Effect Of Food Protein Properties On Film-forming Property

The edible film which made by the biomacromolecular—protein is a new kind of greenpackaging material, no pollution to environment. In this paper, edible composite films basedon gelatin （G）, soy protein isolate （SPI）, whey protein concentrate （WPC）, wheat gluten （WG）,gliadin （GD） and zein （Z） were prepared. The effects of properties and interactions of proteinson the film-forming property were investigated.The effects of properties and interactions among gelatin, soy protein isolate and wheyprotein concentrate on the properties of water-soluble protein films have been studied. Theresults showed that since gelatin, soy protein isolate and whey protein concentrate werewater-soluble proteins, the tensile strength （TS） and transparency （T） of composite films werehigh, whereas the water resistance and water barrier properties were low. The addition ofgelatin to soy protein isolate caused to the increase in the viscosity of film-forming solutionand the values of TS, T and water solubility （SL）. Elongation at break （E） increased firstly andthen decreased. The maximum value of TS, E and T was47MPa,66%,90.6%, respectively.Simultaneously, the water content （MC）, water vapor permeability （WVP） decreased with theaddition of gelatin. The varation tendency of gelatin-whey protein concentrate compositefilms was the same as gelatin-soy protein isolate composite films, but the properties of thelatter were better. With the addition of whey protein concentrate, the TS and E changed much,T reduced, WVP rose, but no significant change in MC and SL. Synthesized the property ofcomposite films,2:3was suitable for the ratio of whey protein concentrate and soy proteinisolate. The TS of WPC-SPI composite films reached the maximum value （17MPa）, E9%, T71%, SL27%.Zein and gliadin are relatively hydrophobic proteins, thus the water resistance and waterbarrier properties of films based on them were perfect. Compared with other protein films,zein-only films are brittle and stiff. However, the gliadin-only films have good extensibilityand low mechanical strength. The results revealed that the addition of gliadin to zein causedto increase in the TS and decrease in the E of zein films. Moreover, there was no significantdifference in MC of composite films. Consequently, zein contributed to strength of compositefilms, and gliadin provided films with extensibility. The yellowness value decreased and T ofcomposite films increased with the addition of gliadin. And SL and WVP values of compositefilms rose a little. Analyses of changes in the dynamic contact angle showed that the increasein gliadin content decreased the surface hydrophobicity of the composite films. The proteininteractions determined by dissolution method suggested that the formation of the networkstructure in protein films was supported mainly by hydrophobic interactions, and thecontributions of ionic bonds and hydrogen bonds were smaller. Simultaneously, the additionof gliadin raised the content of disulfide bonds. The analysis of FTIR showed that the increasein the content of gliadin enhanced the content of-helix and-turn, but the content of-sheetdeclined. And there was no significant difference in the content of random structure. It wasproved that the addition of gliadin increased the flexibility of zein-gliadin composite films.Thermal analysis indicated that the interactions between gliadin and zein facilitated the formation of a homogeneous network structure. However, there was no good relationshipbetween glass transition temperature and the ratio of zein and gliadin. The pictures observedby scanning electron microscopy （SEM） demonstrated that the amount of pores in films wassmaller but the surface of composite films was rougher with the addition of gliadin.Wheat gluten, a mixture of proteins, consists of gliadin and glutenin. In order to realizethe contribution by each component to the property of wheat gluten films, the ratio of gliadinand glutenin in film-forming solution was various by adjusting pH value and ethanol content.The effects of the properties and interactions between gliadin and glutenin, transglutaminase（MTG） modification and preparation conditions on the properties of wheat gluten films wereinvestigated. The results showed that gliadin contributed to the flexibility and extensibility,and glutenin provided wheat gluten films with tensile strength. When concentration of ethanolwas approximately50%, pH value of film solution was11, the ratio of gliadin and gluteninwas suitable for film preparation. Under this condition that the ratio of glycerol and wheatgluten was1:2.5and the plastic plate was choosed as films formation medium, the TS ofwheat gluten films was2.02MPa, E254.8%, and T62%. In order to improve the mechanicalproperty, MTG was used to be a crosslinking agent. When dosage of MTG was20U/g andreaction time was120min, the TS reached a maximum value （2.64MPa）, SL and WVPattained minimum values (24.75%,0.30g·mm·m^-2·h^-1·kPa^-1, respectively). The SEM picturesshowed that the film surface was more delicate and smoother.