Cascading Valorization of Residual Chestnut Biomass: Linking Aqueous Extraction, Reverse Osmosis, Spray Drying, and Solid Reuse

Residual chestnut biomass was evaluated in a cascading valorization route combining hot-water extraction, extractant reuse, reverse osmosis (RO), spray drying, and subsequent use of the extracted solid. Two heterogeneous residual Castanea sativa streams were studied at laboratory and pilot scale. Successive extraction at 100 ∘C recovered up to 141 mg gallic acid equivalents (GAE) g−1 dry biomass as Folin-reactive TPC, with 90–93% of the seven-stage response recovered after four fresh-water stages. Reusing the liquid on fresh biomass increased Folin-reactive TPC to 12.0–24.0 mg GAE mL−1. Ultrafiltration did not provide useful fractionation, whereas RO gave >99.9% apparent Folin-reactive TPC retention and 96.5–97.6% total-solids retention; standard batches achieved 86.7–93.5% volume reduction. Carrier-free laboratory spray drying gave a maximum dry-solids collection recovery of approximately 60–62% in the favorable region near 20% feed solids. A representative powder contained 488±27 mg GAE g−1 dry matter, showed a DPPH IC50 of 13.7±1.1μg mL−1, water activity of 0.5850, and a volume-median particle size d50 of 7.42 μm. SEM showed predominantly spherical/sub-spherical particles together with hollow, concave, and partially collapsed particles. Complementary FTIR, MALDI-TOF, SEC-GPC, and qualitative on-flow ESI-MS provided structural and molecular-size fingerprints consistent with chestnut hydrolysable-tannin-related species, while individual assignments remain tentative. After two extraction cycles, residual biomass retained a net calorific value of 18.1–18.2 MJ kg−1 dry basis, although pellet durability decreased. The results define practical trade-offs linking liquid-product recovery and solid-residue valorization in an integrated chestnut-biomass cascade.