Abstract
Vegetable oils are usually rich in unsaturated fatty acids which are susceptible to oxidation. The oxidation of vegetable oils has been one of the most widely studied fields within lipid chemistry, because it alters their properties and nutritive value, inducing the formation of harmful compounds and off-flavors. Moreover, oxidized vegetable oils display altered physical and chemical properties which are conferred by the newer oxygenated compounds they contain. This is the case of ozonized oils. Ozone is a powerful oxidizing agent that mainly acts on olefinic compounds which generate ozonides and other peroxidic species that can decompose into carbonilic fragments. The action of the oxidant and the later reactions depend on the chemical environment of the reaction as well as the carbonyl termination products resulting from peroxide cleavage. In recent years, sunflower oils with different fatty acid compositions have been developed by breeding and mutagenesis. They displayed higher contents of oleic, stearic or palmitic acids, which mainly alters their triacylglycerol composition. Therefore, four different sunflower oils, common, high oleic, high stearic-high oleic and high palmitic-high oleic, were oxidized with ozone and the progress of the reaction was monitored by measuring the level of oil peroxygenation and the changes in the oils’ fatty acid compositions. The peroxidated species formed during ozonation were studied by FT-IR spectroscopy. The main conclusions of this work were that ozonation caused linear oxidation rates that were similar in all the oils assayed. The addition of water accelerated oxidation, which tended to occur in linoleic polyunsaturated fatty acid The FT-IR pointed to the presence of ozonide-derived peroxides as the major oxygenated species.
Highlights
Vegetable seeds oils consist of a matrix of triacylglycerols (TAGs) plus minor components which could include a broad variety of compounds including sterols, triterpenic alcohols, tocopherols and pigments (Gunstone, 2011)
UV light can promote oxidation though a different mechanism involving the activation of oxygen to the singlet form, which can directly attack the double bonds of fatty acids, and produce peroxyl radicals which are able to start the process of oxidation (Bradley and Min, 1992)
This study indicated that the rates of autoxidation varied substantially among the different oils, with common sunflower oxidizing much faster than high oleic sunflower, which oxidized faster than the highly saturated oils, the kinetics of oxidation displayed the sigmoidal curves typical of the chain reaction processes depicted in the introduction
Summary
Vegetable seeds oils consist of a matrix of triacylglycerols (TAGs) plus minor components which could include a broad variety of compounds including sterols, triterpenic alcohols, tocopherols and pigments (Gunstone, 2011). The oxidation of vegetable oil is usually mediated by the process called autoxidation (Frankel, 1980) This process involves a chain reaction via free radicals in the presence of molecular oxygen. The initiation generally involves the hemolytic abduction of a hydrogen atom from a fatty acid to give place to a free radical that quickly reacts with molecular oxygen to yield a peroxyl radical. The accumulated hydroperoxide are unstable species that tend to break down into carbonilic fragments or react with other oxygenated species, giving way to ketones, epoxides, acids or ethers This is called the secondary oxidation process and is responsible for the production of off-flavors in fats and oils. The oxidation caused by other different chemical agents in these oils is not so well known
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