Introduction
Frying is one of the most conventional methods of food preparation with
annual commercial values of over billions of dollar in the world. During
the process, frying oil is exposed to elevated temperatures, atmospheric
oxygen, and the moisture released from the food which is being fried.
Such a condition results in several deteriorative chemical reactions,
including oxidation in the presence of oxygen, hydrolytic alterations
due to food moisture, and thermal polymerization at high temperatures
(Dobarganes et al., 1988). These could lead to the formation of a wide
range of undesirable compounds influencing the sensory properties,
toxicity, and nutritional quality of fried foods.
Initial quality and chemical composition of frying oils remarkably
affect the reactions occurring during oil deterioration (Farhoosh and
Pazhouhanmehr, 2009). High-quality frying oils have been reported to
initially have free fatty acids less than 0.1% (or an acid value, AV,
of ~0.2 mg g–1), peroxide values (PV)
ranging from 0.5 to 1.0 meq kg–1, and smoke points
above 200 °C (Stevenson et al., 1984). Fatty acids as the major
components of frying media should be on a specified composition.
Polyunsaturated fatty acids (PUFA) are rapidly peroxidized and largely
lower the shelf life of fried products. To avoid polymerization, several
countries have set limits as low as 2% for linolenic acid.
Monounsaturated fatty acids (MUFA, primarily oleic acid), which are
considered to be beneficial from a health standpoint, show high
oxidative stability and provide a light taste. Saturated fatty acids
(SFA, mainly palmitic and stearic) are stable towards peroxidation and
polymerization. However, high SFA levels should be avoided because they
adversely affect the sensory attributes (e.g., waxy mouth-feel and dry
surface) of fried foods (Boskou, 2003). Apart from the fatty acid
composition, tocopherols and phenolic compounds, which are indigenous
minor components found extensively in vegetable oils, have been shown to
exert an important contribution to the protection of frying oils against
thermo-oxidative degradations (Boskou, 2003).
Olive oil is considered to be one of the best candidates for frying
purposes (Molina-Garcia et al., 2017). This is due to possessing a
prominent and well-balanced chemical composition, being associated with
its fatty acid composition constituted primarily of oleic acid (55 –
83%) as well as significant amounts of some tocopherols and phenolic
compounds with powerful antioxidant activity and health-promoting
effects (Bendini et al., 2007). From a commercial point of view, olive
oils are categorized as extra virgin, virgin, refined, and just olive
oil (Firestone, 2005). Extra virgin and virgin olive oils are made
directly from olive fruits with no chemical treatments. Those virgins
with high levels of lipid hydroperoxides, acidity, and off-flavors,
which are called as lampante , are needed to be refined. Refined
olive oils, which are relatively less expensive but lack of any flavor,
could be flavored by adding certain quantity of the virgins, being
commercially categorized as olive oil. Literature review shows a limited
number of comparative studies on the thermo-oxidative stability of
different commercial categories of olive oil. Considering the amount of
polymeric triglycerides generated over heating of a number of virgin and
refined vegetable oils at 170 °C, virgin oils were shown to be of better
oxidative stability than the corresponding refined oils (Gertz et al.,
2000). In a study on the emission of low molecular weight aldehydes from
the oils heated at 180 °C and 240 °C, there was found very similar
results for the extra virgin and refined olive oils (Fullana et al.,
2004). There was observed no significant difference between the
different commercial categories of olive oil fried at 170 °C by
monitoring a range of oxidation products (Casal et al., 2010).
Sesame oil is another super stable oil which is highly resistant to
peroxidation despite its relatively high unsaturation degree. The
exceptional stability of sesame oils has been attributed to the presence
of sesame seed lignans (e.g., sesamin, sesamolin, and sesamol),
tocopherols, and some Millard reaction products (Abou-Gharbia et al.,
2000; Lee et al., 2007; Shahidi and Naczk, 2004). Sesame oil can be
produced from unroasted or roasted sesame seeds, which the latter have
been reported to contain higher contents of sesamol. In fact, sesamol is
formed by thermal hydrolysis of sesamolin, and the rate of this
conversion increases at elevated temperatures and longer durations of
roasting process (Lee et al., 2010). Cold-pressed or virgin and refined
sesame oils are two common commercial types of sesame oil, which both
are consumed as salad dressing due to their high degree of unsaturation
(Wan et al., 2015). Regardless of some reports dealing with sesame oils
blended with other oil sources in order to improve thermo-oxidative
and/or frying stability of the final product, literature review shows no
individual study on the frying stability of virgin or refined sesame
oils. Hence, the aim of this study was to evaluate the frying stability
of virgin and refined sesame oils as compared to a commercial refined
olive oil.