Structural Characterization Of Dietary Fiber From Dried Lychee Pulp-combined Phenol Adducts And Its Laxative And Laxative Mechanism

Lychee is rich in phenolic and dietary fiber.After heat treatment,lychee pulp contains more bound phenolics.Previous studies have focused on the respective benefits of phenolics and dietary fiber.Although dietary fiber has been shown to have a laxative effect,the effect of phenolic on dietary fiber was ignored.In this paper,based on the comparative analysis of the composition and content of phenolic in dried lychee pulp of six cultivars,the structure of dietary fiber-bound phenolic adducts were characterized from different ways,and the laxative mechanism of dietary fiber-bound phenolic adducts was explored.(1)The phenolic composition and antioxidant activity of dried lychee pulp: the total phenolic content,total flavonoid content and the antioxidant capacity of FRAP and ABTS of dried lychee pulp of six varieties were determined.The high-performance liquid chromatography(HPLC)was used to identify the composition of phenolics and analyzed the differences between different varieties of dried lychee pulp.The results showed that after the simulated digestion treatment,the content of phenolic was between 13.96-22.23 mg/g,which were higher than those of the respective control groups(distilled water and methanol extraction group).The total flavonoid content,the antioxidant activity of FRAP and ABTS were 11.78 mg/g,204.98 ?mol/g and 165.45 ?mol/g respectively.Nine kinds of phenolics in dried lychee pulp have been identified preliminarily.The content of procyanidin B2,procyanidin A2 and quercetin-3-rutose-7-rhamnoside are higher.(2)The structural characterization of insoluble dietary fiber-bound phenolic adducts: The structure changes of insoluble dietary fiber-bound phenolic adducts were observed after the release of the bound phenolic from the adducts by alkaline hydrolysis treatment.The results of the scanning electron microscope(SEM)showed that the structure of the insoluble dietary fiber-bound phenolic adducts after alkali hydrolysis treatment became loose and fragmented,which might be related to the removal of phenolics such as ferulic acid(an autofluorescent phenolic).The results of laser confocal microscopy(CLSM)found that after the insoluble dietary fiber-bound phenolic adducts was hydrolyzed by alkaline,ferulic acid was no longer present.The Fourier transform infrared spectroscopy(FT-IR)confirmed that after alkaline hydrolysis treatment,the C-O bond in the insoluble dietary fiber-bound phenolic adducts were broken.Based on the HPLC qualitative and quantitative analysis of the composition of phenolics released from the insoluble dietary fiber-bound phenolic adducts after alkaline hydrolysis treatment,11 types of phenolics were identified.The content of ferulic acid was between 4.79-5.30?g/g,thus confirming that ferulic acid was indeed a bound phenolic in insoluble dietary fiber.(3)The structural characterization of different fractions in dietary fiber-bound phenolic adducts of dried lychee pulp: Different phenolics in lychee pulp during processing may form different adducts with dietary fiber.According to the method of polysaccharide fraction extraction and separation,the dietary fiber-bound phenolic adducts were classified to obtain chelated soluble pectin-bound phenolic,dilute acid soluble pectin-bound phenolic,dilute alkaline soluble pectin-bound phenolic,hemicellulose 1-bound phenolic,hemicellulose 2-bound phenolic and cellulose-bound phenolic.The particle size of adducts in different fractions is between 653.30-1910.50 nm,and the particle size of chelated soluble pectin-bound phenolic was the smallest.The content of uronic acid in the adducts of different fractions was ranked from hight to low: dilute acid soluble pectin-bound phenolic,dilute alkaline soluble pectin-bound phenolic,hemicellulose 2-bound phenolic,hemicellulose 1-bound phenolic,and chelate soluble pectin-bound phenolic.The gas chromatographic results of monosaccharide composition showed that the galactose content was higher in hemicellulose 2-bound phenolic.The chelate-soluble,dilute acid-soluble and dilute alkaline-soluble pectin-bound phenolic contains more rhamnose and fucose,but the above two could not be found in cellulose-bound phenolic.The XRD spectrum showed that hemicellulose 2-bound phenolic and cellulose-bound phenolic exhibited the characteristics of cellulose type I.(4)The laxative effect and mechanism of dietary fiber-bound phenolic adducts of dried lychee pulp in constipated mice: The 7-week-old male KM mice were used,80 mice were randomly and averagely divided into control group,constipation model group,low-dose dietary fiber-bound phenolic adducts group and high-dose dietary fiber-bound phenolic adducts group.The results showed that dietary fiber-bound phenolic adducts of dried lychee pulp could stimulate small intestinal peristalsis by promoting the release of serum excitatory neurotransmitters(Ach,MTL,SP)and inhibiting the release of serum inhibitory neurotransmitters(VIP,NO).The results of the advance rate of the small intestine in control group,model group,low-dose group and high-dose group were 44%,12%,33% and 30%,respectively.The dietary fiber-bound phenolic adducts promoted intestinal peristalsis,released bound phenolic through intestinal microbial metabolism,increased the number of colonic mucus cells and repaired colon damage,and increased the content of short-chain fatty acids(SCFAs).According to the Qt-PCR analysis of gene expression in colon tissue,it was found that dietary fiber-bound phenolic could increase the expression level of aquaporin mRNA(AQP3 and AQP9)in the colon,and then increased the expression of aquaporin,which together exerted a laxative effect.