Fortification of refined soybean oil by hull oil of two Iranian wild pistachios: Improving thermal stability during frying process

Abstract

In this research, various concentrations of Kolkhoung (Pistacia khinjuk) hull oil (KHO), Beneh (Pistacia atlantica) hull oil (BHO), and Sesame (Sesamum indicume L. cv. Dezful) seed oil (1%, 2%, 4%, and 6%) as natural antioxidants were added to refined soybean oil (RSO) and antioxidant effect of these compounds was compared with that of tert-butyl hydroquinone (TBHQ) during 32 h frying process at 180°C. For this purpose, variation of acid value (AV), conjugated diene value (CDV), carbonyl value (CV), total polar compounds, and total tocol (TT) and total sterol contents of RSO was monitored during frying process. Results of oxidative stability assays revealed that 4%KHO had the best antioxidant effect which was fairly equal to that of TBHQ, followed by 4%BHO. It should be mentioned that in contrast to TBHQ, KHO and BHO are not pure antioxidant. The remarkable antioxidant effect of KHO and BHO during thermal process was due to high tocotrienol content and presence of suitable amount of Δ-7-avenasterol and Δ-5-avenasterol, respectively [TT content of KHO (2041 mg/kg) much higher than that of ordinary edible oils and 92.7% of TT content in KHO is formed by tocotrienols]. Antioxidant activity of Δ-7-avenasterol and Δ-5-avenasterol and tocotrienols during frying process was attributed to formation of stabilized tertiary alkyl radicals. These radicals cut down radical chains formed during frying process.