Discriminating the deterioration of walnut kernels stored at different relative humidities by GC-MS coupled with chemometrics

To explore the effects of relative humidity (RH) on the quality of walnut kernels and establish a rapid, effective method/model for identifying their deterioration degree, walnut kernels were stored at 45 ℃ for 90 days under different RH conditions (35%, 50%, 65% and 80%) in this study. Every 15 days, changes in the kernels' color, acid values (AV), peroxide values (POV), fatty acid composition, contents of total phenols and soluble quinones, synchronous fluorescence spectra, and the compositions/contents of volatile organic compounds (VOCs) were analyzed. Partial least squares discriminant analysis (PLS-DA) and variable importance in projection (VIP) were used to conduct differential analysis of VOCs. The deterioration degree of walnut kernels was predicted using Pearson correlation analysis and a Back Propagation Neural Network (BPNN) model. The results showed that RH had a significant effect on the quality of walnut kernels, with 65% RH being the suitable storage condition for them. According to gas chromatography-mass spectrometry (GC-MS) analysis, a total of 40, 34, 23 and 17 characteristic VOCs were identified in the walnut kernels stored at RH of 35%, 50%, 65% and 80%, respectively. Among these VOCs, hexanal, 1-octen-3-ol, 4,5-dimethyltetrahydrofuran-2-one and DL-pantolactone were identified as potential volatile deterioration markers (PVDMs). Based on the POV limit standard of 1.0 mmol/kg for walnut oil, the threshold concentrations of these four PVDMs were 500-1000, 50-100, 10-15, and 30-60 μg/100g, respectively. This research provides a reference for the quality monitoring and evaluation of walnut kernels during storage.