The Research On The Blending Modification Of The Soy Protein Isolate Edible Film

In recent years,there has been a great interest to utilize renewable biomass in the manufacture of good-quality,cost-competitive and biodegradable food packaging materials as a means to reduce the consumption and the dependence on petroleum-based materials and to diminish environmental pollution.Soy protein isolate(SPI)was taken as the main film-forming material.Firstly,the chestnut bur extract(CBE)and the valonia tannin containing hydrolyzed tannin are used as functional additives to improve the antioxidant properties of soy protein isolate films.Secondly,the coconut oil(CO)as a hydrophobic agent and portulaca oleracea extracts(PE)containing saponins as surfactants are used to blend with SPI to improve hydrophobicity of films.Finally,the composite films of hydroxypropyl methyl cellulose(HPMC)and SPI are prepared by surface recombination method to improve the mechanical properties of the SPI film.Tannic acid as protein cross-linking agent are used to further improve the mechanical properties of the composite film.The main findings of this paper are as follows:(1)The CBE at different levels,2,5,8,10%(w/w)of protein was added into the soy protein isolate films respectively.The light transmittance of the SPI/CBE films decreases significantly(p<0.05),as the CBE increased.The transmission of UV light was very low under 300nm for all the films,regardless of the levels of the CBE.With increasing CBE content in the film,the oxygen permeability values(p<0.05)of the film decreased from 3.36±0.11 to 1.38±0.10(cm3 mm m-2 atm-1 day-1),but the moisture content is not changed significantly.In addition,the tensile strength showed optimum values for SPI/CBE films with 8%CBE(w/w),and the elongation is not changed significantly.The antioxidant properties of SPI films were improved with the addition of CBE as determined by 2,2-diphenylpicrylhydrazyl(DPPH)radical scavenging activities.For the same SPI/CBE films,the TP content and DPPH scavenging capacity was higher at 25? and 35? than that at 15?(p<0.05).Fourier transform infrared spectrum indicated that the addition of CBE influenced the SPI film mechanical properties by hydrogen bonding and hydrophobic interaction.The DSC curves indicated that the presence of CBE altered the structure of the SPI by creating new hydrogen bonds and hydrophobic bonds between the macromolecular chains,while the hydrogen bonds stabilizing the network may have been weakened by the repulsion force at the alkaline pH.Thus,a low enthalpy could dissociate the bonds.The cross-section of the SPI/CBE films viewed using scanning electron microscopy showed a coarse texture and coMPact structure.(2)Soy protein isolate(SPI)films prepared at pH 8 and 10 and mixed with four concentrations of valonea tannins(0%,5%,10%,15%,w/w)were analyzed.The addition of valonea tannins increased tensile strength(TS)and decreased percentage of elongation at break(%E)of SPI films(p<0.05).Due to the highly hydrophilic nature of valonea tannins,the water vapor permeability(WVP)of SPI films was not affected;however,oxygen permeability(OP)was reduced(p<0.05).Valonea tannins increased protection against UV light and improved the antioxidant properties of SPI films as DPPH radical scavenging activity.CoMPared to SPI films at pH 8,films at pH 10 had higher transparency,TS,and E%and lower OP,WVP,and DPPH radical scavenging activities.By analyzing the secondary structure of protein,it was found that ?-folding conformation of SPI films of pH=10 was significantly lower than that of pH=8.This reduction in conformation suggested that pH=10 was beneficial to open the soy protein peptide chains and contributed to the cross-linking of valonia tannin and protein.TG curve indicated that the addition of valonia tannin increased the thermal decomposition temperature of the films.When SPI film with the valonea tannin 10%(w/w),the thermal degradation temperature of the film was higher than that of pure SPI film,which also indicated that the addition of the valonia tannin can improve the thermal stability of SPI films.(3)To better understand the effects prepared by different concentrations of CO and PE in the SPI film matrix,three different levels of surfactant(1:1,1:2,and 1:3,CO:PE)and three different levels of CO(0.05:10,0.15:10 and 0.25:10,CO:SPI)were evaluated.The experimental results are as follows:the opacity of SPI emulsified films increased as the CO increase,regardless the proportion of surfactant applied.The tensile strength of the SPI emulsified films decreased as the CO increased,but the elongation increased.In addition,CO could increased the hydrophobicity of films,thereby reducing the water vapor migration through the film.Whereas,the OP of the SPI emulsified films increased with increasing content of coconut oil.Furthermore,when the same proportion of coconut oil was added,the mechanical strength of the SPI emulsified films were improved as the PE increased,the tensile strength showed optimum values for SPI emulsified films with the proportion of CO and PE(1:2).The WVP and OP were improved with the incorporation of PE.While the PE having no significant effect on surface hydrophobicity of SPI emulsified films.The TG curves indicated that the thermal degradation temperature of the SPI films were not decreased as CO and PE added.FTIR spectrum indicated that the addition of CO disrupted the molecular interaction of protein chains in the film,and the hydroxyl group of saponins in the PE can form hydrogen bonds with carbonyl or hydroxyl groups in the protein molecule chain.(4)The blend and bilayer films with the proportion of SPI:HPMC 8:2 and 6:4 were prepared by blend and the solvent casting method.The experimental results are as follows:the light transmittance decreased with the increase of HPMC,regardless it was a blend film or a bi-layer films.A similar "island-like" structure can be seen from the cross section structure of the blend films,which indicated that the SPI was enriched and HPMC was dispersed in the blend films.The contact interface of the two-layer structure of the composite membrane has no obvious separation and void,they were connected tightly.Therefore,the bi-layer films with good adhesion can be obtained by the method of solvent casting.With the increase of the proportion of HPMC,the mechanical strength of the films increased,the moisture absorption of the films decreased.Moreover,the properties of the bi-layer films was better than that of the blend films,but the OP of the bi-layer films is greater than that of the blend films.FTIR spectrum indicated that hydrogen bonding interaction was occurred between SPI and HPMC.And TG curves indicated the thermal stability of blend and bilayer films was significantly improved.(5)The tannic acid at different levels,0.5,1,2,3%(w/w)of protein was added into the SPI/HPMC bi-layer films respectively.The experimental results are as follows:The addition of tannic acid increased the opacity of the bi-layer films.SEM micrographs of SPI/HPMC bi-layer films incorporated with tannic acid had the coMPact cross-section.when tannic acid at a level of 3%,the double layer interface of the bi-layer films is not clear,which indicated that tannic acid as cross-linking agent could make the two substances further dissolve,so that the double layer was not easy to peel off.In addition,the addition of tannic acid increased the WVP of the SPI/HPMC bi-layer films,and had no significant influence on the moisture absorption and water-solubility.The tensile strength showed optimum values for SPI/HPMC bi-layer films with 3%tannic acid(w/w).The oxygen permeatibility decreased as the tannic acid added,which was almost the same as pure SPI film.FTIR spectrum indicated that hydrogen bonding and hydrophobic interaction was occurred between occurred between tannic acid and protein.The TG curves indicated that the thermal degradation temperature of the SPI/HPMC bi-layer films was decreased by the addition of tannic acid.