Phenolic extracts from colored-walnut pellicles: antioxidant efficiency in walnut oil preservation

Abstract

The phenolic extracts of walnut cultivars with a red pellicle (HWE), a purple pellicle (ZWE) and a yellow pellicle (WWE) were compared. The total contents of phenolics, flavonoids and condensed tannins, as well as in vitro antioxidant activities (ABTS and DPPH) of the phenolic extracts were evaluated. The results showed that 25 phenolic compounds were identified in three colored walnuts. The HWE contained the highest contents of total phenolics and condensed tannins, while the ZWE contained the highest content of flavonoids. The DPPH and ABTS assays indicated that the phenolic extracts of colored-walnut pellicles showed a higher free radical scavenging ability than butylated hydroxytoluene (BHT) at the same concentration. Phenolic extracts of colored walnuts were also evaluated as natural antioxidants in walnut oil, substituting for BHT. The efficacy of phenolic extracts in walnut oil preservation was evaluated using accelerated storage (60 °C) for 12 days. After accelerated storage, walnut oil enriched with the phenolic extracts kept better qualities, with significantly lower peroxides, p-anisidine, and TOTOX values. The volatiles were measured and identified using the headspace solid phase micro-extraction gas chromatography to further assess oil quality. Aldehydes such as hexanal and 2-heptenal were identified as representative oxidative volatiles. Hexanal and 2-heptenal content indicated that adding phenolic extracts could prevent flavor deterioration of the oil. This study suggested that phenolic extracts of walnut pellicles could offer an effective alternative to synthetic antioxidants for walnut oil preservation.

Keywords

• Phenolic extracts
• Walnuts
• Walnut pellicle
• Walnut oil