Sustainable Sourcing of l-Rhamnose-Rich Polysaccharides from Natural Biomass Diversity: Extraction, Primary Structural Elucidation, and Antioxidant Activity

This study explores a multi-resource approach for extracting and characterizing l-rhamnose-rich polysaccharides from nine natural biomasses, including green macroalgae (Ulva spp.), sumac species (Rhus spp.), and agro-industrial by-products such as sea buckthorn and sesame cakes. Hot-water and alkaline extractions were performed by biomass type, and the resulting fractions were analyzed using biochemical assays, monosaccharide profiling (HPAEC/PAD and GC/MS-EI), FTIR, and antioxidant activity tests. Extraction yields ranged from <1% in sea buckthorn residues to 15.48% in Ulva spp., which showed the highest recovery. l-rhamnose enrichment varied across biomasses: the highest proportions were found in Ulva extracts and Rhus semialata galls (PRS), reaching up to 44% of total sugars by HPAEC/PAD and 58% by GC/MS-EI. Antioxidant activities also differed markedly. In DPPH assays, the most active extracts were those from sea buckthorn berry cake (PTBA), Rhus coriaria seeds (PRC), and commercial sea buckthorn powder (PPA), with IC50 values of 32, 43, and 42 µg/mL, respectively. Hydroxyl-radical inhibition was also substantial, reaching 83.0% for PTBA, 79.4% for PRC, and 79.9% for Ulva lactuca at 1 g/L, compared with 97.5% for ascorbic acid. These results highlight specific biomasses as promising dual sources of l-rhamnose and natural antioxidants for valorization within a circular bioeconomy.