Tracking transformations of polymeric carbohydrates from green coffee beans to spent coffee grounds: Mannan-dominant structures and valorization implications

This study tracks how carbohydrate polymers evolve along the coffee process chain from green coffee beans to roasted coffee and spent coffee grounds (SCG), with a focus on how process history shapes the mannan-rich residue that remains after brewing. Complementary analyses by acid methanolysis, total hydrolysis, thermogravimetric analysis (TGA), X-ray diffraction (XRD), and solution-state and solid-state nuclear magnetic resonance (NMR) were used to distinguish losses of labile carbohydrates from structural reorganization within the residual polymer matrix. Roasting and brewing progressively depleted sucrose and other accessible low-molecular-weight or weakly ordered carbohydrate fractions, whereas mannan remained the dominant carbohydrate polymer. Total hydrolysis showed that SCG contained approximately 51% mannan and 17% cellulose. At the same time, XRD, TGA, and solid-state 13 C NMR indicated increasing structural order and higher thermal resistance in the spent residue, consistent with selective retention of recalcitrant mannan-rich domains and partial reorganization of cellulose and mannan. These results show that SCG should not be viewed as a generic cellulose-rich feedstock. Instead, their process-dependent, mannan-dominant polymer structure should guide valorization strategies in food, fiber, and composite applications.