Thermal treatment and storage condition effects on walnut paste quality associated with enzyme inactivation

- Initial PV and FFA of walnut paste increased with treatment temperature and time.
- PV and CDV of walnut paste were reduced while FFA increased during storage time.
- Walnut paste was more stable than kernels and cold-pressed oils during storage.
- FFA should be used as a key parameter for heated walnut quality evaluation.

In this study, walnut paste was heated at 60, 75, and 90 °C with three selected times for enzyme inactivation, and its quality stability was determined under accelerated storage conditions by evaluating peroxide value (PV), conjugated diene value (CDV), fatty acid value (FFA) and fatty acid composition. The results showed that the mean PV, CDV and FFA values of the heated paste samples increased with increasing treatment temperature and time. The PV and CDV were reduced or stable, while the FFA values increased significantly during storage period. However, no significant quality difference was observed between controls and heated walnut paste samples with vacuum package after accelerated storage for 20 days. Compared with unheated walnut kernels and cold-pressed oils in vacuum or normal package under same storage conditions, the mean PV of the unheated paste were the lowest, while its mean FFA values were the highest. FFA values of the paste were still lower than the acceptable range (FFA < 0.6%) used by industry for good walnut quality, suggesting that vacuum or normal packages can be used to keep quality of walnut paste for 20 days at 35 °C simulating 2 years of storage at 4 °C or 2 months at 25 °C.