<i>In vitro</i> fecal fermentation characteristics and impact on gut microbiota abundance and metabolism of <i>Brassica rapa</i> L. polysaccharides extracted using natural deep eutectic solvents <i>vs.</i> hot water

In this study, a continuous <i>in vitro</i> fermentation model was employed to systematically compare the structural and functional characteristics of <i>Brassica rapa</i> L. polysaccharides extracted using hot water extraction (HWE) and deep eutectic solvents (DES). After 48 h of fermentation, BRL-E1 exhibited markedly lower molecular weight (1941 Da (BRL-E1) <i>vs.</i> 13 941 Da (BRL-1)), viscosity, and structural complexity compared to BRL-1. Morphological analysis revealed that both polysaccharides underwent progressive depolymerization from their initial honeycomb-like (BRL) and aggregated-cluster (BRL-E) architectures into more uniform, fragmented forms. Importantly, dynamic fermentation parameters, including total sugar concentration, pH, and enzymatic activity, showed significant variation over time for both BRLs, indicating distinct fermentation behaviors. Functionally, both polysaccharides enhanced short-chain fatty acid (SCFA) production, but elicited divergent microbial modulation patterns. Specifically, BRL acted synergistically with the positive control (POS) to selectively enrich SCFA-producing and anti-inflammatory genera, including <i>Bacteroides</i> and <i>Faecalibacterium</i>. In contrast, BRL-E demonstrated a unique prebiotic profile, significantly promoting the proliferation of <i>Bifidobacterium</i> species. Metabolomic profiling further revealed that both BRLs significantly upregulated a range of host-microbiota co-metabolites, including spermidine, indole derivatives (3-indolealdehyde and 3-indoleacetic acid), taurocholic acid, and vitamin B₂. These metabolites are involved in key metabolic pathways-such as purine metabolism, tryptophan/phenylalanine/tyrosine metabolism, the arginine-proline cycle, and glutathione-mediated redox homeostasis-together contributing to the maintenance of intestinal homeostasis. Collectively, these findings demonstrate that the extraction method significantly influences the structural properties and fermentation dynamics of BRLs, which in turn shape their functional capacities.