This study aimed to elucidate the differences in flavonoid composition and gut homeostasis-regulating effects of Shatianyu (Citrus grandis L. Osbeck) whole fruit before and after debittering by Lactobacillus rhamnosus fermentation, thereby providing the theoretical foundation for the precise nutritional processing of Shatianyu.

Shatianyu whole fruit powder (SWFP) and Lactobacillus rhamnosus-fermented Shatianyu whole fruit powder (F-SWFP) were used as test materials. High-performance liquid chromatography (HPLC) was adopted to analyze the differences in flavonoid composition and content between the above-mentioned fruit powder. Subsequently, SWFP and F-SWFP were subjected to in vitro fecal microbiota fermentation for 24 h, respectively. Gas chromatography (GC) and a lipopolysaccharide (LPS) enzyme-linked immunosorbent assay (ELISA) kit were employed to determine the content of short-chain fatty acids (SCFAs) and LPS in each fermentation system, respectively. 16S rRNA sequencing was adopted to determine the structure of fecal microbiota. The correlation between the relative abundance of fecal microbiota among SCFAs and LPS contents was revealed by Pearson correlation analysis.

Eight monomeric flavonoids, namely neoeriocitrin, naringin, melitidin, cigranoside B, cigranoside A, cigranoside C, rhoifolin, and hesperidin, were detected in both SWFP and F-SWFP. Naringin, melitidin, and cigranoside B were the major flavonoid components, accounting for approximately 85% of the total content of detected flavonoids. Fermentation with Lactobacillus rhamnosus significantly reduced the total flavonoid content in SWFP, cigranoside B, neoeriocitrin, rhoifolin, hesperidin and melitidin were the main degraded flavonoids in F-SWFP, with the degradation rates ranging from 28.1% to 78.5%. After 24 h-fecal microbiota fermentation, both SWFP and F-SWFP significantly reduced the relative abundance ratio of Bacillota to Bacteroidota in fecal microbiota, with F-SWFP exhibiting a more pronounced effect. SWFP specifically induced the proliferation of Bifidobacterium and Mediterraneibacter, whereas F-SWFP not only specifically promoted the proliferation of Segatella and g_norank_f_Prevotellaceae, but also inhibited the growth of Collinsella, Blautia, Parabacteroides, and Dialister. 24 h-fecal microbiota fermentation of SWFP or F-SWFP all significantly increased acetic, propionic, and butyric acid content but reduced LPS content in the fermentation system. Notably, F-SWFP fermentation group showed higher SCFAs but lower LPS content than those of SWFP fermentation group, also, the butyric acid and LPS content in F-SWFP group was 1.2 and 0.7 times those of SWFP group, respectively. Correlation analysis revealed that the characteristic microbes in SWFP group were significantly positively correlated with SCFAs content, whereas those in F-SWFP group were not only positively correlated with SCFAs content, but also negatively correlated with LPS content.

Compared with SWFP, F-SWFP showed a lower total flavonoid content, but exhibited greater efficacy in promoting butyrate production while inhibiting LPS production by fecal microbiota. Given the key roles of butyrate and LPS in obesity-related metabolic diseases, F-SWFP serves as a functional ingredient for ameliorating obesity-related metabolic diseases and is suitable for use in the development of healthy foods.

The effects of semi-solid fermentation with different lactobacillus on the bitterness and active components in Shatianyu (Citrus grandis L. Osbeck) fruit powder (SFP) were explored. The aim of the present study was to establish the debitterizing technology and prepare low-bitterness SFP enriched in flavonoids and dietary fiber (DF), for providing the theoretical basis and technical support for the application of SFP in healthy food.

