Effects of heat and UV-C curing on properties of soy protein film

Abstract

This study aimed to investigate the effects of heat curing and UV-C curing on properties of soy protein film. Both the heat and UV-C treatments were applied to either film-forming solution or pre-formed film. In the first part, film-forming solution or pre-formed film was cured at 60, 70, or 80°C for 2, 4, or 6 h. Heat curing had no effect on film thickness (p>0.05), but it did improve tensile strength of the films. Pre-formed film cured at 70°C for 4 h exhibited the highest tensile strength (3.49 MPa), which was 1.8 times higher than the control. Increasing Raman intensity in the S-S stretching region confirmed the formation of heat-induced disulfide cross-links. This increasing degree of cross-linking may also account for a decrease in elongation at break of the film samples. An increase in yellowness intensity was observed in heat-cured samples. Heat curing of film-forming solution significantly increased film transparency but posed the opposite effect on pre-formed films. Heated films exhibited a decrease in water solubility and an increase in surface hydrophobicity. Heat curing of film-forming solution produced a film with increasing water vapor permeability. Meanwhile, water vapor permeability of heat-cured pre-formed films was similar to the control (p>0.05). SEM revealed structural inhomogeneity of heat-cured films. In the second part, the effect of UV-C curing of film formerly heat-treated at 70°C for 4 h was investigated. The radiation doses were varied as 4, 8, 12, and 16 J/cm2. UV-curing did not affect film thickness but did cause a significant increase in tensile strength. Heat-treated pre-formed film undergoing UV-curing at 12 J/cm2 possessed the greatest tensile strength (6.37 MPa), 1.8-fold higher than the film heat-treated alone and 3.4-fold higher than the control. UV-induced dityrosine cross-linking was confirmed using fluorescence spectroscopic technique. In spite of that, UV-C treatment minimally affected elongation at break. Similar to heat treatment, UV-curing also induced an increase in yellowness intensity and a slight decrease in transparency. UV-C treatment produced a film with lower water solubility. UV-curing of heat-treated film-forming solution had no effect on water vapor permeability of the resulted film, but the treatment significantly increased water vapor permeability of heat-treated pre-formed film. UV-C curing at lower doses tended to result in a film with lower surface hydrophobicity. At higher UV-C doses, however, surface hydrophobicity became increasing again. SEM micrographs revealed cracks and pinholes in UV-cured heat-treated film matrices. In conclusion, heat curing and combined heat/UV-C curing were demonstrated as effective techniques for enhancing mechanical strength of soy protein film by promoting the formation of covalent cross-links between protein chains.