Valorization of hop (Humulus lupulus L.) pruning to produce valuable compounds using two biorefinery strategies: Conventional processing and microwave-assisted autohydrolysis

Abstract

The beer industry produces a significant quantity of residues, including hop pruning from the Humulus lupulus L. plant. In this work, two alternative schemes of biorefinery were evaluated for the first time for valorization of this residue. A conventional processing, involving the water extraction of compounds (110 ◦C for 30–60 min), was proposed to obtain phenolic compounds, followed by weak acid treatment to optimize the hemicelluloses solubilization. Alternatively, innovative processing, based on autohydrolysis assisted by microwave was also evaluated for the co-extraction of antioxidants and oligosaccharides. Results obtained from these biorefineries showed that after 30 min of aqueous extraction phenols (33.86 mg GAE/g raw material) and flavonoids (42.50 mg RE/g raw material) were successfully solubilized with an antioxidant activity of 6.09, 43.56, and 29.79 mg TE/g raw material using the DPPH, ABTS, and FRAP methods, respectively. The second stage of conventional process (123.5 ◦C; 1.69 % HCl; 59.6 min) yielded the highest values of xylooligosaccharides and xylose (16.38 g/L) and glucan content (53.25 %). Alternatively, 5.50 g/L of xylooligosaccharides and xylose were obtained along with antioxidant phenolics measuring 31.74 mg GAE/g raw material and 61.06 mg RE/g raw material, using microwave-assisted autohydrolysis (200 ◦C for 5 min). The antioxidant activity of these bioactive compounds was 20.80, 29.82, and 44.01 mg TE/g raw material for the DPPH, ABTS, and FRAP assays, respectively. Overall, this study shows the feasibility of hop pruning processing under two biorefinery schemes, in which between 10.32 and 17.11 g of phenolic compounds and xylan derivatives per 100 g of raw material can be obtained, with high potential to be used in the pharmaceutical, food or chemical industries