| Sweet potato?Ipomoea batatas Linn.Lamarck?is an extremely important tuber crop in Zhejiang Province,sweet potato processing industry had integrated a relatively complete supply chain.However there is still 20%-30%by-products of sweet potato during processing is barely used.There are a lot of fiber and flavonoid polyphenols in sweet potato peels,Therefore,it has the potential to be ideal material for extracting functional dietary fiber.In this study,the dietary fiber was extracted from sweet potato peels by ultrasonic assisted extraction?UE?,subcritical water extraction?SWE?and microwave assisted extraction?ME?,the unassisted hot water extraction method was used as a control to explore the application of three physical extraction techniques in the extraction of sweet potato peels dietary fiber?SPPDF?,the effects of soluble dietary fiber?SDF?extraction yield and total dietary fiber?TDF?extraction yield,physicochemical,antioxidant,polysaccharide composition,morphological structure and in vitro fermentation capacity were used as indicators to analyze the advantages and disadvantages of different physical extraction methods in SPPDF extraction.The results were as follows:?1?The optimal SDF extraction conditions obtained by single factor and orthogonal optimization experiments are:HWE solid-liquid ratio 1:20,extraction temperature 60?,extraction time 3 hours,SDF yield 1.20%,TDF yield 21.72%,SDF/TDF ratio 5.52%;UE solid-liquid ratio 1:30,extraction temperature 70?,extraction power 400 W,extraction time 30 min,the SDF yield 8.21%,TDF yield22.23%,SDF/TDF ratio 36.93%,and extraction time was the most important factor to SDF yield;SWE solid-liquid ratio 1:30,extraction temperature 120?,extraction time30 min,SDF yield 10.43%,TDF yield was 24.68%,SDF/TDF ratio 42.26%,and solid-liquid ratio was the most important factor to SDF yield;ME solid-liquid ratio 1:30,extraction power 500 W,extraction time 6 min,the yield of SDF was 7.74%,the yield of TDF was 21.84%,and the ratio of SDF/TDF was 35.44%.The most important factor affecting the yield of SDF was the solid-liquid ratio.?2?The water holding capacity?WHC?of SWE-SDF was 4.83 g·g-1,which was significantly superior to UE-SDF and HWE-SDF?P<0.05?,there was no significant difference between SWE-SDF and ME-SDF.the highest WHC of ME-TDF was 7.83g·g-1,followed by SWE-TDF;there was no significant difference in swelling capacity?SC?among the SDF extracted by different physical methods?P>0.05?,and the highest SC of SWE-TDF was 7.89 m L·g-1,the highest SDF of oil holding capacity?OHC?was UE-SDF and the highest TDF was SWE-TDF,the viscosity peak?VP?of SDF extracted by different physical methods has no significant difference?P>0.05?.The VP of the TDF extracted by different physical methods was significantly higher than the control group?P<0.05?.The glucose absorption capacity?GAC?of SDF was significantly higher than the control group?P<0.05?,the GAC of SWE-SDF was the strongest,and there was no significant difference among the TDF extracted by different physical methods?P>0.05?,SWE-SDF and SWE-TDF have the highest HRSR and DPPH radicals,the HRSR of SDF is higher than TDF,and the antioxidant capacity of sweet potato peels dietary fiber has the same change trend with its total phenol content.?3?Transmission electron microscopy showed that SWE had the most significant effect on the morphology and structure of TDF,and its specific surface area increased,showing a more complex spatial structure;infrared spectrum analysis demonstrated that the dietary fiber extracted by three physical methods still had the function group of polysaccharide and the polygalacturic acid structure of pectin,the physical method improved the total pectin content of dietary fiber.XRD results indicates that SWE and ME have changed the crystalline form of TDF from cellulose I to cellulose II,and SWE-TDF has the lowest crystallinity,that is to say,SWE has the most serious damage to the crystallinity.?4?The composition of SDF polysaccharide was analyzed by DEAE Sepharose FF column.It was found that SWE-SDF accounted for the highest proportion of neutral polysaccharide in SDF extracted by three physical methods,reaching 58.51%,and acid polysaccharide,HWE-SDF accounts for the highest proportion of washed parts with 0.1,0.2,0.3 and 0.5 mol·L-1Na Cl,reaching 8.48%,19.52%,28.44%and 2.32%respectively.SWE-SDF accounts for the highest proportion of washed parts with 0.4 mol·L-1Na Cl,reaching up to 10.99%,and UE-SDF accounts for the highest proportion of washed parts with 0.6 mol·L-1Na Cl,reaching 38.74%.According to the results of transmission electron microscopy and X-ray diffraction,the proportion of neutral polysaccharide in SDF is related to the degree of destruction of dietary fiber.?5?The results of in vitro fermentation showed that the gas production of UE-SDF was the largest,while the gas production of SWE-SDF and ME-SDF was the lowest.Compared with the commercial INU and FOS,the SDF extracted by physical method did not cause obvious flatulence;the SCFA content was increased most significantly by ME-SDF,and the propionic acid,butyric acid and total acid content were increased most significantly by UE-SDF;the SWE-SDF content was increased most significantly by dispersion analysis,the individual difference in quantity is the smallest,and the individual difference in ME-SDF in increasing propionic acid content is the smallest;the SDF extracted by physical method is better than HWE-SDF in reducing p H value and ammonia content in fermentation environment,the most significant one is UE-SDF and ME-SDF,and the most significant one is ME-SDF;the degradation rate of UE-SDF is the highest,reducing sugar/total sugar is the highest,and the physical extraction method improves the degradation rate of SDF,and also the proportion of raw sugar was consistent with the degradation rate.Conclusion:the GAC,total phenol,HRSR,DPPH-RSR,neutral polysaccharide and 0.4 mol·L-1Na Cl acid polysaccharide content of SWE-SDF are the highest,and the individual difference of promoting acetic acid content in vitro fermentation is the smallest;the SC and OHC of SWE-TDF are the highest,and the crystallinity is the lowest,so SWE has the most serious damage to crystallinity.The content of 0.6 mol·L-1Na Cl acid polysaccharide in UE-SDF was the highest,and it increased the gas production,propionic acid,butyric acid,total acid content and the relative content of reducing sugar in vitro,at the same time,the degradation rate was the highest.ME-SDF increased the acetic acid content most significantly,the p H value and ammonia content were the lowest,and ME-TDF had the highest WHC.SWE has the greatest advantage in improving the physical and chemical properties of sweet potato peels dietary fiber;UE has the greatest advantage in improving the in vitro fermentation of sweet potato peels dietary fiber. |
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