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Oxidation of a functional, CLA-rich oil during storage: determination of volatile and non-volatile compounds

AutorMárquez Ruiz, Gloria ; Holgado, Francisca ; Ruiz Méndez, Mª Victoria ; Velasco, Joaquín ; García-Martínez, M. del Carmen
Fecha de publicación13-oct-2015
CitaciónEuro Food Chem XVIII (2015)
ResumenOils rich in conjugated linoleic acid (CLA) such as Tonalin© oil (TO) are functional ingredients nowadays added to a variety of foods due to their health-promoting effects. However, safety concerns regarding the use of CLA persist and specifically in relation to the formation of oxidized compounds that could lead to adverse physiological effects (1). Conjugated linolenic acid (CLA) is a mixture of positional and geometric isomers of octadecadienoic acid with conjugated double bounds and oils rich in CLA are normally obtained through alkaline isomerization of safflower oil (SO) - an oil rich in linoleic acid (LA). Little is known about oxidation kinetics of CLA and the main variables affecting oxidative stability of CLA-fortified or added foods (2). In this context, the objective of this work was to monitor formation of non-volatile and volatile oxidation compounds in TO as compared with parent SO during storage at 40ºC in the dark. SO and TO contained 74.7% of LA and 76.8% of CLA isomers, respectively. In TO, formation of hydroperoxides was negligible and the first and major compounds formed were polymerization products. Thus, when tocopherols were exhausted (30-days storage), SO showed 120 meq O2/kg oil and 2.5% polymers, values consistent with the expected progress of oxidation in unsaturated oils under these conditions, while TO showed only 7 meq O2/kg oil of peroxide value and as much as 9.1% polymers. In relation to volatile profile, that found in SO was close to that expected from the cleavage of the alkoxyl radicals formed from the LA-derived hydroperoxides, being hexanal the main compound. However, the volatile profile of TO was not that expected from theoretical hydroperoxides formed from CLA but characterized by the occurrence of heptanal and t-2-nonenal, otherwise absent in SO. An alternative route of formation for these distinct volatile oxidation compounds in TO could be the scission of dioxetanes coming from 1,2 cycloadditions of CLA with oxygen. Overall results obtained in this study, both on non-volatile and volatile compounds, support that oxidation kinetics of CLA-rich oils differ substantially from that expected according to the hydroperoxide theory. Oxidation of CLA seems to proceed by the addition mechanism rather by the hydrogen abstraction mechanism, suggesting that oligomeric peroxides are formed from the early events of lipid degradation. It is therefore clearly showed that peroxide value is not valid as oxidation control measurement in functional foods containing CLA-rich oils, as it is nowadays stated in their specifications.
URIhttp://hdl.handle.net/10261/131849
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