Recovery and antioxidant activity of phenolic compounds extracted from walnut press-cake using various methods and conditions

[Display omitted] •Phenolic compounds are extracted at levels of 15−29 mg gallic acid equivalent/gcake at best.•time and solid-solvent ratio are crucial parameters for phenols recovery.•walnut cake extracts significantly delay the oxidation of camelina oil. Walnut (Juglans regia L.) press-cake is a residue of oil processing from which the phenolic compounds were extracted using ethanol 60 % by batch and semi-continuous operations, or by ultrasound-assisted maceration in the case of de-oiled cake, with particular attention to the variables time and solid-solvent ratio. For batch extraction, the conditions maximizing total phenolic yield (TPC) and radical scavenging activity evaluated by 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay were 60 °C without the addition of phospholipids, provided however for phenolic yield, that an extraction time of 60 min and a ratio of 25 mL of solvent per g of cake were applied. Values of up to 14.8 mg gallic acid equivalent (GAE)/gcake and 136.9 μmol Trolox equivalent/gcake were obtained. The semi-continuous variant gave a TPC of 19.8 mg GAE/gcake and an overall extract containing 8% phenolic compounds and 46 % oil. After de-oiling, the cake was subjected to ultrasound-assisted extraction. The TPC and DPPH varied slightly with the applied conditions, between 10.4 and 13.3 mg GAE/gcake and 83–107 μmol Trolox Equiv./gcake respectively, unless the solid/solvent ratio increased to 1/200 (TPC of 29 mg GAE/gcake). It was also shown that the oxidative stability of Camelina sativa L. Crantz oil increased from 24 to 72 h by enrichment with walnut extract.