Effect of Fermentation on Structure, Physicochemical and in Vitro Fermentation Characteristics of Dietary Fiber in Mustard Green Stems

This study explored the effects of three different fermentation starters: Lactiplantibacillus plantarum (Z), Leuconostoc mesenteroides (C) and spontaneous fermentation with the addition of salt (Y) on the structure, physicochemical properties, and in vitro fermentation characteristics of soluble dietary fiber (SDF) and insoluble dietary fiber (IDF) from the stems of heading mustard (Brassica juncea var. capitata). SDF and IDF from the three fermented samples were denoted as Z-SDF/IDF, C-SDF/IDF, and Y-SDF/IDF, respectively. For comparison, SDF and IDF from unfermented samples were prepared and designated as X-SDF/IDF, respectively. The results indicated that fermented dietary fiber (DF) exhibited a porous and fragmented microstructure. Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) analysis revealed similar spectral patterns between fermented and unfermented DF. However, monosaccharide composition was significantly altered after fermentation. The proportion of glucose in the total monosaccharides of C-SDF increased substantially, reaching 62.70%, while xylose was undetectable in Y-SDF. C-IDF showed a 21.47% increase in water-holding capacity, Y-IDF exhibited a 215.46% increase in swelling capacity, and Z-SDF demonstrated a 78.45% increase in oil-holding capacity compared with their unfermented counterparts. Additionally, the cholesterol, sodium cholate, and glucose adsorption capacities of Y-SDF, Z-SDF, and C-SDF were significantly higher than those of X-SDF. After 24 h of in vitro fermentation, total short-chain fatty acid production from Y-SDF, Z-SDF, and C-SDF increased by 35.27%, 61.95%, and 19.13%, respectively, compared with X-SDF. The Firmicutes-to-Bacteroidetes ratio was higher in fermentation systems supplemented with Y-SDF, Z-SDF, or C-SDF than in that supplemented with X-SDF. Moreover, the relative abundance of Bifidobacterium and Collinsella was significantly elevated in the fermented SDF groups compared with the X-SDF group. This study clarifies that fermentation improves properties of dietary fiber from mustard stem, providing a theoretical foundation for the development of fermented mustard products.