Potential of walnut (Juglans regia L.) oil by-products as functional food ingredients: antioxidant and prebiotic properties post the simulated gastro-intestinal tract digestion

The oilcake and oil decantation dregs are primary by-products of the walnut oil industry, currently being repurposed as fertilizers or animal feed1. This study explores their potential application as food ingredients by assessing their bioactivity, specifically antioxidant and prebiotic properties, both pre- and post-simulated gastrointestinal tract (GIT) digestion according to the INFOGEST protocol. Antioxidant activity was evaluated using two in vitro assays: inhibition of lipid peroxidation measured by thiobarbituric acid reactive substances (TBARS) and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging. The prebiotic potential of the samples at a 2% concentration was tested with five probiotic strains. When comparing the EC50 values of the bioaccessible fractions (BF) obtained post-GIT, both the digested oilcake and dregs exhibited enhanced antioxidant activity (an increase of 86% and 84% for the oilcake and 38% and 55% for dregs, in TBARS and DPPH, respectively) compared to the corresponding initial samples. Furthermore, the by-products BF stimulated probiotics growth, surpassing the positive control (fructooligosaccharides) and the samples not subjected to GIT, which underscores the potential of these by-products for gut health. Overall, the bioactivity of the studied by-products was even improved by digestion, which highlights their suitability as functional foods.