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.