Upcycling walnut (Juglans regia L.) by-products: Characterisation of nutritionally relevant bioactive compounds

Abstract

Background: Walnuts (Juglans regia L.) are attracting increasing interest from consumers, industry, and the healthcare community due to their high content of bioactive compounds, including (poly)phenols, carotenoids, tocopherols, and omega-3 fatty acids. Despite the continued growth in global walnut production, the effective valorisation of walnut processing by-products remains a challenge. While most research has focused on the edible kernel, emerging evidence suggests that underutilized walnut fractions may also represent valuable sources of compounds with antioxidant and anti-inflammatory properties. This study aimed to characterize the bioactive compound profiles of green walnuts, defective walnuts, and walnut shells in comparison to commercial whole walnuts and kernels. Methods: Samples collected from an orchard located in Huesca (Spain) were homogenized and analysed in triplicate. Proximate composition was performed following the official methods, fatty acid profile was determined by GC-FID, phenolic profile by UHPLC-ESI-MS/MS, and carotenoids and tocopherols by UPLC-DAD. Results: Green walnuts contained exceptionally high concentrations of carotenoids, especially β-carotene (629.7 ± 39.4 mg/kg). Walnut shells were the richest source of phenolic compounds, notably ellagic acid (713.3 ± 7.6 mg/kg) and catechin (71.9 ± 1.2 mg/kg). Tocopherols were mainly concentrated in the kernel, dominated by γ-tocopherol (226.9 ± 8.1 mg/kg), while defective walnuts showed a compositional profile similar to commercial walnuts. Conclusions: This comparative analysis highlights the significant potential of walnut processing by-products as sources of nutritionally relevant bioactive compounds. The findings support their valorisation as functional ingredients for agro-food and cosmetic applications within a circular economy framework.