Background and Objectives: Walnut green husk is one of agricultural residues that can be considered as natural compound with biological properties because of its phenolic compounds. In the present work, the goal is extraction of aqueous extract of green walnut skin and studying the effect of extraction temperature on the phenolic compounds and antioxidant and antimicrobial properties of extract. Meanwhile, the effects of spray dried encapsulation process of extract in maltodextrin on the biological activity of extract are investigated.

Materials and Methods: In this study, walnut green husk (Sozani variety) was extracted in two temperatures of 25 and 80 °C. Then it was encapsulated in 10% maltodextrin in three different concentrations of extract (0.5, 1 & 2 mg/ml) using spray drying method. Total phenol of the extract was evaluated before and after encapsulation. In addition, the antioxidant activity of the aqueous extract of walnut green husk was determined before and after microencapsulation by the scavenging effect on DPPH (2,2-diphenyl-1-picrylhydrazyl) radicals. Antimicrobial activity of the extract was determined by Agar well Diffusion Method against gram positive (Bacillus cereus, staphylococcus aureus) and Gram negative bacteria (Escherichia coli, Salmonella Typhimurium).

Result: It was revealed that the effect of extraction temperature on total phenolic compounds was significant (p<0.05). The samples extracted in 80°C had more phenol components (39.77 mg GAE/g) than the samples extracted in room temperature (28.88 mg GAE/g). Encapsulation efficiency was increased by increasing the extract concentration. The 2 mg/ml concentration of free and microencapsulated extract obtained at 80 °C had the highest scavenging effect on the DPPH radicals (85.05 & 81.05%, respectively). In addition, 2 mg/ml concentration of free and microencapsulated aqueous extracts of walnut green husk showed the highest inhibitory effect against Gram-positive bacteria. The results further showed no significance differences in the antioxidant and antimicrobial activities of free and microencapsulated aqueous extracts of walnut green husk.

Conclusion: Sozani walnut green husk can be used as a cheap source of antioxidant and antimicrobial compounds due to having high amount of phenolic compounds. Therefore, microencapsulation can be used as an efficient method for increasing the stability of walnut green husk extract against the environmental conditions.

Keywords: Walnut green husk, Aqueous extracts, Antimicrobial activity, Antioxidant activity, Microencapsulation