Comprehensive characterization of walnut oil processing by-products: biochemical composition, bioactive properties, and polyphenol in vitro bioaccessibility and bioavailability.

Underutilized by-products from the walnut-oil industry, namely walnut oilcake (WOC) and walnut oil dregs (WOD), were evaluated for their nutritional composition, phenolic compound profile and digestive behaviour, as well as bioactive properties (antioxidant, antimicrobial, anti-inflammatory, cytotoxic and prebiotic activities). WOC was rich in protein (38.1 g/100 g) and dietary fiber (32.6 g/100 g), while WOD presented high fat (46.8 g/100 g) and carbohydrate content (20.9 g/100 g). Glansreginin A was the predominant phenolic compound in both matrices. Following in vitro digestion using the INFOGEST protocol, higher overall polyphenol bioaccessibility was noticed in WOD (78%) compared to WOC (15%). Bioaccessible fractions exhibited higher antioxidant activity than the undigested samples. Glansreginin A was detected only on the cellular apical compartment suggesting the absence of transport across Caco-2 cells. After in vitro digestion, the non-bioaccessible fractions enhanced the growth of Lactobacillus and Bifidobacterium strains, in some cases surpassing fructooligosaccharides, a standard prebiotic. These findings support the valorisation of walnut by-products as functional ingredients, also contributing to sustainable food systems.