Zein constitutes about half of the endosperm proteins in corn. Recently, attempts have been made to utilize zein for food coatings and biodegradable materials, which require better physical properties, using chemical modification of zein. In this study, zein proteins were modified using citric acid, succinic anhydride, and eugenol as natural cross-linking agents in the wet state. The cross-linkers were added either separately or combined in increment concentrations (0.1, 0.2, 0.3, and 0.4%). The effects of those agents on the mechanical properties, microstructure, optical properties, infrared (IR) spectroscopy, and antibacterial activities of zein were investigated. The addition of cross-linking agents promoted changes in the arrangement of groups in zein film-forming particles. Regarding the film properties, incorporation of cross-linking agents into zein films prepared in ethanol resulted in two- to three-fold increases in tensile strength (TS) values. According to the Fourier-transform infrared (FTIR) spectra and Hunter parameters there were no remarkable changes in the structure and color of zein films. Transparency of zein films was decreased differentially according to the type and cross-linker concentration. The mechanical and optical properties of zein films were closely related to their microstructure. All cross-linked films showed remarkable antibacterial activities against Bacillus cereus ATCC 49064 and Salmonella enterica ATCC 25566. Food spoilage and pathogenic bacteria were affected in a film-dependent manner. Our experimental results show that even with partial cross-linking the mechanical properties and antipathogen activities of zein films were significantly improved, which would be useful for various industrial applications.
Pure zein is known to be very hydrophobic, but is still inappropriate for coating and film applications because of their brittle nature. In an attempt to improve the flexibility and the antimicrobial activity of these coatings and films, Chemical modification of zein through forming Schiff bases with different phenolic aldhydes was tried. Influence of this modifications on mechanical, topographical, wetting properties and antimicrobial activity of zein films were evaluated. The chemical structure of the Schiff bases films were characterized by ATR-FTIR spectroscopy. The results indicate an improvement in mechanical properties with chemically modification of zein to form Schiff bases leading to a reduction in the elastic modulus. An increase in the elongation at break has been observed, but with slight influence on tensile strength. Plasticized zein films have similar initial contact angle (∼40°). An increase in reaction temperature and time increases film's affinity towards water. As shown by contact angle measurements, a noticeable relation was found between film composition and the hydrophilicity. Surface topography also varied by forming Schiff bases, becoming rougher than zein-based films. The antibacterial activities of zein and Schiff bases of zein-based films were investigated against grampositive bacteria (Listeria innocua, Listeria monocytogenes, Bacillus cereus and Clostridium sporogenes) and gramnegative bacteria (Escherichia coli, Yersinia enterocolitica and Salmonella enterica). It was found that the antibacterial activity of the Schiff bases-based films was more effective than that of zein-based films.
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