Establishment of a model substrate oil for antioxidant activity assessment by oil stability index method

Abstract

A model substrate oil using methyl linoleate was established for the determination of the antioxidant activity by Oil Stability Index (OSI) method. OSI values for methyl linoleate with different concentrations (5-100%) in silicone oil were measured at different temperatures (70-120°C). As the temperature increased, the OSI value decreased in each concentration of methyl linoleate. Optimal temperature and concentration of methyl linoleate were established. The effect of concentration of antioxidants, α-tocopherol, and butylated hydroxytoluene on OSI values for 10% methyl linoleate model oil was measured at 90, 100, 110, and 120°C. The logarithmic relationship between temperature and OSI using model substrate oil was similar to that of soybean oil. Furthermore, application of some spice extracts to this model oil system was carried out to give results that compared well with those available in the literature. Thus, the procedure using methyl linoleate-silicone oil as a model substrate oil is available for evaluating the antioxidant activity by the OSI method.The Oil Stability Index analysis method (OSI; AOCS Official Method Cd 12b-92) (1) is one of several methods used to evaluate the stability of fats and oils. It determines stability by measuring conductivity due to volatile organic acids evolved during the oxidation of fats and oils. OSI value is defined as the point of maximal change of the rate of oxidation (1,2). A recently developed oxidative stability instrument allows automatic OSI determinations with a simultaneous analysis of up to 24 samples (3). Although several reports (4,5) showed that there is a close correlation between OSI and the Active Oxygen Method (AOM; AOCS Official Method Cd 12-57) (6), the OSI method has several advantages over the conventional AOM with respect to accuracy and repeatability, and it requires less labor, time, and reagents (2,5). Frankel (7) pointed out the limitations of high-temperature stability tests, such as OSI and AOM in evaluating antioxidants, owing to rapid oxidation, polymerization and cyclization of fats and oils, and decomposition of antioxidants at high temperature. However, antioxidants are commonly used under conditions of high temperature in food processing and cooking. Therefore, it seemed worthwhile to evaluate antioxidative activity at high temperatures by OSI.The OSI method has been used in many studies to evaluate the activity of antioxidants measured in various oils, such as soybean oil (8), lard (9), corn oil (9), whale oil (8), and sardine oil (8). However, the fatty acid compositions of these natural oils vary, which can affect OSI values. Furthermore, these natural oils contain natural antioxidants such as tocopherols which interfere in the determination of the activity of the antioxidants being evaluated. Kajimoto and Murakami (10) revealed that the OSI value of soybean oil stripped of tocopherols decreased as compared with the value of untreated oil. So tocopherols in natural oil had a great influence on the OSI value. Such complications prompted ...

Measurement of Antioxidant Effects on the Auto-oxidation Kinetics of Methyl Oleate – Methyl Laurate Blend as a Surrogate Biodiesel System

“…After an initial warm-up period which lasts for approximately 15 minutes, the curves in Figure 2 exhibit a linear behavior in accordance with first-order kinetics. Indeed, such firstorder kinetic behavior is also observed by Nakatani and co-workers [26]. The rate constants of the primary oxidation stage were determined simply by taking the slope of the ln(Conductivity) vs. time curves in the linear regions, and are compiled in Table 4.…”

Antioxidant activity of phytosterols, oryzanol, and other phytosterol conjugates

“…Methyl esters of vegetable oils should be an ideal system to evaluate antioxidant activity, because they can be purified easily to ensure that no other minor lipid components interfere with the evaluation. Methyl linoleate was successfully used as a model system for antioxidant activity study by OSI (21). The only drawback with soybean FAME was that it did not develop significant viscosity.…”

Effect of temperature on the antioxidant activity of phenolic acids

“…Although the effect of temperature on the activity of different antioxidants in bulk fats and oils has been systematically studied (in the temperature range of 20°C to 100°C) by Marinova and Yanishlieva (1992, 1996, 2003, Yanishlieva and Marinova (1996), and also partly by other authors (Reynhout 1991;Cho 1997;Armando et al 1998;Nakatani et al 2001;Wagner et al 2001;Evans et al 2002;Kolb et al 2002;Zhang et al 2004), there is not a sufficient amount of reliable details about the effect of temperature on the antioxidant activity of different compounds in the available literature. Therefore, a systematic study of the effect of temperature on the antioxidant activity of individual antioxidants was undertaken.…”