| dc.description.abstract | The rising awareness on the environmental impact of conventional plastics and the rapid pace of food innovation have shifted the interest towards biodegradable, smart food packaging materials using biopolymers (e.g. alginate) and natural bioactives based on, for example, polyphenols, organic acids or reactive oxygen and nitrogen species (RONS). In this study we investigated whether the mechanical, barrier, antioxidant, and antimicrobial properties of alginate film (2% w/v) could be improved by addition of citric acid, plasma activated water (PAW), and grape seed extract (GSE) towards the development of bioactive and visually responsive (smart) food packaging materials. Moreover, the effect of citric acid concentration (5, 10, 15 and 20% w/w relative to alginate) and its addition order (either addition of citric acid in alginate solution or addition of alginate in citric acid solution), as well as concentration of glycerol (10 and 30% w/w relative to alginate) in alginate films were evaluated. The tensile strength and modulus, as well as elongation at break were measured to determine the mechanical properties. Barrier properties were measured by water vapor transmission rate (WVTR) and antioxidant activity was evaluated by DPPH assay. The antimicrobial activity of selected film-forming solutions against Escherichia coli and Staphylococcus aureus was assessed, taking into consideration the effect of the incubation temperature (either 10 or 37℃) and the concentration of the test item (either 33.3 or 83.3% v/v).
Addition of 10% citric acid to distilled water and alginate solution resulted in the highest tensile strength of the tested distilled water-alginate films, which was attributed to the crosslinking between alginate and citric acid via ester bonds (FTIR analysis). Higher citric acid concentrations (15 and 20%) led to a decrease in the tensile strength of alginate films, likely due to the plasticizing effect of excess citric acid in the film. Compared to the corresponding film prepared in distilled water, the incorporation of PAW increased the tensile modulus by 32% and decreased WVTR plus elongation at break of alginate-citric acid films by 7% and 26%, respectively, while tensile strength was not affected, hence indicating dual alginate matrix crosslinking with RONS and citric acid. The incorporation of GSE aqueous filtrate (11.66 ± 1.32 g GAE/L) into the alginate film significantly increased the antioxidant and antimicrobial activity, resulting in 2.7 and 5.5 log reductions of E. coli and S. aureus viable counts after 24 h at 37℃, as compared to 2.3 and 2.9 log reductions in absence of GSE, respectively. Replacing GSE by another antimicrobial agent, PAW, resulted in 3.3 and 4.5 log reductions, respectively, for the above-mentioned bacteria. The more pronounced inhibitory effect on Gram-positive bacteria was attributed to the different composition and structure of the cell wall. pH adjustment of the GSE film-forming solution from 2.0 to 10.0 demonstrated the color sensitivity of the film to pH variation, as the UV/VIS spectra absorption peak was shifted from 450 to 500 nm, corresponding to a color change of the film-forming solution from yellow to red. This study has demonstrated the potential of GSE and PAW in combination with citric acid towards enhanced functionality and bioactivity of alginate films for smart food packaging applications. | |