Recovery of lignins with antioxidant activity from Brewer’s spent grain and olive tree pruning using deep eutectic solvents

Agro-food industry generates lignocellulosic residues that can be highly valuable. Nonetheless, lignin is still an underrated biopolymer although it has several potential applications. Lignin can be extracted from biomasses using several pretreatments, being Deep Eutectic Solvents (DES) an environmental-friendly method. Thus, this work proposes to recover lignin from two different agro-food residues – Brewer’s spent grain (BSG) and Olive tree pruning (OTP) – using DES. Experimental designs were carried out for each residue to determine the best lignin extraction conditions based on yield and antioxidant activity. The optimum conditions (120 °C and 5 h) allowed lignin yields of 54.4% and 37.8% for BSG and OTP, respectively, with relatively high purities (>75%). Structurally, both OTP and BSG lignins have similar molecular weights, and FTIR analysis showed that OTP has more aromatic related bands. The OTP lignin showed higher antioxidant activity than BSG. This study demonstrates, for the first time, that DES is a promising solvent for successfully extracting lignins from OTP and BSG biomasses with high purities and valuable properties.