Effects Of Four Modification Methods On The Physical And Chemical Properties And Functional Activity Of Millet Bran Dietary Fiber

Millet bran is a by-product of millet manufacture,which is rich in dietary fiber.In recent years,studies have shown that millet bran dietary fiber has a certain water holding capacity,swelling power,and at the same time has strong adsorption to sodium cholate,nitrite and has potential development and utilization value.However,the content of soluble fiber of millet bran dietary fiber is very low（<4%）,resulting in low physical and chemical properties and functional activity,which limits its application in the food industry.In response to this problem,this article mainly studied the chemical composition,physicochemical properties,functional activity and structure of the four modifications of hydroxypropylation,crosslinking,acetylation and cellulase hydrolysis on the chemical composition,physical and chemical properties,functional activity and structure of millet bran dietary fiber（MBDF）.The aim of focus on the analysis of its effect on the adsorption activity of millet bran dietary fiber,in vitro hypoglycemic activity and antioxidant properties.The contents and results of this article are as follows:（1）Effects of different modifications on the chemical composition and structural characterization of millet bran dietary fiber.The results showed that four modification methods of hydroxypropylation,cross-linking,acetylation and cellulase hydrolysis can significantly increase the soluble dietary fiber content of millet bran dietary fiber（p<0.05）.Among them,acetylation and hydroxypropylation respectively increased the content of soluble dietary fiber in MBDF by 11 times and 5 times.Acetylation,hydroxypropylation and cellulase hydrolysis all significantly reduced the content of fat,ash and protein in MBDF（p<0.05）.Chromaticity analysis showed that the b value（representing yellow）of acetylated millet bran dietary fiber（MBDF-A）and cross-linked millet bran dietary fiber（MBDF-C）was significantly larger than that of MBDF（p<0.05）,indicating that MBDF had browned during the acetylation process,reducing its brightness.However,there is no significant color difference between hydroxypropylated millet bran dietary fiber（MBDF-H）and enzyme hydrolyzed millet bran dietary fiber（MBDF-E）and MBDF.It shows that the two modification methods have no significant effect on the color of MBDF（p>0.05）.Scanning electron microscopy analysis shows that the microscopic surface structure of MBDF is relatively smooth,while the microscopic surface structures of MBDF-H,MBDF-A and MBDF-E are honeycomb,uneven or fragmented,indicating that these modification methods have significantly damaged surface structure damage.On the contrary,the surface microstructure of MBDF-C is more delicate than that of MBDF.It may be that cross-linking improves the links between MBDF cellulose chains and makes the structure more compact.The results of Fourier Infrared Spectroscopy（FT-IR）show that the infrared absorption spectrum of millet bran dietary fiber before and after the four modification methods is very similar,with representative absorption peaks of polysaccharides,but at 3400、1650、2970、1400 cm^-1near,absorption peak of the wavelength has red-shifted or blue-shifted,and the absorption intensity has also changed,indicating that the hydroxypropyl,acetyl and phosphoric acid groups have been successfully linked to the molecular chain of MDBF.At3420、890 cm^-1near,the shift of the maximum absorption wavelength indicates that the cellulose chain of MBDF is cleaved by cellulase.（2）The effect of different modification methods on the physical,chemical and functional properties of millet bran dietary fiber.The section mainly explored the water retention,oil retention,swelling power of different modifications of millet bran dietary fiber,as well as the adsorption capacity of sodium cholate,copper ions,and nitrite.The results have showed that MBDF-H can significantly improve the water retention,oil retention and swelling power of MBDF（p<0.05）.MBDF-C can significantly improve the oil retention and swelling power of millet bran dietary fiber（p<0.05）.MBDF-A can significantly improve its water and oil retention（p<0.05）.The reason is that hydroxypropylation and acetylation can introduce the two very polar groups of hydroxypropyl and acetyl into the cellulose molecules of MBDF,which can significantly increase the polarity of MBDF and increase its affinity for water molecules.Hence,the water holding capacity and swelling power will be improved.The cross-linking modification increases the interaction between the cellulose chains due to the introduction of the phosphoric acid group,and promotes the formation of the intermolecular network structure,thereby increasing the oil retention and swelling properties.At the same time,the adsorption capacity of several modified millet bran dietary fibers to sodium cholate in a simulated gastrointestinal environment was studied.The results showed that compared with MBDF,MBDF-H,MBDF-C,and MBDF-A have higher sodium cholate adsorption capacity,while the adsorption capacity of MBDF-E for sodium cholate is not significantly different from that of MBDF.At the same time,the sodium cholate adsorption of these modified millet bran dietary fibers is better than p H 7.0 at p H 2.0.The adsorption test of nitrite ion showed that the adsorption capacity of MBDF,MBDF-H,MBDF-C,MBDF-A and MBDF-E at p H 2.0 was significantly higher than their adsorption capacity at p H 7.0.At p H7.0,MBDF-H and MBDF-E had higher nitrite adsorption than MBDF,indicating that hydroxypropylation and cellulase treatment increased the nitrite adsorption capacity of MBDF.In addition,the copper ion adsorption test showed that the adsorption capacity of MBDF-H and MBDF-C was significantly greater than that of MBDF（p<0.05）,illustrating that both hydroxypropylation and crosslinking modification can enhance the adsorption capacity of MBDF for copper ions.The reason may be that the hydroxypropylation increases the polarity of the MBDF cellulose chain,while the cross-linking reaction enhances the interaction between the molecules in the MBDF and promotes the formation of the network structure,which is beneficial to the adsorption of copper ions.