Camellia sinensis Leaf Residue from Kombucha Production: Composition, Chemical Markers, and Food Valorization

Kombucha is traditionally produced from a sweetened infusion of Camellia sinensis (L.) Kuntze leaves, the species used to obtain both green and black teas. Green tea retains more catechins, whereas oxidative processing of black tea favors theaflavins and thearubigins. During kombucha production, infusion extracts part of the leaf constituents and subsequent microbial fermentation mainly transforms the solubilized fraction. This study evaluated the solid green/black tea residue as a potential food ingredient. Proximate composition, water activity (aw), 168-h hygroscopicity, and total extractives were determined; hydroethanolic extracts (EtOH:H2O, 70:30) were assessed by total phenolics, DPPH, ABTS, FRAP, ORAC, and LC–ESI–IT3D–MS/MS. The residue contained 65.80 g/100 g total dietary fiber on a dry basis, including 54.40 g/100 g insoluble and 11.40 g/100 g soluble fiber, together with 22.23 g/100 g protein, 1.51 g/100 g lipids, and 4.06 g/100 g ash. Water activity was 0.411, hygroscopicity was 5.08 g/100 g powder, and total extractives represented 14.56 g/100 g dry biomass. Despite previous use of the leaves, antioxidant responses remained at 119.5 µmol TE/g by ABTS, 67.9 by ORAC, 42.8 by DPPH, and 31.3 by FRAP, with 0.82 mg GAE/g total phenolics. MS/MS revealed [M−H]− m/z 169→125, compatible with gallic acid; [M+H]+ m/z 459→289/151/139, compatible with epigallocatechin gallate (EGCG); a nominal [M+H]+ m/z 565 feature yielding products at m/z 427, 425, 409, 389, and 241, compatible with theaflavin; and m/z 195→138, compatible with caffeine. This combination is consistent with the mixed tea source: EGCG represents the green-tea catechin profile, whereas theaflavin is a characteristic black-tea marker; gallic acid and caffeine may occur in both. Retention of fiber, protein, and substances after infusion demonstrates value beyond the fermented beverage. Kombucha fermentation may promote degallation, hydrolysis of conjugates, and release of lower-molecular-weight phenolics. These biotransformations, together with matrix changes during processing, may leave part of the residual substances more exposed and susceptible to subsequent extraction, facilitating their recovery. The results support cascade valorization through recovery of the bioactive fraction followed by use of the fiber-rich solid in flours, bakery products, snack bars, or extruded foods, subject to sensory evaluation and specific quantification of caffeine and catechins.

Acknowledgments: CNPq (PIBIC / PIBITI – 2026 and 400122/2025-1), FAPESP (2025/11374-6) and PIDA/PRPI/USP 1602025/01.