By-Products of Walnut (Juglans regia) as a Source of Bioactive Compounds for the Formulation of Nutraceuticals and Functional Foods

The scarcity of natural resources and higher incidence of diseases related to inappropriate eating habits have prompted the search of food and nutraceutical products with functional properties that are also respectful of the environment. Agro-industrial by-products are a profitable source for obtaining bioactive compounds, with various biological properties, including antioxidant, anti-inflammatory, and anticancer properties, which contribute to immunity and reduce the negative effects of infections, inflammation, and oxidative stress. In the case of the walnut (Juglans regia) oil industry, various by-products are generated, among which we can mention pomace, green shell, shell, skin and leaves. Therefore, there is an opportunity for the recovery of waste, the recovery of target molecules and the formulation of new products, whether they are nutraceuticals, pharmaceuticals, or food additives, contributing to the circular economy and consumer health. Walnut is commonly characterized by its high content of lipids (58–65%), mainly polyunsaturated fatty acids, tocopherols and phytosterols. In addition, the current literature states that its by-products are rich in phenolic compounds, mainly phenolic acids and flavonoids. In accordance, the antioxidant potential of different extracts of nuts, shells and leaves has bee studied using different methods such as the reduction potency assay, scavenging effect of DPPH (2,2-diphenyl-1-picrylhydrazyl) and inhibition of lipid oxidation by β-carotene linoleate system, with EC50 values less than 1 mg/mL. The results obtained showed that all walnut extracts have a strong antioxidant capacity against ROS species. For this reason, this work focuses on the bibliographic review of the bioactive compounds present in the by-products of the walnut industry, as well as mentioning their biological properties and possible applications in the food sector.