Introduction
Osage orange (Maclura pomífera (Raf.) Schneid., family Moraceae) is a
tree native to Texas, Oklahoma and Arkansas and is also known as horse
apple or hedge apple tree (Nečas et al., 2006). This tree is common
throughout the midwestern and southwestern regions of the United States
and also is grown in other parts of the world (Filip et al., 2015). Its
use, however, is limited as a hedge, hardwood and an insect repellant
around homes (Moser et al., 2011). Some studies were conducted to
utilize it for industrial use such as composites (Tisserat &
Harry-O’kuru, 2019). Extracts from fruit and other parts of the tree are
of great interest due to biological activities such as anti-inflammatory
and antinociceptive (Kupeli et al., 2006), antifungal (Peterson &
Brockemeyer, 1953), cytotoxic (Jones & Soderberg, 1979), antimicrobial
(Mahmoud, 1981), anti-tumor (Voynova et al., 1991), estrogenic (Maier et
al., 1995), antiviral (Bunyapraphatsara et al., 2000), and antimalarial
activities (Hay et al., 2004). Among a variety of phytochemicals found
in the Osage orange fruit, the bioactivities of two major isoflavones,
osajin and pomiferin, have been intensely studied. Osajin and pomiferin
were shown to have anti-inflammatory
(Abourashed et al., 2015),
antidiabetic (Bartošíková et al., 2008) and cardioprotective (Florian et
al., 2006; Nečas et al., 2006) activities. Pomiferin has also been shown
to have anticancer (Yang et al., 2011) and antiulcer (Bozkurt et al.,
2017) activities, to promote recovery of kidney functions (Bartošíková
et al., 2010), and to inhibit intracellular oxidative stress (Abourashed
et al., 2015). Other bioactive components in Osage orange fruit extracts
that have been identified include scandenone, auriculasin (Kupeli et
al., 2006) and other prenylated flavonoids (Orazbekov et al., 2018).
Edible oils such as vegetable oils and omega-3 rich oils and
oil-containing food products generally require antioxidants to prevent
their oxidation during manufacturing, transportation and storage.
Synthetic antioxidants such as butylated hydroxyanisole (BHA), butylated
hydroxytoluene (BHT) and tert -butylhydroquinone (TBHQ) have been
employed for over 60 years to prevent oil oxidation (Shahidi &
Ambigaipalan, 2015). However, several studies reported that these
synthetic antioxidants may be toxic and could cause liver problems and
cancer (Khan et al., 2014). Therefore, the use of these compounds in
foods is strictly regulated by State and Federal agencies. For this
reason, the food industry is seeking natural antioxidant replacements
that have comparable activity to these synthetic counterparts.
Studies have shown that Osage orange fruit extracts and components in
them have antioxidant activity. Pomiferin supported defensive reactions
of the body against free radicals and decreased lipid peroxidation
(Bartošíková et al., 2010; Bartošíková et al., 2007). Pomiferin and
osajin had inhibition activity on lipid peroxidation in the rat liver
microsomal fraction and scavenging ability for peroxynitrite and
2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH)
radicals (Veselá et al., 2004).
Orhan et al. (2016) also reported that pomiferin and osajin had DPPH
radical scavenging ability, metal chelating capacity, ferric-reducing
antioxidant power (FRAP), and phosphomolibdenum reducing antioxidant
power (PRAP). Studies by Veselá et al. (2004) and Orhan et al. (2016)
found that pomiferin had higher antioxidant activity than osajin, which
is attributed to the additional hydroxyl group of pomiferin at the ortho
position that stabilizes the phenoxy radical. Increased antioxidant
activity in compounds with an additional hydroxyl group at the ortho
position was also been observed in other antioxidant systems such as
lignans (Hwang et al., 2012).
The previous studies, however, were in vitro antioxidant activity
studies, which provide limited information about the antioxidant
activity on edible oils. Although radical scavenging ability, reducing
power, and metal chelation are the major antioxidant mechanisms, thesein vitro methods often correlate poorly with the actual
protection ability of an antioxidant because the test methods cannot
reflect all of the environmental conditions of actual oil storage
(Decker et al., 2005). Only a few studies have been conducted on the
antioxidant activity of Osage orange fruit extracts in an oil or fat,
but these studies were conducted at considerably higher temperatures
(100-125 ℃) than the typical storage temperature (Clopton, 1953; Schall
& Quackenbush, 1956). The oxidation mechanism and kinetics are
different at different temperatures and, therefore, Decker et
al. (2005) recommended using
temperatures lower than 60 °C for storage studies. Budincevic & Vrbaski
(1991) and Hamed & Hussei (2005) conducted studies on the antioxidant
activity of Osage orange fruit extracts at 55-60 ℃ in linoleic acid
emulsions and in purified sunflower triacylglycerols, respectively.
However, they used only one indicator of oxidation that measures the
concentration of hydroperoxides. Hydroperoxides are primary oxidation
products, which are formed in the early stage of oil oxidation and
decompose or react with other compounds to produce secondary products
including aldehydes, ketones, alcohols, carboxylic acids, dimers, and
polymers. Although the concentration of hydroperoxides is a good
indicator at the early stage of oil oxidation, it cannot provide
information after the value reaches a peak value. For this reason, both
primary and secondary oxidation products should be measured for a better
assessment of oil oxidation (Decker et al., 2005;
Pignitter & Somoza, 2012).
In this study, hexane was used as a solvent to extract Osage orange
fruit. The precipitate in the hexane extract, which was collected by
filtering, had high contents of osajin and pomiferin. The precipitate,
referred to as Osage orange fruit extract (OOE), was evaluated for its
antioxidant activity at 25 ℃ and 40 ℃ in stripped soybean oil (SBO) and
fish oil (FO), in which antioxidants and polar compounds were removed.
Oxidation of oil was monitored with three different analytical methods,
peroxide value (PV), conjugated diene value (CDV) and p-anisidine value
(p -AV) to determine primary and secondary oxidation products.
Headspace volatile analysis was also conducted to examine the
effectiveness of OOE in reducing volatile oxidation products during
storage. The activity of 0.1 wt.%
OOE in oil was compared to a synthetic antioxidant, BHT, at its legal
limit (0.02 wt.%), a leading commercial natural antioxidant, rosemary
extract (RE), at the manufacturer’s highest recommended concentration
(0.1 wt.%), and a widely used natural antioxidant, mixed tocopherols
(Toco), at 0.1 wt.%.