Selective aqueous hemicellulose extraction from hazelnut shells through planetary ball milling

Hazelnuts are among the most widely consumed nuts, but their shells are notoriously difficult to valorise beyond the generation of electricity and heat. Here, we cracked the hazelnut shell valorisation through planetary ball milling, allowing effective aqueous extraction of hemicellulose. Optimized planetary ball milling showed that 4 h milling allowed 50% w/w hemicellulose extraction (80 °C; 24 h), leaving all cellulose and lignin unextracted. The hemicellulose recovered through ethanol precipitation was characterised as 4-O-methylglucuronoxylan (GX; 96% Xyl; 4% MeGlcA) of high DP (M_w 25 kDa). Detailed HSQC NMR analysis of the whole hazelnut shells and isolated hemicellulose fractions revealed a similar sugar composition and pinpointed a highly acetylated xylan, with substitutions at the 2-O and 3-O positions of the xylosyl residues. Importantly, HSQC NMR confirmed acetyl migration from 2-O to 3-O positions during the aqueous extraction. Enzymatic oligosaccharide profiling indicated a random distribution of glucuronyl and acetyl moieties across the xylosyl-backbone. We further demonstrate that the hemicellulose-depleted extracted cellulosic residues can be upgraded to monosaccharides through enzymatic conversion, and a high-quality residual lignin stream. Our research thus contributes to deciphering the structure of hazelnut shell glucuronoxylan and presents an effective hemicellulose-forward valorisation strategy for recalcitrant nut shells.