Studies On Modification, Surface Properties And Film Forming Properties Of Zein

Zein, a hydrophobic protein from corn, is often used as food and non-food packaging materials for its good film forming. But pure zein film has poor elongation, and too brittle and fragile to industrial application. This problem may be solved by chemical methods. In this paper, zein was extracted from corn gluten meal (CGM), a by-product of corn starch production, and was phosphorylated by POCl₃. The physical properties of modified zein were determined. In order to evaluate the effective of phosphorylation to zein, the air-water or hexane/water interface properties of modified or un-modified zein with SDS complex solutions were studied by the methods of adsorption kinetics and interfacial rheology. For estimation the structure of phosphorylated zein, the adsorption characteristics were analyzed by a model. Variations in surface and mechanical properties of zein films with plasticizer or phosphorylated zein were also studied. Surface hydrophilic/hydrophilic of zein films and dynamic wetting changes on its surface were evaluated by static and dynamic contact angle. The more details are as follows:Properties of zein were determinated by SDS-PAGE electrophoresis, zeta potential instrument, differential scanning calorimetry (DSC), circular dichroism instrument (CD). The data showed thatα-zein was the major components in the extraction, and its physical properties are as follows: Isoelectric point pH5.97, Glass transition temperature (Tg) 165.42℃. After phosphorylation, zein solution possessed higher viscosity and negativity, loosed structure and lower isoelectric point. The far-UV CD data indicated that secondary structures of zein have changed and theα-helix reduced. DSC data showed that the glass transition temperature Tg was increased. Both O-phosphate bond and the N - phosphate bond are present in phosphorylated zein and the degree of phosphorylation at pH5.0, 7.0 and 9.0 was 3.31, 6.68 and 5.85 mol P/mol zein respectively.There is variation in surface pressure and absorption kinetics of air-water or oil (hexane)–water interface between zein and modified zein with SDS complex solutions at different concentrations. Surface pressure of all solutions increased with adsorption time. Not only adsorption, penetration and rearrangement in single system, but also competitive, complexation or even incompatibility adsorption in mixed system would be appear during solute adsorbed to interface. The process of adsorption belong to diffusion-controlled adsorption according to the diffusion rate constant kdiff calculated by a formula proposed by Ward and Tordai, and the result of diffusion-determining during adsorption was attained. Interface active of zein was improved after phosphorylation. The rate of penetration and rearrangement of adsorbed zein molecules have also been analyzed by a semi-empirical equation: The data showed that within adsorption time, adsorption at air-water interface has not only penetration but also rearrangement, while only strong penetrations at oil-water interface.Another important feature of protein-based films is rheologiy at interface. With frequency increases, more solute of zein, phosphorylated zein, SDS molecules or zein-SDS complex were adsorbed onto interface of air-water or oil-water, resulting in the increase of elastic modulus (E_d) and the decrease of viscosity modulus (E_v). The flexible interfacial protein film was attained and E_d is greater than E_v. E_d and E_v at the two type interfeaces should be very dependant on the different structure of protein in interfacial films and increased with time. The E_v at air-water interface increased with the degree of phosphorylation increased. Effect of oil salvation at oil-water interface reduced the force between the hydrophobic protein molecules, affecting E_d, E_v value.Surface properties and mechanical properties of zein protein films, including water content, micro-structure and roughness Z of film surface, were changed by adding different proportions plasticizers, which properties of hydrophilic/hydrophobic are different each other. The apparent contact angle of zein film containing glycerol decreased with plasticizer increased, while the film containing oleic acid or polyethylene glycol plasticizer has the opposite result. A exponential equation y = A1×exp(-x/t1) + y0 can be fitted with the rate changes of dynamic contact angle on zein films. Tensil strength (TS) decreased and elongation (EB) increased by adding more plasticizers. Changes on surface and mechanical properties also happen to the phosphorylated zein films, and be affect by the degree of phosphorylation and type of plasticizer. Phosphorylated zein films possessed higher critical surface tensionγc, lower hydrophobicity and higher dynamic contact angle rate. The magnitude of changes was depended on micro-structure and type of plasticizer. Exponential equation y = A1×exp (-x/t1) + y0 also suit to dynamic contact angle of phosphorylated zein films. Elongation of phosphorylated zein films, possessed compact structure, has 3⁴ times than the films with plasticizer only, although its roughness Z is larger than plasticizer films.After phosphorylation, phosphoryl group was introduced into zein molecules, resulting in changes on physical properties, interfacial properties and the film forming properties. The elongation of phosphorylated zein films has improved dramatically and the E_v of interfacial films also increased with the degree of phosphorylation increased. Thus, phosphorylation is an effective, feasible method to improve the flexibility of zein films, suitable for industrial applications.