Structure Characteristics,Functional Properties Of Okra Polysaccharide And Its Modification In Fermented Foods

Okra polysaccharide is a potential food thickener and emulsifying stabilizer,which could be widely used in food processing.This research investigated the molecular structure,antioxidant activities,emulsifying properties of okra polysaccharide?OP?,and conformational changes of complexes between lactoferrin?Lf?and OP before and after heat treatment.Moreover,the protection mechanism of OP on heat-induced structural changes of protein,the effect of OP addition on protein stability in the set yoghurt,antioxidant activity and anti-digestive ability in the bread as well as the turbidity and volatile compounds in the cloudy wheat beer,were investigated.The main research contents and results are as follows:?1?Effect of drying techniques on the fine structure and antioxidant activities of OPThis work investigated the effect of different techniques[freeze-drying?FD?,sun-drying?SD?,oven-drying?OD?and microwave-drying?MD?]on the molecular structure,functional properties and antioxidant activities of the extracted OP.The main composition of OP is galacturonic acid,accounting for about 62.67⁶8.77%of dry basis.The main neutral sugar is galactose,which takes about 53.01⁶4.25%of the total neutral sugars.Although the degree of methylesterification?39.5⁴3.6%?showed no significant difference among OP samples,the contents of galacturonic acid and protein,the number-average molecular weight and apparent viscosity in aqueous solution decreased as the drying temperature increased?FD>SD>OD>MD?.Drying okra at lower temperature resulted in OP with improved water holding capacity,emulsifying activity and emulsion stability,but with lower oil absorption capacity,DPPH and ABTS radical scavenging activity.Rheological results showed that OP is a pseudoplastic fluid.Its main chain was composed of homogalacturonan,xylogalacturonan and rhamnogalacturonan?RGI?.The difference among drying techniques of okra led to significant changes in the molecular structure of OP.The main chain of RGI region in MD-OP had the lowest degree of polymerization,the side chain of RGI region in SD-OP had the lowest degree of polymerization,and the side chain of RGI region in FD-OP had more branching,which was mainly composed of arabinan and galactan.?2?Conjugation between OP and lactoferrin?Lf?and its inhibition effect on thermal aggregation of Lf at neutral pHOP interacts with Lf through electrostatic attraction,which prevented heat-induced denaturation and aggregation of Lf molecule.The turbidity of Lf after heat was 1.618.When the concentration of OP increased from 0 to 0.0075%,the turbidity decreased to 0.113.0.1%Lf was charged positively at pH 7.0 and zeta-potential value was less than 2.5 mV.Lf was charged negatively after OP addition,and the absolute zeta-potential value increased with the increasing OP.The absolute zeta-potential value of Lf/OP solution after heat was significantly higher than the one before heat?OP?0.01%?.Correspondingly,the average size of Lf aggregates decreased from 470 nm to 180 nm when OP concentration increased from 0.005%to 0.0125%.The intrinsic fluorescence peak wavelength for Lf after heat was red-shifted when the OP concentration was less than 0.005%.However,a blue shift was noticed with further increase of OP.The intrinsic fluorescence intensity of Lf/OP solution after heat decreased significantly,but external fluorescence intensity increased significantly.The internal structure of Lf was significantly influenced by the existence of OP.At low concentrations?<0.0075%?,OP resulted in the loss of alpha-helix structure of Lf.While at high concentrations?>0.0075%?,the secondary and tertiary structure of Lf can be protected after heating.?3?Mechanism between OP/casein,and the modification of OP on set yoghurtOP,apple pectin?AP?and konjac glucomannan?KGM?enhanced the water-holding capacity,firmness and rupture distance of set yoghurt.The density and water-holding capacity of yoghurt with 0.06%OP increased from 1.15 g/mL to 1.23 g/mL,and from 37.39%to43.73%,respectively.The firmness and rupture distance of yoghurt with 0.08%OP increased from 9.01 g to 14.86 g,and from 9.01 g to 14.86 g,respectively.The yoghurt with OP achieved the closest transverse relaxation time?T₂?to the control.The free water area(A₂₃)decreased from 1592 for control to 1414 for yoghurt with 0.08%OP addition.All samples exhibited shear-thinning behavior,apparent viscosity increased with increasing concentrations of OP and KGM,with peak values at 0.06%OP and 0.015%KGM.Elastic modulus?G'?and viscous modulus?G''?of yoghurt with OP,AP and KGM were higher than the control,and G'was always higher than G''.Microscopy results showed that addition of OP and AP reduced the porous structure of gel and promoted formation of larger protein clusters,which eventually led to more compact protein network.?4?Effect of okra on antioxidant activities and in vitro starch digestion of wheat breadOP and phenolics are the main functional components of okra,the contents and distribution vary in different tissues.The total dietary fiber?38.61⁴1.75%?and soluble dietary fiber?10.18¹2.55%?showed no significant difference among fine okra pod flour?F-OP,<150?m?,coarse okra pod flour?C-OP,150²70?m?,fine okra seed flour?F-OS,<150?m?,and coarse okra seed flour?C-OS,150²70?m?.Pectin in okra pod ranged from8.42%to 8.89%,which was significantly higher than okra seed?1.94³.47%?.Pectin of F-OS was remarkably higher than C-OS.Okra addition could greatly enhance the antioxidant activities of wheat bread,and breads with C-OS and F-OS showed significantly higher antioxidant activities than breads with C-OP and F-OP.Addition of 5%C-OS significantly increased the amount of extractable?EPP?and non-extractable hydrolyzable phenolics?HPP?of wheat bread,attaining up to 210.8 and 2944.8 mg/100g,respectively.Interestingly,breads made with F-OS and F-OP exhibited a slower digestion rate and extension upon 50 min of digestion,indicating a time-dependence hypoglycemic effect.?5?Effects of okra on the proteins stability and volatile compounds of cloudy wheat beerThe chromaticity of cloudy wheat beer with okra increased from 5.7 EBC to6.0⁶.2 EBC.The turbidity increased from 1.7 EBC to 12.8¹3.8 EBC.The viscosity increased from 1.5 mPa·s to 1.6 mPa·s.The pectin content increased from 15.36 mg/L to26.69²7.90 mg/L,improving yeasts suspending ability.Meanwhile,okra addition significantly affected aroma characteristics of cloudy wheat beer.The higher alcohol such as2-butanol,isobutanol,isoamyl alcohol,1,2-propanediol and phenylethyl alcohol significantly decreased,while isoamyl acetate and octanoic acid significantly increased.The foam in cloudy wheat beer with okra was rich and delicate with improved foam retention.Grass-like odor was outstanding and typically good.Therefore,okra and OP have a broad application prospect in beer industry.