Introduction
The autoxidation of lipid, the reaction of unsaturated fatty acids by
oxygen in lipid systems, is the most common chemical reaction that leads
to severe losses in the sensory attributes, quality of nutrition,
shelf-life, and safety of food systems. The free radicals formed during
lipid oxidation also damage the different body’s cells and cause
countless diseases like cardiovascular disease, cancer, and immune
system deficiencies. In the oil industry addition of antioxidant agents
is the most common way to prevent lipid oxidation. Considering to
carcinogenic effects of synthetic antioxidants, choosing a natural
antioxidant with suitable antiradical capacity is a significant
challenge in the oil industry. Enhancing the understanding of how
physical attributes affect oil peroxidation leading to the development
of new antioxidant technologies that helps to choose a suitable
antioxidant to protect oil substrates against oxidation
(Delfanian et al., 2016;
Delfanian and Sahari, 2020).
Commercial vegetable oils usually contain 0.02-0.05% of water and
various types of surface-active agents like phospholipids, free fatty
acids (1.0-140 mmol/kg oil), sterols, mono- and/or diacylglycerols that
during the refining process are not entirely removed. Bulk oils also
including many other surface-active agents, e.g., hydroperoxides (LOOH),
ketones, aldehydes, and alcohols that are derived from lipid oxidation
(Ghnimi et al., 2017). The water content
increases with the decomposition of LOOH molecules during lipid
oxidation. The LOOH molecules produced during peroxidation tends to
entrap traces of water and form micelles beyond their critical reverse
micelle concentration (CMC). Reverse micelles are generally organized
when transferred lipid oxidation from the initiation to the propagation
phase. The number and size of reverse micelles increased with the
increasing water content and the surfactant molecules as lipid
peroxidation progresses. This can change the structure and properties of
LOOH reverse micelles. Reverse micelles are structurally containing a
water core stabilized by surfactants molecules with nonpolar tails of
the surfactant extending into the oil phase and the polar head groups
extending into the water core. Reverse micelles are efficient
nano-reactors that significantly alter the chemical reaction rates
between water- and oil-soluble components by creating a water-oil
interface (Budilarto and Kamal‐Eldin,
2015b).
Recent studies have shown that the water-oil interfaces created by
reverse micelles are the actual oxidation site in bulk phase oils.
Antioxidant molecules positioning their polar head groups and nonpolar
tails at the water-oil interfaces and improved the oxidative stability
of bulk oils by stabilizes reverse micelles as well as by scavenging
radicals at the interfaces
(Kittipongpittaya et al., 2014;
Lehtinen et al., 2017;
Mansouri et al., 2020). Therefore,
antioxidant efficiency in lipid systems is attributed to its innate
capacity as a chelating agent or radical scavenger, interaction with
other reactants, and locating into the water-oil interface
(Chaiyasit et al., 2007).
In the literature, studies on the homologous series of antioxidants
revealed the nonlinear behavior or cut-off effect for lipophilic
antioxidants (alkyl esters derivatives of phenols) in the lipid system;
antioxidant activity enhances as the length of the alkyl chain is
reached until a critical chain length, after which further chain length
extension causes a drastic decrease in antioxidant activity
(da Silveira et al., 2020;
Kikuzaki et al., 2002;
Sørensen et al., 2015). The nonlinear
behavior of lipophilic antioxidants relates to their location,
partitioning, and mobility in the bulk oil affected by molecular size
and polarity. Therefore, despite the number of hydroxyl groups (-OH),
the antiradical capacity of phenolic antioxidants is also related to the
number of carbon atoms in the alkyl chain
(Shahidi and Zhong, 2011).
Gallic acid (3,4,5-trihydroxybenzoic acid) and its alkyl ester
derivatives, including methyl, propyl, octyl, dodecyl, and stearyl
gallates, have been recognized as natural antioxidants with unique
biological activities. In recent years, a limited number of studies have
examined the antiradical potency of gallic acid alkyl ester derivatives
in chemical and biological systems
(Kikuzaki et al., 2002;
Lu et al., 2006). However, concerning the
interfacial phenomena, there is no reported data on the effect of steric
structure on interfacial performance and mechanism action of gallic acid
alkyl ester derivatives in bulk phase oil.
Therefore, in this paper, the effect of length of alkyl chain on the
interfacial activity of gallic acid alkyl ester derivatives in bulk
phase oil was evaluated that is expected to impact the colloidal changes
of bulk oil during lipid peroxidation.