Abstract:
The objectives of this study were to investigate the effect of phenolic acid addition on properties of soy protein isolate film and to monitor the controlled release of model compounds and antimicrobial agent from such film. In the first part of the study, the effect of type, oxidation state and concentration of phenolic acids was examined. Gallic and tannic acids, either oxidized or unoxidized, were added to the film-forming solution at three different levels (1.3, 2.6 and 4.0 mM). Tensile strength and elongation at break of the phenolic-added films significantly increased (p≤0.05) as compared to the control. Tannic-added films exhibited greater tensile strength and elongation at break than those with gallic acid. Films added with oxidized phenolic acid demonstrated higher tensile strength and elongation at break than those with unoxidized phenolic acid. In general, tensile strength and elongation at break tended to increase with increasing phenolic concentration. The improvement in mechanical properties was a result of phenolic-induced protein cross-linking, as confirmed by the protein pattern investigated using SDS-PAGE. Among the film samples studied, the film added with 4.0 mM oxidized tannic acid possessed the highest tensile strength (2.34 MPa) and elongation at break (322.5%), which were 1.03 and 114.7% greater than those of the control. Phenolic addition produced a film with more hydrophobic surface while water solubility and water vapor permeability became decreasing. Upon phenolic acid addition, transparency and L* decreased while +a* and +b* increased. Hue angle became decreasing, approaching a value representing yellow-orange, while chroma was found to increase. Regarding to moisture sorption behavior, the film containing 4.0 mM oxidized tannic acid displayed type II isotherm. Water activity significantly affected tensile strength and elongation at break of the film sample (p≤0.05). In the second part of this research, release profile of model compounds from the film containing 4.0 mM oxidized tannic acid was monitored. Model compounds with different molecular weight, namely trypan blue (961 Da), FITC-dextran (4 kDa) and FITC-dextran (70 kDa) were used in the study. Trypan blue was added at 1 mg/100 ml of film-forming solution while the FITC-dextrans were added at 10 mg/100 ml of film-forming solution. Time-release profile of the model compounds was monitored in two different dissolution medium conditions, i.e., phosphate buffer (pH6.0) at 4 °C and phosphate-buffered saline (pH7.4) at 37 °C. It was found that the lower the molecular weight of model compound, the greater the amount released and the greater the release rate. Percent maximum release of trypan blue, FITC-dextran (4 kDa) and FITC-dextran (70 kDa) were equal to 59.0, 47.7 and 32.5 in phosphate buffer (pH6.0) at 4 °C and 59.5, 57.6 and 36.0 in phosphate-buffered saline (pH7.4) at 37 °C. The model compounds with different molecular weight demonstrated different time-release profile. In the last part of this study, the films, with or without antimicrobial agent (sodium lactate) was applied to ground pork patties and investigated for their antimicrobial activity. A total of five treatments was used in this study: (1) ground pork patties without wrapping (C), (2) ground pork patties wrapped with soy protein film (S), (3) ground pork patties wrapped with phenolic-added soy protein film (film containing 4.0 mM oxidized tannic acid) (SP), (4) ground pork patties wrapped with sodium lactate-added soy protein film (SL), and (5) ground pork patties with tannic- and sodium lactate-added soy protein film (SPL). Total viable counts, Escherichia coli, Staphylococcus aureus and Salmonella spp. were monitored during 12-day storage at 4 °C. It was found that SP, SL and SPL which were added with tannic acid and/or sodium lactate exhibited antimicrobial activity. Total viable counts and E. coli were lower in number in those treatments. Moreover, sodium lactate-containing SL and SPL also demonstrated antimicrobial activity against Salmonella spp.