Abstract
Oryzanols are frequently found in rice bran oil but almost completely removed in the neutralization step when the oil is chemically refined. In this way, oryzanols can be recovered from the soapstocks to generate a concentrate. Thereby, they could be used as antioxidants in lipids for specific purposes. In the present work the antioxidant power of oryzanols concentrate (33% purity) was studied together with pure oryzanols and butylhydroxytoulene (BHT). Methyl esters were prepared from regular sunflower oil without antioxidants to which the antioxidants before mentioned were added in an effective concentration of 3x10-3 M. The samples were oxidized in a heating block with oxygen flow and the hydroperoxides of linoleic acid methyl ester were analyzed. It was observed that all antioxidants were able to protect the sunflower oil methyl esters from oxidation with respect to methyl esters without antioxidants. Oryzanols presented a notoriously lower protection ability compared to BHT. However, the formation of the linoleic acid methyl ester hydroperoxides and their proportion, for the same oxidation stage, did not show differences between the antioxidants used. Therefore, the oxidation kinetics were similar between the different antioxidants studied.
Highlights
It is interesting to study the evolution of the composition of a system subjected to a process of oxidative rancidity and to compare the variations that occur in the presence of an antioxidant whose antioxidant power is to be analyzed
The ordinates axis is showed in "arbitrary units" (UA), since the values were determined as the total area "normalized" with respect to methyl esters concentration (g/mL) in the sample analyzed
To reach a hydroperoxides rate of 1.2x106 U.A. took 60, 188 and 235 h for Sunflower oil methyl esters (SFOME) without antioxidants, pure oryzanols and oryzanols concentrate, respectively. These results indicated that all the antioxidants protected the SFOME under the conditions of the analysis effectively and that the BHT was the one with the highest antioxidant power
Summary
It is interesting to study the evolution of the composition of a system subjected to a process of oxidative rancidity and to compare the variations that occur in the presence of an antioxidant whose antioxidant power is to be analyzed. The study of the products formed from linoleic acid showed that hydroperoxides predominate in positions 9 and 13 while the hydroperoxide in position 11 is not formed in significant amount (Bergström, 1945; Bergström, Blomstrand, & Laurell, 1950) This is due to the fact that the formation of the conjugated double bond gives the free radical greater thermodynamic stability and, the free radicals available for the subsequent reaction with oxygen and with the free hydrogen radical will be located at carbon 9 and in carbon 13 of the linoleic acid molecule (Bascetta, Gunstone, & Walton, 1983; Brash, 2000)
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