Oxygen consumption of aqueous suspensions of wheat wholemeal, bran and germ: Involvement of lipase and lipoxygenase

Abstract

The oxygen consumption values (µmol O 2 /10 min/g at 25°C) of aqueous suspensions of wheat wholemeal (0·1–3·0, bran (1–18) and germ (1–11) are substantially higher than those of white flours (0·01–0·03). The actual values depend upon storage history; the O 2 consumption value of materials stored 2–4 weeks at 20°C, 65% r.h. are many-fold higher than those of the same materials from freshly-milled grain. The O 2 consumption of mixtures of finely-ground ( 2 uptake is due primarily to oxidation of unesterified, polyunsaturated fatty acids, catalysed by lipoxygenase that is concentrated in the germ fraction. The increased O 2 demand of stored materials is due to higher levels of polyunsaturated fatty acids, released during storage, by hydrolysis of triacylglycerols, catalysed by a triacylglycerol hydrolase (lipase) that is concentrated in the bran fraction. The triacylglycerol-hydrolase activity of wheat germ is relatively low. Thus, O 2 -uptake by aqueous suspensions of wholemeal flour can be explained in terms of the combined effects of the bran component, causing lipolysis during storage of wholemeal, and the germ component, catalysing the O 2 -dependent peroxidation of polyunsaturated fatty acids when the wholemeal is added to water; both bran and germ components contribute triacylglycerols as the substrates for lipolysis.