progress depends on the development of selective pretreatment strategies, standardized hydrolysate characterization, and continuous reactor configurations capable of maintaining metabolic stability under chemically complex conditions. In addition, the incorporation of techno-economic analysis and life cycle assessment is essential to evaluate process feasibility beyond laboratory-scale performance. Advancing toward industrial implementation will require a shift from yield-oriented studies to integrated design frameworks that simultaneously address chemical, metabolic, and engineering constraints, enabling more efficient carbon utilization and energy recovery across the entire process chain.