Hemicellulose structure, dispersibility, and emulsification stability of holocellulose nanofibers from spent coffee grounds

Spent coffee grounds (SCGs) are a promising source of holocellulose nanofibers (HCNF) due to their high polysaccharide content, predominantly composed of galactomannan, arabinogalactan, and cellulose. Our previous study revealed that the HCNF isolated from SCGs contains highly crystallized mannan I, which has not been reported from any other biomass, and exhibits exceptional mechanical nano-fibrillation into 2-3 nm microfibril widths. As an extension of our previous report, we explored the molecular origin of complete nano-fibrillation with respect to hemicellulose structure. We also examined the morphological dimensions of condensed dispersions, the effect of alkaline treatment on mannan, and the detailed rheological behavior of dispersions compared with redispersions obtained from freeze-dried HCNF. Additionally, emulsification performance was also investigated as a potential application. Gel-state 2D NMR analysis confirmed the presence of galactomannan and arabinogalactan in the SCG holocellulose, with β-_D-glucuronic acid in arabinogalactan likely contributing to the high carboxyl content and excellent aqueous dispersibility. This resulted in condensed 5 and 10% HCNF dispersions that were completely nanofibrillated mechanically to approximately 3 nm in width and 0.7 μm in length. Alkaline treatment at room temperature decreased carboxy content and electrophoretic mobility but did not dissolve highly crystallized mannans, indicating partial removal of residual proteins and glucuronic acid in arabinogalactan. Rheological measurements of HCNFs revealed hysteresis loops at concentrations above 5%, gel-to-sol transition at higher shear strains, and overshoot in G" curves. Oil-in-water Pickering emulsions stabilized by 2% HCNFs showed excellent long-term stability up to 55% oil ratio over eight months stored at either room temperature or 40 °C.