Extraction-Dependent Structural Diversity of White Kidney Bean Cotyledon Polysaccharides Determines Their Effects on Ex Vivo Fermentation of the Elderly Gut Microbiota

Background/Objectives: Aging is accompanied by profound alterations in gut microbiota composition, reduced microbial diversity, and impaired metabolic resilience. Dietary polysaccharides with different structures exhibit varying efficacy in alleviating these age-related disorders. In this study, we aimed to extract and characterize polysaccharides from white kidney bean (Phaseolus vulgaris L.) cotyledon using different methods and to evaluate their fermentation behavior by the elderly gut microbiota. Methods: White kidney bean cotyledon polysaccharides were extracted using ultrasound-assisted acid (WKBC-P1), alkaline (WKBC-P2), and chelator (WKBC-P3) methods. Ex vivo fermentation of WKBC-P1, WKBC-P2, and WKBC-P3 was performed using pooled fecal microbiota from the elderly aged 65–70 years old. Results: Monosaccharides profiling revealed that WKBC-P1 and WKBC-P3 were rich in arabinose, whereas WKBC-P2 contained high levels of glucose. Molecular weight analysis indicated that acidic extraction led to smaller, more fragmented polysaccharides, while chelating extraction produced the highest molecular weight and most uniform fractions. Ex vivo fecal fermentation showed that WKBC-P1 and WKBC-P2 significantly increased the relative abundance of Bifidobacterium and Bacteroides_H while suppressing potentially harmful Proteobacteria. Correspondingly, both fractions induced robust production of short-chain fatty acids (SCFAs), while reducing branched short-chain fatty acids (bSCFAs), indicating a metabolic shift toward carbohydrate fermentation rather than protein catabolism. Conclusions: Overall, these findings demonstrate that acidic and alkaline extractions yield fractions that most effectively support beneficial microbial growth and SCFAs generation in aging microbiota. This knowledge might indicate that white kidney bean cotyledon polysaccharides represent promising prebiotic candidates for maintaining gut homeostasis and metabolic health in elderly populations.