A Comparative Review on the Extraction, Antioxidant Content and Antioxidant Potential of Different Parts of Walnut (Juglans regia L.) Fruit and Tree

Abstract

As a valuable tree nut, walnut is a well-known member of the Juglandaceae family. The fruit is made up of an outer green shell cover or husk, the middle shell which must be cracked to release the kernel, a thin layer known as skin or the seed coat, and finally, the kernel or meat. The nutritional importance of walnut fruit is ascribed to its kernel. The shell and husk are burned as fuel or discarded away as waste products. In the past two decades, the evaluation of the phenolic content and antioxidant activity of different parts of walnut has received great interest. In this contribution, the recent reports on the extraction and quantification of phenolic content from each part of the walnut tree and fruit using different solvents were highlighted and comparatively reviewed. The current review paper also tries to describe the antioxidant content of phenolic extracts obtained from different parts of the walnut tree and fruit. Additionally, the antioxidant and antiradical activities of the prepared extracts have also been discussed.

Keywords: Juglandaceae; antioxidants; antiradical activity; flavonoids; phenols; walnut.

Figure 1

Botanical nomenclature of walnut fruit different parts.

Figure 2

Different parts of the walnut tree and fruit, as illustrated in the schematic image. Fruit, branch, leaf, shoot, and bark are considered as the main parts of the tree which have been comprehensively investigated. Walnut fruit is the most crucial portion of the plant because it contains the kernel. Walnut fruit is composed of five main parts: the outer green husk, the middle part as the shell and, finally, the kernel covered with a brown leathery layer as skin. Inside the nut, the seeds of walnut are separated from each other by a brown woody layer, namely, the pellicle (not shown here).