Physicochemical Properties and Antioxidant Activity of Enzymatic Modified Soy Protein Isolate Films with Lignin
In this study, a sustainable packaging system was developed to provide food safety and security. Soy protein isolate (SPI) was enzymatically modified by transglutaminase under different conditions to ensure desirable and optimized enzyme crosslinking activity before film preparation. Physicochemical properties including viscosity and molecular weight distribution of the modified proteins and films were measured. Results confirmed the enzymatic treatment is an effective way to modify the SPI based biopolymeric film. Modified films with the enzyme had significant increases in tensile strength (TS), percent elongation (%E), initial contact angle, and a reduction in swelling and protein solubility properties compared to the control films. FTIR and XRD spectra revealed that the enzyme treatment modified the structure of SPI film matrix. The optimal film preparation conditions achieved in this part were protein denaturation temperature 80 °C, and enzyme incubation time 2hr.
We attempted to enhance antioxidant activity of enzymatically modified SPI film with the addition of two types of lignin, alkali lignin (AL) and lignosulphonate (LSS), at different concentrations. Results indicated that AL carried higher radical scavenging ability than LSS. Films containing AL showed high absorption in the UV region, and this UV-blocking ability increased with increasing lignin concentration. Deconvoluted FTIR spectra and XRD results suggested that the addition of lignin caused some changes in secondary structure of the protein matrix. The addition of lignin improved TS and thermal stability of films, but reduced %E as a function of lignin concentration. Radical scavenging activity and UV-blocking ability alongside improvement in physicochemical properties of enzymatic modified SPI film with lignin motivated us to apply this bioplastic in two types of oil, soy oil and fish oil. Results revealed that applying enzymatically modified SPI film with AL and LSS in the inner layer of a soy oil packaging system, decreased oxidation rate to around 75%, and pentanal production to about 40% of control. UV-blocking ability of AL caused reduction in oxidation rate for more than 75% compared with the normal packaging system. The effectiveness of this active packaging system in soy oil was greater than fish oil. Thus, the developed biopolymeric materials may have application to food packaging.