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.