Structural Changes,in Vitro Fecal Fermentation Properties And Gut Microbiota Regulation Of Resistant Starch Spherulites

Gut microbiota plays a crucial role in host metabolism,neuromodulation and immune protection,and the composition of the microbial community is highly related to the proportion of carbohydrates that are available to bacteria in the diet.Resistant starch spherulites(RSS)has received extensive attention in carbohydrate nutrition,for its good thermal and processing stability as well as the favorable ability of producing butyrate and proliferating beneficial bacteria,which is of great contribution to maintain colon health and improve metabolic diseases.Recent studies have shown that the structural differences of resistant starch might have different regulatory effects on the fermentation behavior and community structure of gut microbiota.In this study,RSS with different crystalline structures were prepared to investigate the effect of different crystalline structures on in vitro human fecal fermentation properties and gut microbiota composition.At the same time,progressive structural changes of RSS were also observed.Elucidating the relationship between starch structure and in vitro fermentation properties,additionally with the fermentation mechanisms would shed new light on the design of functional ingredients with enhanced gut health.The study was conducted as the following two aspects:We fabricated A-and B-type RSS with crystallinity of 47%-79%and 40%-64%through enzymic debranching of waxy corn starches(WCS),and investigated the fermentation characteristics of RSS and subsequent impact on human gut microbiota composition using in vitro batch fecal fermentation model.The A-and B-type polymorphic RSS displayed similar fermentation rate throughout the whole fermentation,the final gas production of A-and B-type RSS(per 50 mg)was respectively 11.8-13.2 m L and 12.1-12.6 m L,suggesting that the crystalline type and crystallinity of starches are not key factors in controlling the fermentation rate.At the end of fermentation,the concentration of acetate and butyrate decreased by 8.4 m M and 3.3 m M with the increase of A-type RSS crystallinity,respectively.However,the high crystallinity of B-type RSS reduced the production of acetate and propionate,with the concentration of which decreased by 8.4 m M and 2.1 m M,respectively.RSS with different crystalline types showed similar overall microbial community structure but distinct key bacteria species.A-type RSS promoted butyrogenic bacteria(i.e.,Roseburia faecis and Lachnospiraceae),whereas B-type RSS tended to promote another key bacteria species Prevotella copri with high-level propionate production.To elucidate the fermentation mechanisms of RSS,progressive structural changes during in vitro fecal fermentation were also conducted.The structure characteristics such as morphology,molecular chain length distribution,crystallinity and thermodynamic properties were determined by scanning electron microscopy,size exclusion chromatography,X-ray diffraction and differential scanning calorimetry.In terms of WCS,the molecular chain length distribution and crystallinity did not show significant changes with the prolonged fermentation time,and enthalpy decreased from 10.8 J/g to 8.1 J/g,suggesting that the amorphous and crystalline regions of natural starch granule were fermented at nearly the same rate.In terms of RSS,DP_AP1and h _AP2/h _AP1 of A-type RSS decreased and there was no obvious AP₂ peak of B-type RSS.At the 12 h of fermentation,the crystallinity of A-type RSS significantly decreased from 55%to 39%,and the enthalpy decreased from 13.8 J/g to 5.8 J/g.Similarly,the crystallinity of B-type RSS significantly decreased from 52%to 35%,and the enthalpy decreased from 12.3 J/g to 4.1 J/g,indicating that long-chain amylopectin and crystalline regions of RSS were preferentially utilized by gut microbiota,possibly due to the local ordered regions provided suitable attachment sites or surfaces for bacteria.