Properties of silflower oil extracted with hexane
Unhulled silflower seeds were extracted with hexane at 55 °C, and for side-by-side comparisons, unhulled sunflower seeds were purchased and extracted under the same conditions. Table 1 shows the oil contents and properties of silflower oil in comparison with sunflower oil. The oil content in unhulled silflower seeds (15.2%, w/w) was slightly lower than that in unhulled sunflower oil (18.0%, w/w). TPC in silflower oil was significantly higher (12.3%, w/w) than sunflower oil (4.1%, w/w). It also had a higher content of total phenolics (1.12 mg GAE/g) than sunflower oil (0.46 mg GAE/g). In addition to the major components, triacylglycerols, plant oils contain polar lipids such as phospholipids and galactolipids, monoacylglycerols, diacylglycerols, free fatty acids, sterols and sterol derivatives (for example, phytosterols), tocopherols, carotenoids, chlorophylls, other phytochemicals (Zambelli et al., 2015). Apparently, silflower oil contained more of these compounds than sunflower oil. High total phenolics of silflower oil may be beneficial to its oxidative stability since many phenolic compounds are known to have antioxidant activity. It is also possible that silflower oil may contain large amounts of biologically active compounds that are beneficial to human health, which should be further studied. The ABTS assay is a widely used method to assess the radical scavenging ability of antioxidants in oil (Ilyasov et al., 2020; Re et al., 1999). The ABTS assay of the methanol extract of crude silflower oil was slightly, but not statistically significantly lower than that of crude sunflower oil despite having higher total phenolics content.
The fatty acid composition of silflower oil (Table 1) analyzed in this study is similar to those in previous studies (Evangelista et al., 2023; Kowalski & Wierciński, 2004) and that in sunflower oil indicating that this oil has potential as a new edible oil. As shown in Table 1, the linoleic acid (C18:2, c9,12) and oleic acid (C18:1, c9) contents in silflower oil (65.53 and 19.86%, respectively) were similar to those in sunflower oil (65.48% and 22.49%, respectively). Some minor fatty acid contents were different in the two oils. For example, contents of myristic acid (C14:0) and palmitic acid (C16:0) were higher in silflower oil while those of stearic acid (C18:0) and behenic acid (C22:0) were lower. Total contents of unsaturated fatty acids were similar in the two oils with 86.2% in silflower oil and 88.5% in sunflower oil.
It should be noted that silflower oil contained 4.34% (the peak area in GC) squalene. Since this finding is very important for the application of silflower oil as an alternative source of squalene, the isolation of squalene was conducted using column chromatography on silica gel. Squalene was isolated from silflower oil in the yield of 4.89% (w/w), and its structure was confirmed with 1H NMR in comparison with previous results (Shi et al., 2019). Squalene has antioxidant, anticancer, immune stimulating, and emollient activities, and its use in nutraceutical, cosmetic, pharmaceutical, and vaccine industries is growing (Rani et al., 2018). Shark liver oil is the most abundant source of squalene (up to 79%) and has been the most common source (Lozano-Grande et al., 2018). However, due to the international concern on overfishing as well as potential risks of polychlorinated biphenyls (PCBs), heavy metals, and other contaminants in the oil, consumers and industries are seeking alternative sources. Currently, the best plant sources of squalene are olive oil (0.15-0.75%) and amaranth oil (6-8%) (Gutiérrez-Luna et al., 2022; Lozano-Grande et al., 2018). Therefore, the high content of squalene in silflower oil indicates the potential application of this oil as a promising alternative source of squalene.
Analysis of all the tocopherols including α-, β-, γ, and δ-tocopherols was attempted, but their signals were overlapped with other unknown signals, and only the level of α-tocopherol could confidently be determined in this study. The level of α-tocopherol was lower in silflower oil (305.5 μg/g) than in sunflower oil (660.1 μg/g). Free fatty acid content in silflower oil (1.00%, w/w) was higher than in sunflower oil (0.47%, w/w). It contained a very large amount of chlorophyll (1002.8 mg/kg).