The purpose of this research was to improve the properties of functional edible oils with potential health promoting effects, enriched with phenolic-rich extracts obtained from pistachio and walnut (5.1 and 27.4% phenolic contents respectively), by means of emulsion and micro emulsion systems. Stable water-in-oil (W/O) emulsions were obtained employing polyglycerol polyrhizinoleate (PGPR) as emulsifier (0.5, 2% H2O in oil), despite having a whitish and opaque appearance; transparent and stable microemulsions were prepared using proper proportion (e.g., 97:3) between the oily phase and the mixture of aqueous phase and emulsifiers (3:2 lecithin-distilled monoglycerides (DMG). Total polar phenolics contents ranging between 257 and 835 mg/kg were obtained in the novel functional edible oils' formulations, reaching higher content using walnut as compared to pistachio extracts. Antioxidant capacity determined by the 2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazyl (DPPH) method increased approx. 7.5 and 1.5 times using walnut and pistachio extracts respectively. An emulsion using gallic acid and a microemulsion employing hydroxytyrosol, two well-known antioxidants, were also studied to compare antioxidant capacity of the proposed enriched oils. Furthermore, the oxidative stability of these products-very relevant to establish their commercial value-was measured under accelerated testing conditions employing the Rancimat equipment (100 °C) and performing an oven test (at 40 °C for walnut oils and 60 °C for pistachio and refined olive oils). Rancimat oxidative stability greatly increased and better results were obtained with walnut (2-3 times higher) as compared to pistachio extract enriched oils (1.5-2 times higher). On the contrary, under the oven test conditions, both the initial oxidation rate constant and the time required to reach a value of peroxide value equal to 15 (upper commercial category limit), indicated that under these assay conditions the protection against oxidation is higher using pistachio extract (2-4 times higher) than walnut's (1.5-2 times higher). Stable emulsions and transparent microemulsions phenolic-rich nut oils (250-800 mg/kg) were therefore developed, possessing a higher oxidative stability (1.5-4 times) and DPPH antioxidant capacity (1.5-7.5 times).
Keywords: antioxidant capacity; functional oils; phenolic extracts; pistachio; walnut.