A biorefinery approach for the valorization of spent coffee grounds to produce antioxidant compounds and biobutanol

Abstract

Spent coffee grounds (SCG) has been proposed as a potential material within the biorefinery context. The optimization of the microwave assisted extraction of antioxidant compounds with natural deep eutectic solvents (NADES), a promising green solvent, was conducted in this study. Under the optimal extraction conditions (120 °C, 15 min, 20% water in NADES and 0.53 ChCl:Glyc molar ratio), the obtained extract contained: total phenolic content (TPC), 0.48 mg of gallic acid equivalents (GAE) g−1 SCG, coumaric acid being the most abundant; total flavonoid content (TFC), 0.44 mg of catechin equivalents (CE) g−1 SCG; antioxidant activities: DPPH, 0.55 mg trolox equivalents (TE) g−1 SCG; ABTS, 3.17 mg TE g−1 SCG; and FRAP, 1.52 mg TE g−1 SCG. Moreover, the residual NADES extracted SCG was microwave pretreated with dilute sulfuric acid, enzymatically hydrolyzed and fermented by Clostridium beijerinckii, generating 7.1 g L−1 butanol (81 kg butanol t−1 SCG and 126 kg acetone-butanol-ethanol (ABE) t−1 SCG). In this way, SCG displayed its potential as a source to be used in an integrated biorefinery approach.