Effects of ingredients on the oxidation of linoleic acid by lipoxygenase in bread doughs

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Abstract
Flour and water doughs containing 1‐14C‐linoleic acid (18:2) and various ingredients were prepared to study the oxidation of linoleic acid by lipoxygenase in bread doughs. Lipids were extracted, treated with diazomethane, and 14C‐labelled fatty acid methyl esters separated by thin‐layer chromatography. Radioactivity was determined in silica gel bands containing unoxidised 18:2, hydroperoxy acids (L1), hydroxy acids (L2), hydroxyepoxy acids (L3) and trihydroxy acids (L4). Minor components detected by autoradiography were present mainly in L3 and L4. Recoveries of total radioactivity were always > 95%. Untreated flour‐water dough was mixed aerobically for ˜4 min, rested, and the lipids extracted after 10 min total dough time. Yields of 14C products were unoxidised 18:2 = 28.6 μmol, L4 = 93.9 μmol/100 g dry flour. Similar yields were obtained from ClO2‐treated flour, both after 10 min and 60 min dough time. Salt, salt + yeast, or salt + yeast + ascorbic acid in the dough did not reduce 18:2 oxidation significantly, but increased L3 at the expense of L4. Soya flour preparations inhibited linoleic acid oxidation by 25–44%, but pure soya lipoxygenase had no effect at all. Heat treatment reduced the inhibitory effect of soya flour. Accessible thiol groups were not essential for lipoxygenase activity or for the reduction of L1 to L2 since adding cysteine or N‐ethyl maleimide had negligible effects on the 18:2 oxidation products. Most of the flour carotenoids (xanthophylls) were bleached by wheat enzymes in non supplemented doughs, and all were bleached in doughs supplemented with soya flour. 14C‐labelled triglyceride was not oxidised except in doughs containing soya flour mixed in air (1.5% oxidation) or oxygen (3 % oxidation). Soya flour contains lipoxygenase isoenzymes (principally lipoxygenase‐2) which oxidise linoleate in triglycerides. This isoenzyme is evidently not present in wheat.