Research on the physicochemical and characteristic odor indices during the oxidation process of peeled walnut kernels

Peeled walnut kernels, rich in lipids, are prone to oxidation, resulting in undesirable odors such as rancidity and pungency due to the formation of aldehydes, ketones, and acids. This study examined kernels oxidized at 60°C over varying periods, assessing odor profiles through sensory evaluation and principal component analysis (PCA) using a gas-phase electronic nose. Threshold values for physicochemical indices such as acid value (AV), peroxide value (POV), p-anisidine value (p-AnV), total oxidation value (TOTOX), and malondialdehyde (MDA) content, were determined based on correlation analysis and oxidation kinetic models. Headspace solid phase microextraction-gas chromatography-mass spectrometry (HS-SPME/GC-MS) was utilized to analyze the volatile components of peeled walnut kernels during different oxidation times, and characteristic odor indices were determined by combining relative odor activity value (ROAV) and partial least squares regression (PLSR). The oxidation process was categorized into five stages (0–10, 15, 20, 25, and 30 days), with significant quality deterioration noted at 15 days, marked by a consumer acceptance score below 6 and dominant rancid odors. The determined threshold values were 0.9 mg/g for AV, 1.5 mmol/kg for POV, 0.9 for p-AnV, 6.0 for TOTOX, and 3.5 μg/mL for MDA, respectively. Volatile compounds such as (E,E)-2,4-decadienal, hexanal, and (E)-2-octenal were identified as key contributors to the odor profile. Among these, hexanal and (E)-2-octenal demonstrated strong predictive accuracy for POV and p-AnV, with threshold values were 1.8 and 0.9 mg/kg, respectively. This indicates that physicochemical indicators reached the threshold values at the same time as pronounced lipid oxidation and rancidity.