Antioxidant Effect on Kinetic Parameters
The peroxidation behavior of pure TAGs and TAGs containing 1.2 mM of gallic acid and alkyl gallates at 60 °C are shown in Fig 2 and Fig 3. The TAGs were oxidized to the end of the termination phase to plot the kinetic curve of the LOOH accumulation accurately. The combinational kinetic model was fitted very well on the PVs changes over time and provided accurate kinetic parameters. The kinetic oxidation parameters representing the inhibitory effects of the antioxidant components in bulk oil are shown in Table 3. The oxidation rate of soybean oil TAGs (Ki ) was significantly reduced by adding gallic acid and alkyl gallates. Considering kinetic parameters F , ORR, and A , which is the ratio of F to ORR, the greater extents of strength and effectiveness were observed for methyl gallate. The antioxidant consumption \({\overset{\overline{}}{W}}_{\text{AH}}\) for TAGs containing methyl, propyl, and octyl gallates during lipid peroxidation was lower than gallic acid. In general, the results indicated that the esterification of gallic acid and the addition of carbon atoms to the alkyl chain (< 8) improved antioxidant capacity. Considering to higher value of F and A , and the lower value of ORR parameters, the inhibitory effects of antioxidant compounds in TAGs were as follows: methyl gallate > propyl gallate > octyl gallate > gallic acid > dodecyl gallate > stearyl gallate. Comparison of the mechanism of antioxidant action of methyl gallate and gallic acid in bulk fish, canola, and olive oils showed that methyl gallate was the most effective antioxidant in preventing lipid oxidation (Mahdavianmehr et al., 2016). Although, in another study Mansouri et al. (2020) reported that the inhibitory activity of gallic acid in bulk sunflower oil was much better than methyl gallate.
As can be seen in Table 2, the hydrophobicity of alkyl gallates (log P) was higher than the gallic acid. Therefore, the increased antioxidant potency of the methyl, propyl, and octyl gallates in bulk phase oil compared to gallic acid can be attributed to improve surface-active characteristics by the increase in the alkyl chain length and the precise placement of antioxidants in the actual site of oxidation. Lu et al. (2006) reported that the impact of alkyl gallates depends more on their molecular polarity and solubility that affect their availability to the reactive center. The nonlinear antioxidative activities were observed for alkyl gallates in bulk oil peroxidation. So that, the inhibitory activity of dodecyl gallate and stearyl gallate during lipid peroxidation was lower than the gallic acid. The size of antioxidants affects their activity by changing their mobility in the bulk phase oils, leading to a cut-off effect. Lipophilic antioxidative components by long alkyl chains have lower mobility, so decreased diffusibility toward the reactive centers. Moreover, the increase in the alkyl chain length enhances the possibility of hydrophobic interaction (Budilarto and Kamal‐Eldin, 2015a).
Considering the mechanism of the free radical chain of bulk oil oxidation, antioxidant molecules of higher effectiveness take part more in chain termination reactions blocking peroxyl radicals (LOO) than in chain initiation reactions creating hydroperoxyl (HOO) and alkoxyl (LO) radicals. The higher potency indicates that the hydrogen-donating molecules (AH) provide radicals (A) of less possibility to participate in chain propagation reactions producing reactive radicals, such as LOO, L, and AOO(Marinova and Yanishlieva, 2003). Mechanistically, fewer tendencies were observed for methyl, propyl, and octyl gallates to participate in the side-chain reactions (data not shown). While the participation of gallic acid and stearyl gallate in the side-chain reactions were much higher than other antioxidant compounds, which is the main reason for the poorer performance of gallic acid and stearyl gallate compared to other phenolic antioxidants in lipid systems.
CMC’s kinetic parameter marks the transition from the initiation stage where micelles are stable to the propagation stage with extensive micellar collisions. The CMC provides a logical explanation regarding the interfacial performance of alkyl gallates in protecting the oils from peroxidation. The addition of antioxidant compounds decreased the CMC value of the oil samples (Table 3). This denotes that free LOOH molecules are present in lower concentrations in the reaction medium and organized as more stable reverse micelles by the antioxidant molecules at the water-oil interfaces. In this respect, the analyses of micelle size and interfacial tension provided helpful evidence.