（3）Effects of different modification methods on the functional properties of millet bran dietary fiber.Focus on the analysis of the effects of four different modification methods on MBDF emulsification,viscosity and cation exchange capacity.The results have showed that the emulsification activity index of MBDF-H,MBDF-C and MBDF-E was significantly higher than that of MBDF（p<0.05）,indicating that hydroxypropylation,cross-linking and enzymatic hydrolysis can significantly improve the emulsification ability of MBDF.Among them,the emulsifying ability of MBDF-H is 2.67 times that of MBDF.The main reason is that hydroxypropylation can significantly increase the SDF content of MBDF.However,acetylation modification significantly reduced the emulsifying ability of MBDF（p<0.05）.The emulsification stability experiment showed that the four modification methods had no significant effect on the emulsification stability of MBDF（p>0.05）.Viscosity experiments have showed that,compared with MBDF,the hydroxypropylated millet bran dietary fiber has a higher viscosity,while the other three modified millet bran dietary fibers have a lower viscosity.The viscosity of MBDF-H,MBDF-C,MBDF-A,MBDF-E is positively correlated with ion concentration.The exchange capacity of cation showed that the cation exchange capacity of MBDF-H and MBDF-C was significantly higher than that of MBDF（p<0.05）,while the capacity of cation exchange of MBDF-E was significantly lower than that of MBDF（p<0.05）.This indicates that hydroxypropylation and cross-linking modification can enhance the cation exchange capacity of MBDF,but cellulase hydrolysis treatment can decrease the cation exchange capacity of MBDF.（4）Effect of different modification methods on the antioxidant activity of MBDF.It was used was used to extract polyphenols by 80%methanol solution from MBDF and four modified millet bran dietary fibers,and then the free radical scavenging ability,reducing power,metal ion chelating ability and the effect of liposomes of these polyphenols were studied.Inhibition of oxidation,systematic evaluation of the effects of four modification methods on the antioxidant capacity of MBDF.The research results have showed that the four modification methods all reduced the polyphenol content in MBDF.At the same time,both changed and unchanged MBDF showed higher hydroxyl radical scavenging ability and superoxide radical scavenging ability.Compared with MBDF,the ABTS⁺scavenging capacity（total antioxidant capacity）of MBDF after modification treatment was significantly reduced（p<0.05）.On the contrary,MBDF-H,MBDF-C,MBDF-A,MBDF-E have significantly higher ferrous ion complexing ability than MBDF（p<0.05）.Among them,MBDF-A showed the highest complex rate（74.26%）,followed by MBDF-H（64.37%）.Among the four modification methods,only the hydroxypropylation modification significantly improved the reducing power of MBDF（p<0.05）.It may be that the hydroxypropylation reduces the polyphenols of MBDF,but it can produces new resistance substances with stronger oxidizing power.In addition,MBDF and differently modified MBDF showed a certain ability to inhibit the auto-oxidation of linoleic acid.Among them,MBDF-H,MBDF-C,and MBDF-A have higher inhibitory power than propyl gallate and MBDF.As these results show that hydroxypropylation,cross-linking and acetylation can significantly enhance the partial antioxidant capacity of MBDF,and it has the potential to be used in the field of food preservation and storage.（5）The effect of different modification methods on the hypoglycemic activity of MBDF（millet bran dietary fiber）.The section mainly studied MBDF and four kinds of modified MBDF’s ability to adsorb glucose,glucose dialysis retardation index（GDRI）,inhibitory ability toα-amylase,and the relationship between various millet bran dietary fiber andα-amylase the role of time.The results show that MBDF and its four modified products have greater ability to inhibit glucose dialysis.Among them,cellulase hydrolysis significantly increased the GDRI of MBDF,and cross-linking modification reduced the GDRI of MBDF.At the same time,acetylation,cross-linking and hydroxypropylation all significantly enhanced the adsorption capacity of MBDF to glucose,which may be related to its higher SDF content and porous microstructure.On the contrary,cellulase hydrolysis reduces the adsorption capacity of MBDF to glucose.In addition,MBDF and the four modified MBDF all showed a certain ability to inhibitα-amylase activity.Among them,cross-linking modification improves theα-amylase inhibitory ability of MBDF,and acetylation,cellulase hydrolysis and hydroxypropylation reduce theα-amylase inhibitory ability of MBDF.Fluorescence spectra showed that after combining with MBDF,the fluorescence absorption intensity and maximum absorption wavelength ofα-amylase changed.Among them,MBDF-H,MBDF-A,and MBDF-E have a significant quenching effect on the fluorescence absorption ofα-amylase.At the same time,afterα-amylase is combined with MBDF,its maximum absorption wavelength also has a significant shift.These results indicate that MBDF and other four modified of MBDF can inhibit the activity ofα-amylase by affecting the structure ofα-amylase and changing the micropolarity around its binding group.Therefore,four modification methods such as hydroxypropylation can improve the in vitro hypoglycemic activity of MBDF from different angles,and improve its application in health foods,especially hypoglycemic products.