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.