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

Biomass Conversion and Biorefinery https://doi.org/10.1007/s13399-023-04778-6 REVIEW ARTICLE Recent development in the application of walnut processing by‑products (walnut shell and walnut husk) Summaia Fordos1 · Namrah Abid1 · Muhammad Gulzar1 · Imran Pasha1 · Fatih Oz2 · Arashi Shahid1 · Muhammad Kashif Iqbal Khan1 · Amin Mousavi Khaneghah3 · Rana Muhammad Aadil1 Received: 6 June 2023 / Revised: 13 August 2023 / Accepted: 19 August 2023 © The Author(s) 2023 Abstract 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 WSderived 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. Keywords Walnut · Walnut Shell · Walnut Husk · Dyes · Biochar’s · Catalysts 1 Introduction Walnuts are one of the four most consumed dry fruits worldwide and are thus considered an important oil tree, economic and ecological tree species. Walnut fruit comprises three major parts, i.e., walnut kernel, walnut shell (WS), and walnut husk/hull (WH). Even though WSs and husks/hulls are considered processing waste, recent studies have shown * Imran Pasha * Amin Mousavi Khaneghah ; * Rana Muhammad Aadil 1 National Institute of Food Science and Technology, University of Agriculture, Faisalabad 38000, Pakistan 2 Department of Food Engineering, Faculty of Agriculture, Ataturk University, Erzurum, Türkiye 3 Department of Fruit and Vegetable Product Technology, Prof. Wacław Dąbrowski Institute of Agricultural and Food Biotechnology, State Research Institute, 36 Rakowiecka St, 02‑532 Warsaw, Poland many benefits of these by-products showing their potential in the medical, nutrition, agriculture, and industrial field. Mates et al. [1] studied the antioxidant capacity of the walnut septum and reported that the septal hydro-alcohol extracts showed a high antioxidant capacity compared to butylated hydroxyl toluene. The overall composition and application of WS and WH (Fig. 1), while the pictorial representation of WS composition and its application is given in Fig. 2. The WS is made up of lignin (52.3%), cellulose (25.5%), and hemicellulose (22.2%). The tough, non-toxic, recyclable, and renewable nature of the shell is made possible by its structural makeup. The shell and tissues that have stopped growing are supported by parenchymatous cells. The lignified cells that make up the secondary wall of shells are dense, robust, and hard. WSs can be used to create biological oil rich in organic compounds like phenol, ketone, and ester through pyrolysis. They may be used to develop biological carbon using modified manufacturing techniques which is a key component in creating active carbons with specific surface areas and high porosity [2]. WSs have unique application capabilities in producing supercapacitor electrodes and industrial wastewater treatment. Since WSs are tough and fragile, they can polish delicate 13 Vol.:(0123456789) Biomass Conversion and Biorefinery Fig. 1  The overall composition and application of WS and WH Fig. 2  WS Composition and its application equipment and passivate ultra-hard blades after superfine grinding. Additionally, after processing and employing specialized technologies, this shell may be utilized as a composite filler, industrial reductant, material for extracting biomass compounds, and a tool for separating oil from water [3]. 13 The greenish color covering of the walnut surrounding the fruit is known as WH. It is a by-product of the walnut processing business that, if used correctly, can bring many benefits and find its uses in various fields of life [4]. WH not only contains many important polyphenols and Biomass Conversion and Biorefinery phytochemicals but the chemical juglone present in the WH finds its use as a sustainable dyeing agent. Flavonoids are polyphenolic compounds with antioxidant, anti-inflammatory, and anti-carcinogenic properties [5]. Nutraceuticals have a range of benefits, including growth promoters, antibiotics, immunostimulants, antioxidants, flavorings, colorants, and natural substitutes for artificial compounds [6]. Li et al. [7] identified 75 phenolic compounds, including flavonoids, ellagic acid, and gallotanins, in the methanolic extract of the walnut septum. They reported that walnut septum contained 122.78 mg GAE/g dimers and trimers of procyanidins. Walnut septum extracted on human A172 glioblastoma cells. A 70 µg/ml dose significantly reduced the A172 cell viability without altering HFF-1 cell viability [8]. The pictorial representation of WH composition and its application is given in Fig. 3. This review aims to explore different by-products of walnut produced during processing and identify various biochemicals and components present in the WS and WH and their potential uses. Through the comprehensive research exploration, this review highlights the potential applications of these two by-products in different industries as ecofriendly materials and energy sources for pharmaceutical, nutraceutical, and agriculture applications. 2 Search study We thoroughly searched the literature in PubMed, Google Scholar, Science Direct, and Scopus for publication addressing the chemical composition and potential application of walnut by-products from 2017- 2023 (January). The keywords used for searching literature were ''walnut'', ''walnut husk'', ''walnut hull'', ''walnut shell'', ''walnut byproducts'', ''industrial application of walnut by-products'', "agriculture application of walnut husk", "agriculture application of walnut hull", "agriculture application of walnut shell", "agriculture application of walnut by-product", "medicinal application of walnut husk", "medicinal application of walnut hull", "medicinal application of walnut shell", "medicinal application of walnut by-product", "food application of walnut husk", "food application of walnut hull", "food application of walnut shell", "food application of walnut by-product", ''nutraceutical application of walnut (...truncated)