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Abstract
In oil, free fatty acids (FFAs) are thought to be the preferred substrate for lipid oxidation although triacylglycerols (TAGs) are the predominant lipid class. We determined the preferential oxidation substrate (TAGs versus FFAs) in soybean oil heated at 100 °C for 24 h, after validating a method for quantifying esterified and free lipid oxidation products (i.e., oxylipins) with mass-spectrometry. Reaction velocities and turnover (velocity per unit substrate) of FFA, and free and TAG-bound (esterified) oxylipins were determined. FFA hydrolysis rate and turnover were orders of magnitude greater (16-4217 fold) than that of esterified and free oxylipin formation. The velocity and turnover of TAG-bound oxylipins was significantly greater than free oxylipins by 282- and 3-fold, respectively. The results suggest that during heating, TAGs are preferentially oxidized over FFAs, despite the rapid hydrolysis and availability of individual FFAs as substrates for oxidation. TAG-bound oxylipins may serve as better markers of lipid oxidation.
Highlights
In the US, soybean oil is the primary source of the essential polyunsaturated fatty acids (PUFAs)[1,2], linoleic acid (LA; C18:2n-6) and α-linolenic acid (ALA; C18:3n-3)[3,4]
We evaluated whether TAGs or free fatty acids (FFAs) are the preferred substrate for lipid oxidation in soybean oil
Concentrations of total LA- and ALA-derived oxylipins listed in Supplementary Table 1 were measured in different soybean oil volumes (1, 2, 5, and 10 μL) following Na2CO3 hydrolysis, using ultra high-pressure liquid chromatographytandem mass spectrometry (UPLC-MS/MS)
Summary
In the US, soybean oil is the primary source of the essential polyunsaturated fatty acids (PUFAs)[1,2], linoleic acid (LA; C18:2n-6) and α-linolenic acid (ALA; C18:3n-3)[3,4]. The main substrates for lipid oxidation are triacylglycerols (TAGs) and free fatty acids (FFAs), which make up 95–97% and 0.3–0.7% of oil, respectively[10,11]. Both TAGs and FFAs have been shown to oxidize during thermal treatment of oil[8,12,13,14]. We evaluated whether TAGs or FFAs are the preferred substrate for lipid oxidation in soybean oil This was achieved by measuring the rate of free and esterified (mostly TAG) oxylipin formation in soybean oil heated for up to 24 h, after developing a method for separating and hydrolyzing esterified oxylipins in oil.
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