2.8. Antioxidant Assessment

The antioxidant effects of different materials can be measured in several ways. Multiple studies (Table 2) have assessed the antioxidant activities of compounds by measuring the degree of lipid oxidation of oxygen-sensitive foods in packaging materials. The antioxidant activities of plastic bottles are different to those of antioxidant films, which cannot be used for packaging meat and vegetables. Therefore, the antioxidant activity of the caps was tested using three methods: DPPH, ABTS, and FRAP.

Methods for assessing antioxidant activities in active packaging systems.

Note: Peroxide values (PV), conjugated dienes (CD), conjugated triene hydroperoxides (TH), thiobarbituric acid index (TBARS), metmyoglobin (MetMb).

The antioxidant effectiveness of the caps was measured according to the 1,1-diphenyl-2-picrylhydrazyl (DPPH) method. DPPH radical-scavenging activity in the presence of the cap extract solution was monitored at 517 nm using a UV/Vis spectrophotometer (Hitachi Ltd., Tokyo, Japan). The film was extracted in methanol at 55 °C for 3 h. The antioxidant activities of the films were determined using the DPPH method. The DPPH scavenging activity was determined according to Equation (2).

where A_i is the absorbance of the DPPH solution mixed with the cap extract solution, and A₀ is the absorbance of the DPPH solution mixed with methanol.

The ABTS was dissolved in water to reach 7 mM concentration. It was then mixed with 2.45 mM potassium persulfate solution in a 1:1 ratio for 16 h. The solution was then diluted with methanol until it reached the absorbance of 0.70 ± 0.02 at 734 nm. A volume of 40 mL cap extract was added to 3960 mL of the diluted solution. After 6 min incubation in the dark at room temperature, the absorbance was measured at 734 nm.

Free-radical-scavenging activity of the antioxidant caps was determined using the ABTS assay, according to Equation (3).

where B_i is the absorbance of a mixture of ABTS and cap extract solution, and B₀ is the absorbance of the ABTS and methanol mixture.

Antioxidant activities of the caps were assessed using the FRAP method. A calibration curve was plotted as a function of the concentration of standard (Vc) solutions (0–1000 ppm). The result was expressed as micrograms of Vc equivalent per gram of film (mg AAE/g film).