*2.9. Inhibition of Hazelnut Oil Oxidation Assay*

The oil-in-water emulsion was prepared according to a previous study. To obtain oil without any antioxidants, hazelnut crude oil was stripped by silica gel, activated charcoal, and sucrose [28]. The aqueous phase of the emulsion was prepared by dispersing 0.5 wt% Tween 20 in 10 mM phosphate buffer at pH 7.0 followed by stirring at room temperature for 20 min to ensure complete dispersion. Hazelnut oil-in-water emulsions were prepared by homogenizing 10 wt% oil phases with 90 wt% aqueous phases at ambient temperature using a high-speed blender for 2 min, followed by ultrasonic vibration for 20 min. Peptides or TBHQ (tert-Butylhydroquinone, as positive control) were added to this emulsion with a final concentration of 0.02% to test the inhibition ability of peptides against oil. Sodium azide (NaN3, 0.02% (*w*/*w*)) was used as an antibacterial agent.

Incubation temperature for the oil-in-water emulsion system was generally set at 37 ◦C [29] or 40 ◦C [30]. In this work, a longer incubation time was required for analyzing the possible oxidative products of oil-in-water emulsion compared to that of linoleic acid oxidation, so an incubation temperature of 37 ◦C was selected. During incubation at 37 ◦C for 14 days, the levels of lipid hydroperoxides as primary lipid oxidation products were monitored at regular intervals to assess the degree of oxidation of hazelnut oil. In brief, 0.3 mL of emulsions were breakdown by 1.5 mL of isooctane/2-propanol (3:1, *v*/*v*), vortexed, and then centrifuged at 1000× *g* for 2 min. Next, 200 μL of organic solvent phases were collected and mixed with 2.8 mL of methanol/1-butanol (2:1, *v*/*v*), followed by 15 μL ferrous iron solution (prepared by mixing 0.132 M BaCl2 and 0.144 M FeSO4, dissolved in 0.4 M HCl), and 15 μL ammonium thiocyanate solution (3.94 M, dissolved in water). The absorbance was measured at 510 nm after 20 min. Lipid hydroperoxides (μmol/g oil) were calculated using a standard curve prepared by cumene hydroperoxide (0, 20, 40, 80, 160, 300, and 400 μM) [31].

## *2.10. Statistical Analysis*

IBM SPSS 26.0 software (IBM Corporation. Armonk, NY, USA) was used for statistical analysis between groups. The data are expressed as the mean ± standard deviation (*n* = 3).
