INTRODUCTION
Perennial crops are known to have many advantages over annual counterparts such as deeper rooting depth, better drought tolerance, and less tillage required (Crews & DeHaan, 2015; Glover et al., 2010). They can reduce soil erosion, increase nutrition retention, increase carbon sequestration, and enhance agricultural sustainability (Crews & DeHaan, 2015; Schiffner et al., 2020). Recently, Silphium integrifoliumMichx. (silflower), a perennial plant in the sunflower family native to the central United States, has drawn great interest as a potential oilseed crop, and studies have been conducted to domesticate crops and to understand the properties of crops, seeds, and seed oil (Evangelista et al., 2023; Price et al., 2022; Van Tassel et al., 2014; Van Tassel et al., 2017).
Silflower seeds are known to be high in protein (33.53%), fat (22.05%), and fiber (22.07%) contents (Kowalski & Wierciński, 2004), which are similar to sunflower (Helianthus annus L.) seeds. Fatty acid composition of the silflower seed oil with linoleic (62.3-63.0%) and oleic (18.7-19.6%) acids as the major fatty acids is close to that of sunflower oil (Evangelista et al., 2023; Kowalski & Wierciński, 2004). Although dehulling is challenging for silflower seed, studies found that its high-protein meal had potential for food and industrial applications (Evangelista et al., 2023).
Since silflower has many advantages as a crop and its oil has a great potential in the food industry, more studies are needed to utilize it as an oilseed crop. Especially, no study has been conducted on the oxidative stability of the seed oil although oxidative stability is one important property of edible oils. Silflower oil has high contents of unsaturated fatty acids, which are beneficial to human health but susceptible to oxidation. Oxidative stability of oil not only depends on the fatty acid composition, but also the inherent antioxidants in oil (Madhujith & Sivakanthan, 2019). Therefore, it is also important to study compounds other than triacylglycerols in an oil and their effects on the oxidative stability of the oil.
In this study, oil extracted from unhulled silflower seeds was analyzed for total phenolics, 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) assay, total polar compounds (TPC), free fatty acids (FFA), tocopherols, and chlorophyll, and its oxidative stability was compared with the oil extracted from unhulled sunflower seeds. Oxidative stability index (OSI) and thermal stability measured by thermogravimetry analysis (TGA) were used to evaluate the oxidative stability of oils. Since the oxidative stability of oil can be affected by refining processes, which change the levels of antioxidants such as tocopherols and prooxidants such as free fatty acid and chlorophyll, different refining processes were evaluated to improve the oxidative stability and appearance of silflower oil.