Recent development in the application of walnut processing by-products (walnut shell and walnut husk)

Walnut is among the four most consumed dry fruits around the globe. Apart from the edible walnut kernel, walnut fruit consists of a walnut shell (WS) and walnut husk/hull (WH), usually discarded in walnut processing and consumption. These walnut by-products are filled with beneficial compounds that find their use in different fields. This review summarizes recent developments and research on functional aspects of walnut waste (shell and husk/hull) in various fields. WS has many important bioactive compounds, including lignin, cellulose, oleic, and palmitic acids. The creation of WS and carbon-based materials, such as activated carbons and unmodified/modified WS, as adsorbents have been explored. Possible uses for WS-derived by-products include all-natural but powerful adsorbents for eliminating hazardous substances, such as heavy metals, dangerous compounds, and synthetic industrial colors. Similarly, WH also has many beneficial compounds like juglone. WH has antioxidant properties and can be used as textile and protein strainers. These wastes are used in agriculture, laboratory, medical, and food industries, which can be employed as sustainable and environment-friendly alternatives.