Status of linusorbs in cold-pressed flaxseed oil during oxidation and their response toward antioxidants

Linusorbs (LOs, cyclolinopeptides) are a class of naturally occurring cyclic hydrophobic peptides found in flaxseed oil, whose oxidation states indicate the oxidative stability and bitterness of flaxseed oil. Subjected to 63 °C accelerated oxidation, most Met-containing LOs in cold-pressed flaxseed oil entirely depleted by the 6th day except CLP, and MetO2-containing LOs became the dominant ones. However, no MetO2 form of Trp-containing LOs, such as CLD, CLF and CLG, were detected. Given their oxidative kinetics, methionine sulfoxide (MetO) residue in some LOs was less sensitive toward oxidation in the presence of Trp (Tryptophan) group, and the oxidative stability of Met-containing LOs was CLP < CLB < CLL ≈ CLM < CLO, as compared to MetO-containing LOs: CLD < CLE < CLC < CLF ≈ CLG. When antioxidant was added into cold-pressed flaxseed oil to assess the additives' antioxidant effect, no significant difference was observed on oil oxidative indices in early stage except α-tocopherol, where they vary dramatically in suppressing Met oxidation of LOs: L-AP (L-ascorbyl palmitate) > TBHQ (tert-butyl hydroquinone) > γ-tocopherol > carnosic acid > α-tocopherol. Besides its ability to suppress oxidation of Trp-containing LOs, L-AP also exhibits superior antioxidant effect on non-Trp-containing LOs due to its amphiphilic property. Due to the prooxidative effects of both α- and γ-tocopherol on LOs that contain Trp, it has been suggested that tocopherols may repair Trp residue on LOs, leading to increased tendency of MetO residues to oxidize. The findings of this research are critical for elucidating the antioxidative mechanism of LOs, which can further lead to the establishment of strategies in suppressing bitter after taste to produce high-quality flaxseed oil.