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.