## 4 Materials and Methods
### Test Conditions for GC-MS Analysis  
The tuna peptide powder with the best sensory evaluation after ozonation was selected for volatile compound analysis using GC-MS. Volatile compounds from the untreated and ozonated tuna peptide samples were analyzed by GC-MS and identified through library matching and retention index confirmation. Specifically, we compared each mass spectrum to entries in established spectral databases (the NIST/EPA/NIH Mass Spectral Library and the Wiley Registry) to assign tentative compound identities [47]. Headspace solid-phase microextraction (HS-SPME) was employed for sample pre-treatment. Briefly, 0.2 g of the sample, 1.44 g of NaCl, and 8 mL of deionized water were mixed, transferred to a headspace vial, and sealed. The vial was incubated at 60 \({}^{\circ}\)C with agitation for 15 min to reach equilibrium, followed by adsorption using an activated DVB/CAR/PDMS fiber for 40 min. The extracted volatiles were then desorbed at the GC injection port at 250 \({}^{\circ}\)C for 5 min. GC conditions: An HP-5 MS capillary column (30 m \(\times\) 0.25 mm \(\times\) 0.25 \(\upmu\)m) was used, and the temperature of the KC interface was maintained at 240 \({}^{\circ}\)C. He (99.99%) was used as the carrier gas at a flow rate of 1.0 mL/min without diverting. The initial temperature of the column was set at 40 \({}^{\circ}\)C, and the holding time was 2 min. The temperature of the machine was raised to 160 \({}^{\circ}\)C at 4 \({}^{\circ}\)C/min, increased to 250 \({}^{\circ}\)C at 8 \({}^{\circ}\)C/min, and maintained for 10 min. MS conditions: Solvent removal time, 1 min; ion source, electron bombardment source (EI); ion source temperature, 230 \({}^{\circ}\)C; ionization voltage, 70 eV; scanning mass range, 30-500 \(m\)/\(z\); interface temperature, 280 \({}^{\circ}\)C; four-stage bar temperature, 150 \({}^{\circ}\)C. Analysis conditions: carrier gas: high-purity helium (purity \(\geq\) 99.999%), in constant flow mode, with no sample splitting, and a column flow rate of 1.0 mL/min; injection port temperature at 250 \({}^{\circ}\)C; ion source temperature at 230 \({}^{\circ}\)C; quadrupole temperature at 150 \({}^{\circ}\)C; programmed temperature conditions: maintained at 40 \({}^{\circ}\)C for 2 min, then ramped at 30 \({}^{\circ}\)C/min to 180 \({}^{\circ}\)C, and finally ramped at 10 \({}^{\circ}\)C/min to 270 \({}^{\circ}\)C for 3 min. SCAN mode is employed, covering a scanning range of 45-450 u. Solid phase microextraction conditions: extraction head 30/50 \(\upmu\)m DVB/CAR/PDMS, extraction duration of 40 min. Quantitative ion: 57 [47; 48]. For additional validation, an alternative extraction method was conducted by dissolving 2.0 g of pre-baked NaCl in 10.0 mL of the sample within a 20 mL headspace vial. The mixture was incubated at 50 \({}^{\circ}\)C with agitation at 450 rpm for 40 min before GC-MS analysis. The column temperature program included an initial hold at 40 \({}^{\circ}\)C for 2 min, followed by a ramp at 30 \({}^{\circ}\)C/min to 180 \({}^{\circ}\)C, and a final increase at 10 \({}^{\circ}\)C/min to 270 \({}^{\circ}\)C for 3 min. Retention times and parameters for key olfactory compounds and internal standards are summarized in Table 6[49]. To evaluate the sensory impact of individual volatile compounds identified by GC-MS, the Odor Activity Value (OAV) was calculated [50]. The OAV for each target aldehyde was determined using the formula: OAV = C/OT, where 'C' is the concentration of the compound measured by GC-MS and 'OT' is the odor threshold of that compound, typically in water. Odor threshold values for n-hexaldehyde, nonaldehyde, and octanal were obtained from published literature and are listed in Table 5[51]. A compound is generally