Walnuts (Juglans regia) are a source of fat (oil) known for its distinctive nutty flavour, and it is a versatile ingredient in culinary applications. Walnut oil (WO), rich in mono- and polyunsaturated fatty acids (MUFAs and PUFAs), undergoes oxidation reactions and produces undesirable flavours when exposed to light and air. Commercially available WO is stored in glass bottles and kept under ambient temperature and fluorescent light conditions. The shelf life for walnut oil under such conditions is relatively short. This study compares the properties of oil that has been freshly extracted from mature kernels with commercial walnut oil just before its expiration date.
Oil extraction from nuts was performed using Soxhlet apparatus. The fatty acid composition determined using gas chromatography confirmed that unsaturated fatty acids are predominant in the composition of WO, with linoleic (58.0-59.9 %) and oleic (16.5-18.2%) acids being present in high amounts. The acid (AVs) and peroxide (PVs) values were evaluated using potentiometric and iodometric titrations in correspondence with ISO standards. The AV was slightly higher for the commercial than the extracted oil (2.85 mg KOH/g and 2.24 mg KOH/g, respectively). A similar trend was observed for peroxide values. The total phenolic content in the sample was quantified using Folin–Ciocalteu's reagent. Thermal analysis of WO was performed using pressure differential scanning calorimetry (PDSC). The maximum oxidation time (tmax), the time to reach the maximum heat flow, varied from 209 to 20 min for different experimental temperatures (100 to 130 °C) and was much lower than for the oil from hazelnuts.
A comparison of the data obtained for freshly extracted and commercial walnut oil does not show significant changes, proving that the cold-pressed oil purchased and used before the expiration date meets the requirements and maintains the quality after a few months of storage on the shelf.
