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).