Abstract
Little is still known about both the effect of amino acids on the oxidation course of edible oils and the modifications that the former may undergo during this process. Bearing this in mind, the objective of this work was to study the evolution of a system consisting of soybean oil with 2% of l-lysine under heating at 70 °C and stirring conditions, analyzing how the co-oxidation of the oil and of the amino acid affects their respective evolutions, and trying to obtain information about the action mechanism of lysine on soybean oil oxidation. The study of the oil progress by 1H Nuclear Magnetic Resonance (1H NMR) showed that the presence of lysine noticeably delays oil degradation and oxidation products generation in comparison with a reference oil without lysine. Regarding lysine evolution, the analysis by 1H NMR and Liquid Chromatography–Mass Spectrometry of a series of aqueous extracts obtained from the oil containing lysine over time revealed the formation of lysine adducts, most of them at the ε position, with n-alkanals, malondialdehyde, (E)-2-alkenals, and toxic oxygenated α β-unsaturated aldehydes. However, this latter finding does not seem enough to explain the antioxidant action of lysine.
Highlights
The antioxidant ability of proteins and peptides [1] has long generated great interest among food scientists, especially in the last two decades, when numerous studies have been conducted to investigate the potential of protein hydrolysates of diverse origin as antioxidants in food systems [1,2,3]
These results indicated that lysine exerts a clear antioxidant effect and it is to be expected that this will affect the formation of oxidation compounds such as hydroperoxides, epoxides, and aldehydes
Where N is the number of moles of each compound, k the proportionality constant between the area of the 1 H NMR signal and the number of protons that generate it, and ALC and ALA’ the respective areas of signals LC and LA’. The evolution of this ratio throughout the refined soybean oil (RSO) + 2LYS sample oxidation process, which is shown in Figure 4B, revealed that, in accordance with the results provided by the Liquid Chromatography followed by Mass Spectrometry (LC–MS) study, the proportion of Nε-formyl-lysine relative to lysine increased as the process advanced
Summary
The antioxidant ability of proteins and peptides [1] has long generated great interest among food scientists, especially in the last two decades, when numerous studies have been conducted to investigate the potential of protein hydrolysates of diverse origin as antioxidants in food systems [1,2,3]. In this context, the monitoring of lipid oxidation is usually carried out by means of classical methodologies such as peroxide value, conjugated dienes, and/or TBARS (2-Thiobarbituric Acid Reactive Substances) assay, in some cases combined with the determination of volatile aldehydes like propanal or hexanal. Functional groups of proteins can react with lipid oxidation products like hydroperoxides or aldehydes [5,6]
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