Combined Microwave And Enzymatic Method Of Grapefruit Peel Dietary Fiber To Improve The Release Of Bound Polyphenols And Its Product Development

With the gradual deepening of the research on grapefruit,the active substances in grapefruit peel have attracted more and more attention.However,there is still a big gap in the research on the active substances in insoluble dietary fiber(IDF).Therefore,in this study,the IDF would be used as the research object,and the bound polyphenol(BP)was released through the method of modified by the microwave combined enzyme,and the soluble dietary fiber.Both of them were explored their activity and practical application value.Firstly,the conditions of microwave modified grapefruit peel insoluble dietary fiber(GP-IDF)were optimized by response surface methodology to improve the yield of BP.The optimal conditions were microwave power 600 W,temperature85 ℃ and treatment time 37 min.Then,the structure of GP-IDF modified by microwave,enzyme and microwave enzyme were characterized to preliminarily explain the mechanism of releasing bound polyphenols.According to the data of scanning electron microscope,GP-IDF became abnormally loose due to microwave combined enzyme treatment.From the X-ray diffraction pattern,the lignin,hemicellulose in GP-IDF was removed by microwave enzyme treatment,which was consistent with the results obtained by Fourier transform infrared spectroscopy.Then,the bound phenols(M-BP,E-BP and ME-BP)released by the three treatment methods were determined qualitatively and quantitatively.27 bound phenols contained in GP-IDF were identified by UPLC-ESI-QTOF-MS/MS.According to LC-ESI-Qq Q-MS/MS,the higher content of bound phenols in grapefruit peel insoluble dietary fiber was respectively gallic acid(42.50 ± 3.21 μg/g DW),ferulic acid(18.46 ± 1.65 μg/g DW)and protocatechuic acid(6.16 ± 0.72 μg/g DW).Among the three bound polyphenols,ME-BP showed better antioxidant capacity in vitro and showed good inhibition of α-glucosidase.The results demonstrated that combined microwave and enzyme modification was an effective method to extract the BP from GP-IDF.Appropriate modification methods not only can increase the proportion of soluble dietary fiber(SDF),but improve the structural and functional properties of SDF,and make it have excellent processing properties.In the process of BP releasing from IDF,combined microwave and enzyme method used in this study would promote the conversion of IDF to SDF.SDF was extracted by ethanol precipitation and recorded as GP-IDF-SDF.Then its structure was characterized.It was found that its micro surface structure was flat and smooth,with some filaments.Also,XRD and TGA data displayed that it was a typical type I cellulose crystal and had good stability below 200 ℃.GP-IDF-SDF had strong water holding capacity(13.43 ±1.19 g/g),oil holding capacity(22.10 ± 0.85 g/g)and glucose adsorption capacity(14.49 ± 0.068 mg/g).In addition,galacturonic acid,arabinose and glucuronic acid were the main monosaccharide components,indicating that pectin may be the main component of GP-IDF-SDF and the average molecular weight was from 0.15 to 167 k Da.Finally,the effects of GP-IDF-SDF on color,rheology,texture and sensory properties of blueberry jam were compared with commercial low methoxy pectin.The results showed that the jam(GPSJ-1)with GP-IDF-SDF had lighter color and less red and yellowish.The addition of GP-IDF-SDF also had little effect on the viscosity and gel strength of jam,but in practical production,it could improve the stability of jam and obtain the soft jam.According to the sensory evaluation analysis,adding GP-IDF-SDF or pectin might improve the color,texture and spreading of jam,which was gladly accepted by people.As a result,GP-IDF-SDF might be expected to play an active role in the development of new food ingredients.