Changes in CDV, CV, and TPC
All the oils studied showed linear trends of change (R2 > 0.97) in CDV, CV, and TPC with frying time (32 h at 180 °C) (Fig. 1). CDV represents the oxidizability of lipid systems containing methylene-interrupted dienes, which become conjugated during peroxidation (Farmer and Sutton, 1946). Regardless of the slight differences in the initial values, the rate of change in CDV was significantly lower for ROO (0.72 mmol l–1h–1) than for VSO (1.00 mmol l–1h–1) and RSO (1.84 mmol l–1h–1) (Fig. 1A). This can be due to the very much lower level of PUFA in ROO than in the sesame oils (Table 1), which makes it less prone to produce conjugated diene hydrperoxides. The better resistance of VSO than RSO to the formation of these oxidation products naturally arises from the greater antioxidant potency of the former (Table 1). Traditional deodorization of sesame oils has been reported to remove sesamol almost completely and more than 85% of sesamolin and related isomers of sesaminol, leading to markedly reduced oxidative stability of the final product (Kochhar, 2002).
Despite CDV, which stands for conjugated diene hydroperoxides as primary oxidation products, CV represents a wide variety of secondary oxidation products known as aldehydes and ketones. Its level in frying oils is of extreme importance because carbonyls often cause rancidity and off-flavor, and reduce nutritional value of fried foods (Endo et al., 2001). While there was no significant differences among the initial values, ROO revealed the highest susceptibility to secondary oxidation, so that the change in CV for it (2.31 μmol g–1h–1) was significantly much faster than those for RSO (1.89 μmol g–1 h–1) and VSO (1.17 μmol g–1 h–1), respectively (Fig. 1B). This indicates well the greater quantitative and/or qualitative contribution of the predominant antioxidant fractions in the sesame oils (γ-tocopherol and lignans) than in ROO (α-tocopherol, hydroxycinnamic acids and hydroxytyrosol) to prevent carbonyls from being formed.
TPC is one of the most well-known and reliable indicators for the extent of chemical deterioration in frying oils. It represents a range of polar components (see below) which are considered to be toxic to human health (Firestone, 1996). With respect to the rates of change in TPC (Fig. 1C), VSO was still of better quality than RSO (0.70 vs. 1.02 % h–1) but there was no much difference between the refined oils in inhibiting the production of polar compounds.