Mannan-rich Holocellulose nanofibers mechanically isolated from spent coffee grounds: Structure and properties

Spent coffee grounds (SCGs) contain abundant polysaccharides consisting of mannose (29 %), galactose (11 %), and glucose (11 %) and are a promising source of holocellulose nanofibers (HCNFs). In this study, the mannan-rich HCNFs were isolated from the SCG holocellulose in the yield of 52 % SCGs using an ultrahigh-pressure wet jet mill. The HCNF was refined by passing the suspension through a high-pressure homogenizer with a 95 μm nozzle 1-15 times. The 5-pass HCNFs were 2.4 nm wide and 0.7 μm long with 143 viscosity-average degrees of polymerization and contained mannan I crystals (5-10 nm in size) on cellulose microfibrils. The delignification process in water at 75 °C based on the Wise method allowed recrystallization of mannan on a cellulose microfibril substrate. The once-freeze-dried HCNFs had 30-50 nm widths and were not fully nanofibrillated in water when shaking the HCNF/water suspensions but exhibited comparable viscosities to those prepared by mechanical milling. The SCGs-derived HCNFs have a high potential for application in the food industry.