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.%.