Effects Of Different Drying Methods On The Volatile Flavor,Polysaccharides Of Okra And The Basic Structure And Rheological Characteristics Of Polysaccharides From The Pericarp

Okra(Abelmoschus esculentus(Linn.)Moench)is an annual herbaceous plant.Okra has a unique flavor and rich in nutrition.It contains a variety of chemical components,especially polysaccharides.In this study,okra pods were treated with different drying methods to evaluate the effects of drying methods on its flavor substances,as well as the chemical properties of polysaccharide.Then,the pericarp which was the main part of okra was applied for the polysaccharides,who were extracted by hot water,boiling water and acidic solution in series.After that,their structural characteristics were investigated and compared.Moreover,rheological property of the main polysaccharide fraction was characterized.Main contents and conclusions are as follow.1?Fresh okra fruits harvested at different growth stages were subjected to hot-air drying after fixation(SHD),hot-air drying(HD),natural drying(ND)and freeze drying(FD),separately.The effect of these drying methods on the contents of ash,moisture,amino acid and volatile substances in dried products were investigated.The results showed that the moisture content of okra was relatively low after SHD and HD.However,FD had less effect on flavonoids as compared with other drying methods.Dimethyl sulfide was the characteristic volatile substance of fresh okra.It was found that the four drying treatments could contribute to the increase number of volatile substances to different extents,and also result in changes in the relative contents of volatile substances including dimethyl sulfide.The most abundant volatile substances were alcohols and aldehydes after SHD,while the relative contents of alcohols were higher after ND.The relative content of aldehydes was higher after FD and HD.These findings suggested that the characteristic volatile substances of fresh okra fruit were retained,and at the same time new volatile substances appeared after all drying treatments.Combinations of these substances in different proportion imparted distinct aromas to okra fruit by different drying methods.2?Polysaccharides were extracted from okra with different drying methods at 60? by water extraction,followed by alcohol precipitation method.Its structure characteristics and rheological properties were investigated.The results showed the four polysaccharides were mainly composed of type I rhamnogalacturonan(RG-?)pectin,but linear degree was a little different.The infrared spectra of the four polysaccharides were similar,while solid morphology and molecular weight distribution was a little different.In addition,apparent viscosity and gelling properties were measured.The four polysaccharides exhibited weak gel properties.3?Okra pods(pericarp)obtained after removing seeds were applied for polysaccharide extraction.Pericarp was firstly extracted with water at 60? followed at 100?,and then extracted with acid solution(pH 4.0,60?).The gained polysaccharides were named as OPW-60,OPW-100 and OPA-60,successively.Then,structural characteristics and antioxidant activities investigations were performed on the these polysaccharides.The results showed that monosaccharide composition of three polysaccharides was different.OPW-60 was mainly type ? rhamnogalacturonan(RG-I)pectin,while OPW-100 and OPA-60 consisted of RG-? and homogalacturoan(HG)pectins.1H NMR analysis confirmed the existence of rhamnose and acetyl group among them.The structure diversity resulted in the difference of viscosity,molecular weight,surface morphology and antioxidant activities.OPW-60 exhibited the highest apparent viscosity,which might be attributed to the differences in molecular weight distribution and monosaccharide composition.OPW-60 was most effective in scavenging 2,2-Diphenyl-1-picrylhydrazyl radicals and chelating ferrous ion ability,while OPW-100 exhibited the highest hydroxyl radical scavenging activity.4?As the main polysaccharide of okra pericarp,OPW-60 was applied for rheological property characterization.Effects of different mass fractions,temperature changes,salt ions and freeze-thaw on the steady and dynamic rheological properties of OPW-60 were investigated,as well as texture characteristics.The results showed that OPW-60 had shear thinning behaviour in the range of 0.25%?2.0%,which was a pseudoplastic fluid and behaved like a weak gel.The viscosity of okra polysaccharide OPW-60 was not sensitive to temperature changes,which was similar during the freeze-thaw process.The addition of salt ions(Na+,Ca2+)could reduce the apparent viscosity of OPW-60,and 0.001 M Na+ increased the viscosity slightly.The hardness and gel strength of OPW-60 was concentration dependent.