Effects of ferulic acid and UV curing on properties of soy protein film

The objective of this study was to explore the effect of UV-C curing on the properties of ferulic acid-added soy protein film. The films were fabricated from soy protein isolate and added with 1.5% ferulic acid. UV-C radiation was applied at four different doses (0.32, 1.56, 4.00, 12.00 J/cm2) to either preformed film or film-forming solution. The mechanical, physicochemical, and morphological properties of the film samples were investigated. Ferulic acid addition and UV-C curing at 0.32 J/cm2 posed a significant effect on film thickness while film density was slightly affected by ferulic acid addition and/or UV-C treatment. UV-C irradiation of ferulic acid-added film resulted in an increase in tensile strength and elongation at break. The films irradiated at the highest dose (12.00 J/cm2) exhibited about 1.3-fold increase in tensile strength and a 1.7-fold increase in elongation at break from the control. UV-C treatment on preformed film did not produce any difference in tensile properties from the treatment on the film-forming solution. Protein cross-linking via C-N and dityrosine bonds was confirmed using FTIR and fluorescence spectroscopic techniques. Apart from the mechanical properties, ferulic acid addition and UV-C curing also posed a significant effect on the film's optical properties, including transparency and colour. UV-C irradiation made the ferulic acid-added film become lower in transparency and higher in chroma, as the films appeared more opaque and more intense in yellowness to the naked eye. As compared to the control, UV-C treatment of ferulic acid-added films caused a slight increase in water vapour permeability. However, similar water vapour permeability was observed among the UV-treated ferulic-added films regardless of the UV-C dose used. Ferulic acid addition and/or UV-C irradiation also minimally affected the water solubility of the film samples. In spite of that, an increase in surface hydrophobicity was observed with increasing UV-C dose, especially for the treatments on preformed film. In conclusion, UV-C irradiation was demonstrated as an effective technique for improving the tensile properties of ferulic acid-added soy protein film. It should be noted that, upon utilizing this technique, the transparency and colour of the soy protein film were also affected.