Phytochemical Content, Minor‐Constituent Compositions, and Antioxidant Capacity of Screw‐Pressed Walnut Oil Obtained from Roasted Kernels

Abstract

The screw‐pressed walnut oil produced from unroasted kernels and roasted kernels subjected to roasting at 140, 160, and 180 °C for 5, 10, and 15 min is analyzed and compared. The effects of roasting on the chemical compositions (acid value, peroxide value, fatty acid composition, tocopherols, phytosterols, squalene, and polyphenols), oxidative stability index (OSI), and free radical scavenging capacity (via DPPH, ABTS, FRAP, and ORAC assays) of walnut oil is investigated. It is observed that roasting yields slight changes in the acid values and fatty acid contents of walnut oils. Upon roasting walnut kernels at 180 °C for 10 and 15 min, the peroxide values (5.10 and 5.87 mmol kg−1) of oil are significantly increased. Roasting is observed to cause an increase in the amount of tocopherols (470.16–508.68 mg kg−1), phytosterols (1740.91–2048.05 mg kg−1), and polyphenols (30.08–44.82 mg GAE/kg) in the oil whereas the level of squalene (3.17–5.90 mg kg−1) decreases. In addition, the oxidative stability and antioxidant capacity of walnut oil increases with roasting, which could increase the amount of natural antioxidants in walnut oil, and produce Maillard reaction products with antioxidant effects.Practical Applications: Cold‐pressed oil is a good source of bioactive components and is favored by consumers. Screw‐pressed walnut oil obtained from roasted kernels has higher amounts of micronutrients, such as tocopherols, polyphenols, and phytosterols. These compounds exhibit strong antioxidant properties and thus are considered as health‐promoting substances. In effect, roasting is a suitable cold‐press pre‐processing method to produce walnut oil for application in healthcare.An analytic comparison is made of cold‐pressed walnut oil produced from unroasted kernels and roasted kernels that had previously undergone roasting. The qualitative and quantitative effects of roasting temperature and duration on the content of natural antioxidants in walnut oil are determined. Roasting walnut oil has significantly higher oxidative stability and free radical scavenging capacity than unroasted walnut oil.