Modulating in vitro fecal fermentation behavior of sodium alginate by Ca²⁺ cross-linking

Slow fermentable dietary fibers can be utilized by human gut microbiota in the distal region of the colon and thus exert a sufficient short-chain fatty acids (SCFAs) supplement in the distal region of the human colon. Alginate (Alg) based microgels are widely fabricated and used to control their digestion by digestive enzymes releasing active substances site-specifically. Herein, sodium alginate microgels with gradient calcium-ion (Ca²⁺) cross-linking densities were developed, restricting their degradation by gut microbiota. Alg microgels were prepared using high-speed shearing after Alg was cross-linked with 10, 40, and 60 mmol/L Ca²⁺, respectively (named 10-Alg, 40-Alg, and 60-Alg). The fluorescence and atomic force microscopic results showed that the 40-Alg particle has the densest structure among the three cross-linked Alg. In vitro human fecal fermentation results revealed that the Ca²⁺ cross-linking exerted more restricting effects than delaying effects on the fermentation of Alg, and the 40-Alg exhibited the slowest fermentation rate and the least fermentation extent, by characterizing the residual total carbohydrate content, residual monosaccharide content, pH, and total short-chain fatty acids. The 16S rRNA gene sequencing results indicated that cross-linking structures shaped a high specifical Bacteroides-type microbial community and that OTU205 (Bacteroides_xylanisolvens) highly correlated to the cross-linking density (R = 0.65, p = 0.047). In sum, Ca²⁺ cross-linking generated a dense and compact structure of sodium alginate that facilitated a more restricted fermentation property and specificity-targeting microbial community structure in comparison to the original sodium alginate.