Optimization and kinetics modeling of phenolics extraction from coffee silverskin in deep eutectic solvent using ultrasound-assisted extraction

This research investigates the effect of extraction parameters on total phenolic content (TPC) and the antioxidant capacity of coffee silverskin (CS) extract using ultrasound-assisted extraction (UAE) in deep eutectic solvent (DES). The optimization was carried out in two stages: (i) the optimization of the UAE condition with the highest TPC; and (ii) a four-factor Box-Behnken design (BBD) to optimize the UAE condition with the optimal TPC; 2,2 diphenyl-1-picrylhydrazyl (DPPH) radical scavenging capacity; and ferric reducing antioxidant power (FRAP). The results showed that the optimal UAE condition with the highest TPC was 150-250 & mu;m CS particle size; 1,6-hexanediol as hydrogen bond donor (HBD); 1:7 HBA:HBD molar ratio; and 30% (w/w) water content, given choline chloride (ChCl) as hydrogen bond acceptor (HBA), 30 min extraction time and 30 degrees C extraction temperature. The BBD-based optimal UAE condition was 30% w/w water content, 45 mL/g liquid/solid ratio, 90 min extraction time and 85 degrees C extraction temperature, given the CS particle size of 150-250 & mu;m and the HBA:HBD molar ratio of 1 (ChCl): 7 (1,6 hexanediol), achieving 19.19 & PLUSMN; 0.20 mg GAE/g CS for TPC, 24.06 & PLUSMN; 1.77 mg TE/g CS for DPPH radical scavenging capacity, and 59.13 & PLUSMN; 4.55 mg Fe (II)/g CS for FRAP. The experimental results were in good agreement with the BBD-based predicted results (22.40 mg GAE/g CS for TPC, 24.09 mg TE/g CS for DPPH, and 59.43 mg Fe(II)/g CS for FRAP). The two-site kinetics model best fitted the experimental data, with R2 of 0.991-0.999.