| The dietary fiber(DF)was extracted from millet bran by enzymatic method in this research,and the soluble dietary fiber(SDF)was extracted from DF by complex enzyme(cellulose-xylanase)modification method to optimize the process,the remaining residue is insoluble dietary fiber(IDF).Then the structure and physical-chemical properties of the millet bran and dietary fibers were studied.And further the antioxidant and adsorption capacities of glucose and cholesterol of the samples were explored.Finally,the DF was mixed into the dough to measure the farinographical properties and tensile properties and the millet flour steamed bread enriched with dietary fiber was developed.The study concluded that:Enzymatic preparation and modification of millet bran dietary fiber: the DF was extracted from millet bran by thermostable ?-amylase,glucoamylase and neutral protease,which the purity of DF was 89.75%.Then the SDF was extracted from DF by complex enzyme(cellulose-xylanase)modification method,and single factor experiment and response surface design were executed to optimize the preparation technology of the SDF.The optimum extraction conditions of SDF were as follows: the total amount of complex enzyme was 1.73%,the ratio of cellulose and xylanase was 1:1.74,the hydrolysis temperature was 50? and the hydrolysis time was 1.44 h.And the SDF and IDF yield was 12.33% and 85.12% under this condition,respectively.The structure and physical-chemical properties of the millet bran,DF,SDF and IDF were studied.The monosaccharide composition analysis showed that the content of monosaccharides among the dietary fibers were significantly difference(P<0.05).The content of xylose was the highest in the DF,but the content of glucose was higher than that of xylose in SDF and IDF.According to the scanning electron microscope,it was found that laminated structure of DF was showed clearly,and spherical starch granules were less adhering to it.The SDF was characterized by irregular mass and mutual adhesion,then the structure of IDF was loose and porous.FT-IR analysis displayed that all of the three samples had characteristic absorption peaks,such as O—H,C—H,C=O and C—O,and the SDF characteristic absorption peaks of polysaccharides were relatively stronger.The results of XRD showed that the crystallinity of dietary fibers were higher than that of the millet bran.The XRD patterns of the millet bran,DF and IDF were similar in shape,and the crystal type was cellulose type I,while the SDF pattern appeared different,there was a number of strong diffraction peaks between 25° and 50°.The weight average molecular weight of SDF was 3.034×107 g/moL.The solution of SDF was pseudo-plastic fluid,and the elastic behavior was stronger than that of viscous behavior.The water holding capacity,oil holding capacity,swelling property and solubility of SDF were significantly difference with other samples(P<0.05),but the cation exchange capacity was not detected.The contents of phenolic and flavonoid of DF and millet bran were analyzed and the antioxidant activity was researched.The results showed there were significantly differences in the contents of polyphenols and flavonoids among the millet bran and several dietary fibers(P <0.05).Total polyphenols and total flavonoids were arranged in descending order: millet bran> DF> IDF> SDF.The types of polyphenols in DF included bound,free and covalent bonding,and the content of bound phenolic was higher than soluble phenolic acid in DF,especially the content of protocatechuic acid in acid hydrolysis bound phenolic(ABPA)and the content of coumaric acid in alkaline hydrolysis bound phenolic(BBPA)was much higher than other phenolic acids.The DPPH· scavenging ability and ferric reducing ability of millet bran were significantly higher than those of other dietary fiber(P<0.05).The OH-· scavenging ability,O2-· scavenging ability and Fe2+ chelating ability of SDF were significantly higher than those of other samples(P<0.05).As for ABTS+· scavenging ability,millet bran and DF were better.The glucose and cholesterol adsorption capacity of millet bran and dietary fibers were compared,the result showed that glucose dialysis retardation index of DF and SDF were higher,but there was no significant difference among the samples on the glucose adsorption capacity(P>0.05).The inhibition capacity against ?-amylase,cholesterol adsorption capacity and sodium cholate adsorption capacity of millet bran were lower than that of the three dietary fibers,and the effect of SDF was the best.25% steamed millet flour and different contents of DF(2%,4%,6%,8% and 10%)were added to wheat flour,then the effects of DF on the dough rheology and quality and digestibility of steamed bread were studied.The result showed that 25% steamed millet flour and increasing content of DF had significant difference(P<0.05)on dough farinographical properties and tensile properties.When compared with wheat flour steamed bread,it was found that the color of steamed bread with millet flour and DF was dark and yellow,and the ?E* value was increased gradually.With the content of DF added,color of steamed bread was deepened and the hardness was increased significantly but the springiness was decreased significantly(P<0.05),which indicating the sensory acceptability of the steamed bread was gradually decreased.In vitro digestion results showed that with the increase of DF content,the RDS and SDS contents of millet flour steamed bread were decreased gradually,conversely,the RS content increased.Meanwhile,the starch hydrolysis rate,HI and GI were also declined significantly(P<0.05).Moreover,the protein digestibility decreased gradually,so the DF content should not be added too much.Based on the overall evaluation results,the sensory property of 2% DF sample was good and was evaluated as the food with medium glycemic index,which was beneficial to control the blood glucose level.In addition,its abundant nutrition and good functional properties could meet the requirements of healthy diet. |
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