Study on the antioxidant properties of robusta and arabica coffee silverskin by-product with different degrees of grinding

This study evaluated the effects of coffee silverskin cultivar (Arabica and Robusta), grinding degree (<95 μm, 95–132 μm, >132 μm), and extraction solvent (water, ethanol) on the recovery of caffeine, chlorogenic acids, and antioxidant activity. Samples were analysed using HPLC, Folin-Ciocalteu, and antioxidant assays: DPPH and FRAP (ethanol-dependent), ABTS and metal chelation (water-dependent). Robusta contained significantly higher caffeine (6604–10,534 μg/g) than Arabica (4550–8550 μg/g), while caffeine content did not differ with solvent. Intermediate particle size (95–132 μm) maximized caffeine extraction, while coarse particles (>132 μm) yielded more chlorogenic acids. Cultivar and grinding degree had minimal influence on Total phenolic content (TPC) of water extracts; however, Robusta exhibited higher TPC (44.3–79.9 mg GAE/g dry matter) in ethanol extracts. Robusta exhibited the highest antioxidant activities in all assays: metal-chelating (49.2 mg EDTA/g) and ABTS (462.3 mg Trolox/g) in water extracts; DPPH (142.5 mg Trolox/g) and FRAP (30.5 mM Fe²⁺/g) in ethanol extracts. Robusta silverskin showed significantly higher soluble dietary fiber (SDF) (13.1–15.4 g/100 g dry matter), while insoluble fiber did not differ with cultivar. These findings provide a basis for optimizing silverskin valorisation and its use as a sustainable bioactive source in functional food applications.