Abstract
The objective of this study was to know the evolution of the oxidation of soybean oil and biodiesel under the conditions of the oxidation stability test (110 °C) using the Rancimat apparatus. Samples were analyzed at different periods of time until the end of the induction period. The analytical determinations related to the changes in oxidation include peroxide value, anisidine value, natural tocopherols and polar compounds. Acid value, kinematic viscosity, polymers and ester content were also analyzed because of their relevance in the evaluation of biodiesel quality. Results showed that only peroxide value and the group of polar compounds including hydroperoxides, i.e. oxidized monomeric TAG in the oil and oxidized monomeric FAME in the biodiesel increased significantly during the early oxidation stage. The end of the induction period was marked by a rapid increase in polymerization compounds and the exhaustion of tocopherols. Significant changes in acid value, viscosity and ester content were only observed after the end of the induction period.
Highlights
Lipid oxidation takes place through a set of autocatalytic reactions that produce a high number of new compounds
Results showed that only peroxide value and the group of polar compounds including hydroperoxides, i.e. oxidized monomeric TAG in the oil and oxidized monomeric Fatty acid methyl esters (FAME) in the biodiesel increased significantly during the early oxidation stage
The oxidation of oil is influenced by numerous variables, i.e. the fatty acid composition, processing, temperature, minor compounds with prooxidant or antioxidant activity, etc., so that it is difficult to foresee the evolution of oxidative deterioration (Choe and Min, 2006)
Summary
Lipid oxidation takes place through a set of autocatalytic reactions that produce a high number of new compounds. Hydroperoxides, the primary oxidation compounds, decompose and give rise to a variety of secondary oxidation products. The evaluation of the oxidation status in oils is complex and requires the application of more than one analytical method for a better understanding of the level of both primary and secondary oxidation compounds present in a certain sample (Frankel, 2005). Biodiesel is an alternative fuel derived from fats and oils obtained by a transesterification reaction with an alcohol, usually methanol and etanol (Pighinelli et al, 2011; Rashidet et al, 2012). Since biodiesel has the same fatty acid composition as its parent oil or fat with considerable amounts of unsaturated fatty acids, the evolution and evaluation of oxidation in biodiesel present similar problems to those mentioned above for oils
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