Role of the Green Husks of Persian Walnut (Juglans regia L.)—A Review

Abstract

Green husks are the outer layer of walnut fruits. They form part of the agro-residues discarded away upon nut maturity in the walnut industry. Although it is not used, research studies show that green husk is a rich source of natural bioactive phytochemicals. A total of 83 individual phenolic compounds were identified in walnut husks, mainly consisting of naphthoquinones, flavonols, and hydroxycinnamic acids. There is no standard profile of phenolic compounds in walnuts; the quantity and quality of phenolic compounds vary by cultivar. Walnut extracts exhibit strong antioxidant activities and play an important role in the plant’s defence mechanisms against diseases, attacking different plant organs. The study provides a review of total phenolic content and individual phenolic compounds identified in green husks of different walnut cultivars as documented in different studies. It also explores the role and application of green husks in various industries such as traditional medicine, textile, wood, leather, beverage, and agriculture. In addition, the effects of phenolic compounds on biotic and abiotic factors are also evaluated.