| The development of microencapsulation technology make microcapsule technologymore widely used in the food industry, including the following aspects: powder spice flavors,powdered fats; microencapsulation of food additives; immobilized enzymes or cells. Wallmaterial as the microcapsule shell, plays a very important role on the the microcapsuleproduct features and property. The rheological properties of the wall material directly affectsthe properties of microencapsulation, the rheological properties of the wall material wasdetermined bye the polymer structure. It has great significance to investigate the molecularstructure of the basic theory by the rheological properties of, and the mechanism of effect ofthe wall material rheological properties to physical and chemical properties of themicrocapsules. In this experiment, modified starch, gum arabic, sodium carboxymethylcellulose, soy protein isolate(SPI), gelatin were selected as the model wall material, to explorethe impact of the molecular structure and rheological properties of wall materialimpact on thephysical and chemical properties of the microcapsules.High performance liquid chromatography(HPLC) and IR spectrometer were used tocharacterize molecular weight distribution and molecular groups of the wall material, theresults showed that: Mw (weight average molecular weight) were, modified starch3.069×105g/mol, gum arabic2.493×105g/mol, carboxymethyl cellulose1.351×105g/mol, soyprotein isolate6.368×103g/mol, gelatin5.526×103g/mol, the Mw of polysaccharide wallmaterial significantly larger than the Mw of protein wall material. Mw/Mn (molecular weightdispersion coefficient) wre: modified starch2.780, gum arabic1.73363,gelatin1.32391,carboxymethyl cellulose1.28241,soy protein isolate1.04206. The greater thedispersion coefficient, the wider the molecular weight distribution. The characteristics groupsof the wall material were analysised by IR spectra, amide with proteins wall material wasanalysised by peakfit software, the secondary structure of SPI and gelatin were compared, theresults show that: the secondary structure of the two proteins are quite different, there aremore ordered structure in the secondary structure of the SPI molecules, and this may meanthat the SPI molecules having a relatively rigid structure.The emulsion stability of the five kinds of wall material were detected by the emulsionstability index and polarizing microscope, the experimental results show that the ESI (emulsion stability index) were:gum arabic3898.33, modified starch2706.25, gelatin2432.22,SPI23.72,carboxymethyl cellulosic0. The emulsion droplet size distribution observed bypolarized light microscopy wre: gum arabic25.4-39.0pix, modified starch25.5-31.1pix,gelatin21-40.2pix, SPI41.0-136.1pix, carboxymethyl cellulose19.3-156.1pix. The smallerthe average particle diameter of the droplets, the more narrow distribution range, the better thestability. Therefore polarizing microscope observation is consistent with the experimentalresults of emulsion stability index method.The wall material film was maded, and fuzzy comprehensive evaluation method for thewall material film elongation at break, tensile strength, water vapor coefficient, porosity andpermeability coefficient, cumulative weighting membership value were: modified starch film0.658, carboxymethyl cellulose film0.5847, gum arabic film0.5047, soy protein isolate film0.4818, gelatin film0.4008.The apparent viscosity, dynamic modulus and creep compliance of the solution of thewall material were measured using a rotational rheometer. The results show that: therheological curves of the apparent viscosity-shear stress and shear stress-shear rate curveswere in compliance with the characteristic curve of a pseudoplastic fluid, theHerschel-Bulkley equation for the rheological curve fitting results current characteristic indexn <1means that the shear thinning, the solution being measured as a pseudoplastic fluid,showing a shear-thinning phenomenon. With the temperature rise of the solution the apparentviscosity were gradually decreased. The experimental results of the dynamic modulus showedthat: the elastic modulus G ’and viscous modulus G’’ are increasing with increasing oscillationfrequency, where the SPI G’> G’’, while other wall material G’’> G ’. The creep recovery studyresults showed that the wall material creep compliance J (t) were: modified starch>gumarabic>gelatin>carboxymethyl cellulose>SPI.In order to determine the moisture content, yield, efficiency, and the apparentmorphology, particle size distribution and bulk density and other physical and chemicalproperties of microcapsules, the microcapsules were maded by spray drying. Experimentalresults showed that: the physicochemical properties of the microcapsules had a big differencebetween different wall materials, relatively low apparent viscosity of the wall material in thespray drying may be formulated into a relatively high solids content of the feed solution, themoisture content of prepared microcapsules will be smaller, the volume average particlediameter will be lager, while the bulk density will be smaller. The yield and efficiency of the microcapsules were: gum arabic95.12%and90.77%, modified starch94.06%and89.64%,gelatin92.15%and82.27%, SPI86.09%and78.50%, carboxymethyl cellulose77.60%and75.31%.The yield and efficiency of the microcapsules had a large revelance with emulsionstability of the wall material, the higher emulsion stability the higher the yield and efficiencyof microcapsules. A great difference in the morphology of the microcapsules prepared bydifferent wall material: the microcapsules with modified starches as wall material was aspherical and smooth surface; While the microcapsules with gum arabic, gelatin and SPI aswall material had depressions on surface; carboxymethyl cellulose as wall material hadwrinkled surface. The one hand, is caused by the spray drying itself; other hand, associatedwith the viscosity and the molecular structure of the wall material. |
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