Changes in the polar components
Figure 2 shows the changes in the contents of thermo-oxidative (oxTGM, TGD, and TGP) and hydrolytic (DG and FFA) polar components during the frying process. This two sets of components differ not only in polarity or molecular weight but also in nutritional impact (Dobarganes et al., 1988), and therefore, it is of crucial importance to specify their distribution for any frying medium over the process.
VSO showed extraordinarily the highest potency in inhibiting the creation of thermo-oxidative polar components during the frying process (Fig. 2A,B,C). Apart from the antioxidant capacity of the tocopherol and phenolic fractions, sesame oils have been shown to include significant amounts of ∆5- and ∆7-avenasterols capable of an anti-polymerization effect during prolonged heating at high temperatures (Mohamed and Awatif, 1998; Sims et al., 1972). Considering oxTGM as a measure of total oxidation, encompassing both oxygenated primary and secondary oxidation products (Ruiz-Mendez et al., 1997), ROO was of slightly weaker strength than RSO to inhibit oxidation in total (Fig. 2A). The refined oils behaved almost similarly with respect to the contents of TGP (Fig. 2C) but ROO was significantly the better frying oil from TGD point of view (Fig. 2B). TGD and TGP, which are oxTGMs linked together preferentially through oxygenated linkages, are very complex and structurally not fully recognized. Nowadays, their summation (TGDP) and a cutoff point of 10% based on it have absorbed much attention on a health ground (Farhoosh and Tavassoli-Kafrani, 2011). All the oils presented linear trends of change (R2 > 0.96) in TGDP with frying time (Fig. 2D). On the basis of 10% TGDP, the maximum frying time for VSO was calculated to be 32.1 h, which was considerably higher than the statistically different values for RSO (13.9 h) and ROO (15.4 h).
Hydrolytic polar components are also of crucial importance because they are likely to exert pro-oxidant effects and lower increasingly the oxidative stability of frying oils. Monitoring the contents of FFA and DG appeared quite different frying performances for the oils studied (Fig. 2E,F). VSO contained significantly the highest contents of FFA and DG during the frying process and RSO exhibited more resistance than ROO to hydrolytic alterations. As can be seen in Fig. 2E, besides the much smaller quantities compared to DG, FFA showed no given change pattern over time, making it less reliable than DG to determine hydrolysis rate during frying. Higher volatility of FFA than DG causes them to be lost more easily at elevated temperatures (Dobarganes et al., 1988). The more is the content of FFA as well as the tendency to their formation, the lower smoke point for an oil will be. The oils with lower smoke points (especially < 200 °C) are less useful for frying operations because they are more prone to the emission of potentially toxic volatile organic compounds (Katragadda et al., 2010). The fresh VSO had a smoke point of 182 °C which was drastically lower than those of the fresh RSO (231 °C) and ROO (208 °C). As shown in Fig. 3, the smoke points of VSO, RSO, and ROO significantly decreased and reached a plateau (~161 °C, ~206 °C, and ~190 °C, respectively) after 8 h of the frying process.