| Chinese yam is the Dioscorea opposita Thunb.rhizome and has been recognized as medicinal and food plant in China.Polysaccharide is one of the main components of yam,which has various effects such as antioxidant,anti-aging,hypoglycemic,antitumor,and immune enhancement.Studies showed that extrusion and expansion could improve the extraction rate of polysaccharide and had some effects on their processing characteristics and biological activity.Polysaccharide is difficult to be directly digested and utilized by the body,and the mechanism of its biological function is inseparably related to digestion,fermentation and intestinal microbial.Therefore,in this study,the powder of yam was used as raw material to prepare the powder of yam by twin-screw extrusion technology.Then,unextruded yam polysaccharide(YP)and extruded yam polysaccharide(EYP)were extracted by hot water extraction.The basic compositions and structure,in vitro digestion and glycolysis of the two polysaccharides were studied,so as to provide some theoretical guidance for yam extrusion processing and utilization.The results were as follows:(1)The optimal extraction process of EYP was optimized by response surface experiment based on single-factor test using the polysaccharide yield as the index:extraction temperature 87℃,extraction time 3 h,solid-liquid ratio 1: 18,extraction for two times.Under these conditions,the yield of EYP was 6.82%.The yield of YP was3.18% using the above optimal process.(2)The contents of total sugar,protein and uronic acid of EYP and YP were determined by the phenol-sulfuric acid method,the coomassie bright blue method and the m-hydroxybiphenyl method.The contents of total sugar,protein and uronic acid of EYP were 75.61%,0.69% and 15.53%,respectively,while the contents of total sugar,protein and uronic acid of YP were 42.81%,3.16% and 10.94%,respectively.PMP precolumn derivatization HPLC showed that YP contained rhamnose,galacturonic acid,galactose,arabinose,mannose,glucuronic acid,glucose and xylose with molar percentages of 0.38: 3.83: 4.04: 1.32: 3.23: 0.34: 86.51: 0.35,respectively.The monosaccharide composition and molar percentages of EYP were: rhamnose:galacturonic acid: arabinose: galactose: mannose: glucuronic acid: glucose = 0.66: 8.25:1.32: 5.89: 1.27: 0.57: 82.03.The weight average molar mass of YP and EYP were determined by high performance liquid gel permeation chromatography(HPGPC).The Mw of YP was 9.11 kDa.EYP consisted of two fragments: the Mw of peak 1 was 8.94 kDa and that of peak 2 was 5.92 kDa.The infrared spectral of YP and EYP were basically similar.The surface of YP was relatively smooth,with irregular geometric shape and thicker particles.The surface of EYP was irregular scaly shape,sharp angles and flaky structure.(3)The digestive characteristics of YP and EYP in simulated gastric juice and intestinal juice were studied by in vitro simulated digestion models.The results indicated that the relative molecular weight,the contents of total sugar and reducing sugar of YP and EYP did not show significant changes during digestion,and no free monosaccharides were released during the digestion process.These results suggested that YP and EYP were not degraded by simulated gastric juice and small intestinal juice.(4)Human fecal flora was used as a model to study the in vitro fermentation behavior of YP and EYP.The results showed that the changes of pH,the contents of total sugar and reducing sugar of all groups decreased gradually with the increase of fermentation time.After 24 h of fermentation,the glycolysis solution pH of YP and EYP decreased from 8.52 and 8.66 at the beginning of fermentation to 5.70 and 5.03,respectively,which were lower than the blank group(9.06 and 7.48)at the same time.The contents of total sugar of YP and EYP groups decreased from 2.66 mg/mL and3.80 mg/mL at the beginning of fermentation to 0.43 mg/mL and 0.11 mg/mL,respectively,and the contents reducing sugar decreased from 0.54 mg/mL and 0.67mg/mL at the beginning of fermentation to 0.17 mg/mL and 0.13 mg/mL,respectively.Both YP and EYP were able to be metabolized by intestinal microorganisms to produce acetic acid,propionic acid and n-butyric acid.The yields of total short-chain fatty acids in EYP and YP groups reached the highest value at 24 h,which were 20.38 mM and15.32 mM,respectively,which were higher than that in the blank group(9.36 mM).16 s RNA high-throughput sequencing showed that both YP and EYP could regulate the composition of intestinal flora.At the phylum level,YP significantly upregulated the relative abundance of Firmicutes and Bacteroidetes,and EYP significantly increased the relative abundance of Firmicutes.At the genus level,YP significantly increased the relative abundance of Megamonas and Prevotella,while EYP significantly upregulated the relative abundance of Megamonas and Bifidobacterium.YP and EYP significantly down-regulated the relative abundance of Fusobacterium and Lachnoclostridium,suggesting intestinal probiotic function. |
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