Upcycling of Waste Hop Stems into Cellulose Nanofibers: Isolation and Structural Characterization

Abstract

Hop (Humulus lupulus) is cultivated to harvest female flowers that lend a deep flavor, aroma, and bitter taste to beer. However, the rest of the plant is burned or land filled as agro-industrial waste. This work upcycles hop stems (HS), which contain 44% cellulose, and demonstrates their suitability as raw materials for the isolation of cellulose nanofibers (CNFs). The Wise method followed by alkaline pretreatment removed lignin and hemicellulose. 2,2,6,6-Tetramethylpiperidine-1-oxyl radical-mediated oxidation fibrillated CNFs from pretreated and non-pretreated HS. A uniform height distribution was inferred from atomic force microscopy, with a median of ∼2 nm for pretreated and non-pretreated HS-derived CNFs. Solid-state nuclear magnetic resonance and X-ray diffraction characterizations indicated that the pretreatment enhanced the purity and crystallinity of the CNFs, though traces of triacylglycerols and calcium oxalate monohydrate remained. The two CNF samples exhibited similar two-step thermal degradation at 255-260 and 300 °C, though less char residue was produced by the pretreated CNFs.