Shatianyu septum and pulp were dried and crushed to generate SFP, then, the bitterness of SFP and the equivalent main flavonoid glycosides (naringin and melitidin) in SPF were evaluated by sensory evaluation to clarify the bitter substance base. Lactobacillus plantarum GIM 1.1516 (LP1.1516), Lactobacillus casei GIM 1.411 (LC1.411), Lactobacillus acidophilus GIM 1.731 (LA1.731) and Lactobacillus rhamnosus GIM 1.325 (LR1.325) were used to ferment Shatianyu homogenate. During fermentation, the reducing sugar content, pH and acidity were dynamically monitored, the naringin and melitidin degradation rate were analyzed by HPLC, and the bitterness of Shatianyu homogenate after fermentation was also evaluated to determine the optimal fermentation conditions. Then, the fermented SFP was prepared, and its bitterness, together with total phenolic, total flavonoids, total DF (TDF), soluble DF (SDF), and insoluble DF (IDF) content were determined, respectively. Furthermore, the oxygen radical absorbance capacity (ORAC), ferric reducing antioxidant power (FRAP), ABTS free radical scavenging ability, together with water retention (WRC), water swelling (WSC), and oil adsorption capacity (OAC) of fermented SFP were determined, respectively.

SFP exhibited a more bitter taste, which was mainly caused by the co-flavor of naringin and melitidin, the main flavonoid components in SFP. During the first 12 h-fermentation, LP1.1516, LC1.411, LA1.731 and LR1.325 could use the reducing sugar in the homogenate and produce acids, while during 12-24 h-fermentation, only LR1.325 could still use the reducing sugar and produce acids. Shatianyu homogenate fermented by LR1.325 for 12 h showed the highest total degradation rates of naringin and melitidin (41.1%), also showed the highest sensory bitter removal rates (66.0%). Compared with unfermented SFP, the taste of SFP prepared by 12 h-LR1.325 fermentation changed from bitterer to between little bitter and slightly bitter, with a sensory bitter removal rate of 60.8%. Furthermore, the later showed higher total flavonoid content (increasing 19.8%), lower IDF (decreasing 9.6%), and higher SDF content (increasing 19.6%). Also, SFP prepared by 12 h-LR1.325 fermentation showed higher ORAC antioxidant activity (increasing 24.1%), higher water retention (increasing 20.8%) and water swelling capacity (increasing 32.0%).

12 h-LR1.325 fermentation of Shatianyu homogenate consisting of septum and pulp generates low-bitterness SFP enriched in flavonoids and DF, which could be used in the development of related functional foods.

The effects of soluble and insoluble dietary fiber (DF) from Shatianyu (Citrus grandis L. Osbeck) pulp on gut microbiota were explored, so as to enrich the theoretical basis of fruits and vegetables DF in regulating gut microbiota, therefore guiding their precision nutrition processing.

In vitro fecal microbiota fermentation model was used to ferment soluble DF (SDF), insoluble DF (IDF) and total DF (TDF, IDF and SDF mixed according to their proportion in Shatianyu) for 24 h, respectively. The content of SCFAs in each fermentation broth was analyzed by gas chromatography. The structure of fecal microbiota in each fermentation group was analyzed by 16S rRNA sequencing. Microstructure of Shatianyu DF and their microbial attachment status were analyzed by scanning electron microscopy. Pearson correlation analysis was used to analyze the correlation among the relative abundance of bacteria and SCFAs content in fermentation broth.

IDF, SDF and TDF in Shatianyu pulp all improved acetic and propionic acid content in fermentation broth. IDF and TDF group showed higher acetic and propionic acid content than those in SDF group, and IDF also improved butyric acid content. After 24 h-fermentation, IDF and TDF group showed similar microbial community composition on the OTU level, which was obviously different from those of SDF group. Many rod-shaped bacteria gathering in the cracks on SDF surface were observed, however, a large number of spherical or ellipsoidal bacteria gathering in the cracks and folds on IDF surface were found. LEfSe analysis revealed that SDF specifically induced the enrichment of Veillonellaceae, unclassified_k__norank_d__Bacteria and Butyricicoccus, while IDF specifically induced the blooming of Streptococcus, Faecalibacterium, Paraprevotella, Lachnospira, and TDF specifically induced the enri-chment of Prevotella, unclassified_p__Firmicutes, Lactobacillus and Turicibacter. Furthermore, the abundance of characteristic microbes in IDF and TDF group was significantly positively correlated with SCFAs content in fermentation broth.

It was different from the traditional view that IDF mainly played a role in promoting intestinal motility, and Shatianyu pulp IDF was also an important energy source of gut microbiota, which was more effective in regulating gut microbiota producing SCFAs than those of SDF. Thus, Shatianyu pulp IDF was a potential functional ingredient of healthy food for improving intestinal microecology.