Synergistic antioxidant mechanism of camellia oil filter cake polyphenols and phospholipids

Natural antioxidants can be expected to derive from the Camellia by-products after seed oil processing. Taking the Camellia oil filter cake as the object, this study aims to investigate the effects of the polyphenol extracts and phospholipids on the oxidative stability of the Camellia oil (CO). The polyphenol extracts were also selected as the phosphatidylethanolamine (PE), and phosphatidylserine (PS) in the stripped Camellia oil (SCO). The critical micelle concentrations (CMCs) of the PE and PS in the SCO were determined by tetracyanoquinodimethane (TCNQ) ultraviolet–visible spectrophotometry. An accelerated oxidation test was carried out to evaluate their synergistic inhibitory with the polyphenol extracts on the SCO. Quantum chemical calculations through Open-source Computational Chemistry Application (ORCA) were also performed to analyze the frontier molecular orbitals, electrostatic potentials, and average local ionization energy of the 10 types of polyphenol compounds (benzoic acid, homovanillic acid, 2,3,4-trihydroxy benzoic acid, vanillic acid, gallic acid, quinic acid, catechin, protocatechuic acid, kaempferol, and quercetin) from the polyphenol extracts. The results showed that the CMCs of the PE and PS in the SCO were 409.2 and 462.4 µmol/L, respectively, while the CMCs of the PE and PS in the CO were 399.8 and 424.3 µmol/L, respectively. Furthermore, the 500 µmol/L PE and PS with 0.02% polyphenol extracts were synergistically inhibited to form the lipid hydroperoxide (LOOH) in the SCO. Camellia oil filter cake polyphenol extracts were used to significantly inhibit the formation of LOOH in the Camellia oil during the early stage of accelerated oxidation (150 min). At the end of the induction period (390 min), there was no significant difference in the LOOH content between the Camellia oil filter cake polyphenol extracts and the blank control group, while the experimental groups with the 500 µmol/L addition of the PE, and PS. The filter cake extracts still exhibited significantly lower LOOH levels, compared with the control group. The phospholipids were used to synergistically suppress the LOOH production when combined with the polyphenol extracts. The Area under the curve (AUC) index was used to indicate the LOOH concentration over time. The AUC index of the PE, PS, CFC, and HFC extracts were 78, 91, 92, and 95 µmol/L, respectively. The quantum chemical calculations showed that the lower HOMO-LUMO (Highest occupied molecular orbital, HOMO; The Lowest unoccupied molecular orbital, LUMO) energy gap was associated with the enhanced antioxidant capacity. The minimal HOMO-LUMO gap was quercetin (3.822 eV) ,with kaempferol showing the next smallest gap (3.914 eV). Quinic acid was the compound with the highest energy gap (6.683 eV). The minimum electrostatic potential points of quercetin and kaempferol were 14 and 17, respectively. While the minimum electrostatic potential values were -2.43 and -2.53 eV, respectively. The electron loss of the quercetin and kaempferol was stronger than the rest compounds. The minimum average local ionization energy of the quercetin and kaempferol were 8.44 and 8.55 eV, respectively. The quantum chemical calculation revealed that the quercetin and kaempferol acted as the electron donors, thus reducing free radical formation and then inhibiting the lipid oxidation in camellia oil. The addition of 500 µmol/L of the PE and PS into the SCO can be expected to form the micellar structures inside the camellia oil. The PE/PS formed the more stable micellar structures in the camellia oil with the LOOH and highly reactive phenolic compounds from filter cake, such as quercetin and kaempferol, thereby delaying the oxidation of Camellia oil. Phospholipids play a critical role in the oxidative stability of the camellia oil. This finding can provide a theoretical foundation to develop the natural antioxidants from Camellia seed oil processing.