Chemical changes of lipid oxidation compounds under simulated gastric conditions

Trabajo presentado en el World Congress on Fats and Oils & 28th ISF Congress, celebrado en Sydney (Australia) del 27 al 30 de septiembre de 2009.Chemical changes occurring to dietary lipid oxidation compounds throughout the gastrointestinal tract are practically unknown. In this regard, the first site for potential chemical modifications is the stomach due to the strong acidic conditions. In this study, model lipid oxidation compounds, samples of oxidized oils and fractions concentrated in oxidized compounds were subjected to in vitro gastric conditions. A simulated gastric fluid (SGF) containing 0.03 M NaCl and 3.2% w/v pepsin in 0.1 M HCl (pH 1.0) was used and samples were stirred with SGF for 3 hours at 37 ºC. For the assays on model lipids, compounds representative of the most important groups of oxidation products in diet were studied. Thus, methyl linoleate hydroperoxides were selected as representative of the major oxidation compounds formed during oil and food storage, i.e. hydroperoxides, while methyl 9,10-epoxyestearate, methyl 12-oxoestearate and methyl 12-hydroxyestearate were selected as model compounds representative of the functional groups epoxy, keto and hydroxy that bear the main compounds forming during frying and baking. For quantitative determination, methyl heneicosanoate (C21:0) was used as internal standard. For the assays on oxidized oils and fractions, various edible oils oxidized at 40 ºC were used. GC-FID/MS was applied in all the assays, HPLC-UV served to determine the remaining hydroperoxides and to detect the keto and hydroxydienes, and HPSEC-RID was used to detect formation of polymerization compounds. Results showed the instability of hydroperoxide and epoxide structures under simulated gastric conditions, while keto- and hydroxystearate remained unaltered. The major new compounds formed were diols from epoxyestearate, and hydroxydienes, ketodienes and volatiles from linoleate hydroperoxides. Losses of epoxystearate and linoleate hydroperoxides depended on sample concentration and ranged from 15 to 52 % and from 40 to 70%, respectively, under the conditions applied. With regard to changes found in oxidized oils and fractions under simulated gastric conditions, unoxidized samples remained practically intact while oxidized samples showed significant changes in peroxide values and polymerization compounds. Overall, results showed that significant modifications may occur to dietary oxidized lipids under gastric conditions, giving rise to structural changes and/or differences in the amounts of the oxidized compounds originally ingested