Evaluation the chemical composition, phenol, flavonoid and antioxidant activity of walnut skin extract (Juglans regia L.)

Today, consumers are more inclined to use healthy and natural additives. Phenolic compounds obtained from natural products are a good option to minimize lipid oxidation. Walnut is one of the most important nuts that is widely cultivated all over the world. Due to the fact that the green skin of the walnut contains about 64% of the wet weight of the walnut fruit, an amount equal to 240 thousand tons of green skin of the walnut is produced from this fruit every year. The walnuts used in this study were purchased in 1400 from the gardens of Saman city located in Chaharmahal and Bakhtiari province. After powdering the dried green peel, extracting with methanol solvent was done by Soxhlet method. In order to identify chemical compounds and functional groups, GC-MS and FTIR tests were performed. Also, the amount of phenol and flavonoids in the extract of walnut skin was measured. Finally, the antioxidant activity of green walnut skin extract was investigated by measuring the reduction of radical capacity with the help of 2-2-diphenyl-1-picrylhydrazyl (DPPH). A total of 89 compounds were identified, representing 99.9% of the total compounds in the extract. The most important identified compounds that had the highest area under the curve included polyphenols, organic compounds and phytosterols. FTIR results indicate the presence of hydroxyl, alkane and aromatic functional groups of lignin. In the present study, the amount of phenolic compounds in walnut peel extract was 96.07±0.22 (mg of gallic acid per gram of dry weight of the sample). Also, the flavonoid compounds of green walnut skin extract were 349 μg/g of quercetin. EC50 values obtained in this study were equal to 0.15. The results showed that the methanolic extract of green walnut skin can be introduced as a substitute for synthetic antioxidants.