Valorization of coffee silverskin by cascade extraction of valuable biomolecules: preparation of eco-friendly composites as the ultimate step

This study presents a multidisciplinary approach for dealing with the environmental problems related to agro-industrial coffee residues. The exploitation of these residues allows biomolecules to be obtained from renewable sources and enables the preparation of CO₂-neutral biocomposites, with the advantages of reducing fossil depletion, avoiding climate-altering emissions, and limiting plastic pollution. Coffee silverskin (CSS), a by-product deriving from coffee bean roasting, was subjected to different eco-friendly extraction processes, such as ultrasound-assisted, CO₂-supercritical, and water-subcritical extractions to recover caffeine, phytosterols, and polyphenols. The residues remaining after the extractions were further valorized as fillers into biocomposites based on poly(1,4-butylene succinate) (PBS). Biocomposites (filler content up to 30 wt%) were prepared by melt mixing, and they were characterized in terms of their thermal, mechanical, and morphological performance. The effect of the presence of residues derived from different extraction procedures on the resulting properties of biocomposites was assessed and discussed, and the ultrasound-assisted treatment was found to leave the CSS residue as the most compatible with the PBS matrix. The results of this study indicate that the proposed bio-refinery could successfully and fully valorize the CSS agro-industrial residues, even in its ultimate step, producing biocomposites characterized by low economic and environmental impact; these new materials will be a possible bio-alternative to the traditional polymers commonly used by the packaging industry.