Electric field treatment improving physicochemical properties of zein film produced by casting

Abstract: Zein is the major storage protein of corn and comprises 45%-50% of total protein in corn, and possesses good property of forming film in virtue of abundant sulphur amino acids. High content of hydrophobic amino acid grants hydrophobicity to zein film, but film made of natural casting retains its crispness. In order to improve the flexibility of zein film, plasticizers are added into the film solution. Though plasticizers can significantly enhance mechanical properties of zein film, they affect their security and edibility. Equipments such as pulsed electric field used in processing of casting can improve the properties of zein film by taking appropriate measures, without affecting the security and edibility of zein film. But the popularization of these equipments is hindered for their high price. In this paper, the combination of DC power (DYY-8C, Beijing) and conducting glass coated with indium tin oxide was used to set up paralleled uniform electric field, and then the electric field was introduced into the processing of zein film. This method was of simple equipment, easy operation and good selectivity. Coupled with the paralleled uniform electric field (1-5 A/m2), the soft zein films with complete shape were obtained by casting. The properties of zein film such as tensile strength, elongation at break, water vapor permeability and water absorption were measured and analyzed to evaluate the influence of the paralleled uniform electric field on zein film. Meanwhile, thermostability difference between films before and after treatment was studied by differential scanning calorimetry. By analyzing the data, tensile strength and elongation at break increased with the increase of electric current density while water vapor permeability and water absorption decreased. When electric current density was at 4 A/m2, properties of zein film were optimal. Tensile strength and elongation at break reached the maximum value (73.09 MPa and 9.68%, respectively); water absorption and water vapor permeability reached the minimum value (14.87% and 2.54×10-8 g·m/(m2·h·Pa), respectively). Contact angle declined slightly (within 5°), and tended towards stability when electric current density was above 2 A/m2. Experiments indicated that the presence of electric field force facilitated interaction between zein molecules, and these molecules were organized in a network pattern contributing to the improvement of mechanical properties. Meanwhile, polar amino acid residues tended to move forward to the topside of the network, resulting in the slight decline of contact angle. There existed significant difference among the data obtained from the experiments by Duncan analysis (P<0.01). During the processing of zein film, no hazardous substance was added into the film solution. By adjusting the electric current density, film or edible packing material with flexibility, fixed shape, mechanical strength, certain mechanical properties or contact angle can be obtained, which are biodegradable, edible and non-toxic toward humans and environment.