Characterization of the oils
Fatty acid composition of VSO, RSO, and ROO was in agreement with those usually reported for sesame and olive oils (Table 1). The oils were of statistically the same amount of SFA (mainly palmitic, C16:0, and stearic, C18:0, acids). ROO was constituted of almost twice amount of MUFA (mainly oleic acid, C18:1) than the sesame oils. Instead, the sesame oils of roughly the same content of PUFA (mainly linoleic acid, C18:2) were considerably more polyunsaturated than ROO. With respect to the relative rate of oxidation reported for linoleic, oleic, and stearic acids as 1200:100:1 (Hsieh and Kinsella, 1989), ROO was definitely to have the most oxidatively stable fatty acid composition.
The sesame oils had acceptable PVs (Table 1) in the range recommended (0.5 – 1.0 meq kg–1) for high-quality frying oils (Stevenson et al., 1984) but the quite higher value for ROO was in the permitted range (~10 meq kg–1 for ROOs) promulgated by the International Olive Oil Council (IOOC). Olive oils have been postulated to be comprised of certain components that interfere with the conventional PV measurement, so that even freshly expressed olive oils have PVs of ~10 meq kg–1, or higher values under dry climatic conditions (Firestone, 2005). The AV of VSO was significantly higher than those of RSO and ROO which were statistically the same. The value for high-quality frying oils and refined olive oils should not exceed ~0.2 mg g–1 (Stevenson et al., 1984) and ~0.6 mg g–1 (Firestone, 2005), respectively.
The highest contents of total tocopherols and phenolic compounds were found in VSO, followed by in RSO and ROO (Table 1). Conventional refining processes normally cause significant decreases in the contents of TT (Tasan and Demirci, 2005) and TP (Zacchi and Eggers, 2008). α- and γ-Tocopherols are recognized as predominant homologues (~95%) in olive (Firestone, 2005) and sesame (Hwang, 2005) oils, respectively. The antioxidant activity of tocopherols decreases in the order of δ > γ > β > α in fats and oils, while in vivo , that is vitamin E activity, decreases in the order of α > β > γ > δ (Wanasundara and Shahidi, 2005). Hydroxycinnamic acids and hydroxytyrosol are known as the main phenolic compounds of the minor fraction of olive oil chemical composition (Boskou, 1999; Pellegrini et al., 2001). Sesamin is the major lignan in sesame oil (Wu et al., 2016), which is in turn transformed into sesamol, episesamin, and sesaminol during refining processes and/or at high temperatures (Wan et al., 2015).
As can be seen in Table 1, ROO exhibited an OSI of about two folds those of the sesame oils. This is likely due to the more contribution of the fatty acid compositions than the anti-oxidative minor components to resist against thermal oxidation under the harsh conditions normally occurring in Rancimat. OSI may essentially not provide a correct estimation of frying stability of an oil, resulting from some inherent differences in the reaction environments established during frying processes and Rancimat test (Habibi-Nodeh et al., 2019).