Ultrasonic-assisted acid/alkaline extraction of prebiotic polysaccharides from Brassica rapa L. residues for targeted modulation of gut microbiota and metabolites via in vitro digestion and fermentation

ABSTRACT This study proposes a high-value utilization strategy for Brassica rapa L. processing residues under varying pH extraction conditions to achieve resource upgrading of this agricultural byproduct. Using an acid/alkaline extraction method, two structurally distinct bioactive polysaccharides were obtained: acid-extracted BRL-S and alkali-extracted BRL-J. Structural characterization revealed that BRL-J exhibited a lower molecular weight ( Mw ) and superior particle size homogeneity, whereas BRL-S showed a higher Mw and larger particle size distribution. After 48 h of in vitro fermentation, both polysaccharides underwent targeted depolymerization and exhibited distinct antioxidant activities. Microbiological analysis demonstrated that BRL-S1 (BRL-S after 48 h of fermentation) significantly suppressed Firmicutes, whereas BRL-J1 (BRL-J after 48 h of fermentation) selectively enriched Bacteroidetes, thereby synergistically promoting short-chain fatty acid production. Metabolomic analysis further revealed that polysaccharide intervention activated host–microbial co-metabolism, significantly upregulating nine key metabolites involved in alanine metabolism, the tricarboxylic acid cycle, and tryptophan metabolism. These metabolic shifts contribute to gut–brain axis regulation, energy metabolism, and immune homeostasis. Overall, this study provides a novel perspective on the high-value utilization of B. rapa L. processing by-products and establishes a theoretical foundation for the development of functional foods targeting gut microbiota modulation.