Both the natural lipids of flour and added fats are known to play an important role during the production of bread. In this review, the chemical and physical interactions of fat have been assessed in an attempt to explain these technological functions. Particular emphasis has been placed on the "binding" or complexing of lipid by flour proteins during the development of dough. While publications in this field have frequently been contradictory, evidence now indicates that observed lipid binding may involve lipid mesophase transformation and the nonspecific occlusion of lipid phases within the gluten network. The significance of these suggested events has been compared with current theories of lipid function in the breadmaking process.

The mechanism by which soya lipoxygenase enzyme action improves the rheological properties of wheat flour doughs during mechanical development in air has been investigated further. Free-lipid extraction, reconstitution and replacement experiments have shown that the rheological effect of lipoxygenase action, which is consistent with an oxidative improvement of the dough proteins and may also result in extended mixing tolerance, only occurred in the presence of an oxidisable, polyunsaturated, free-lipid substrate. Addition of this substrate in an oxidised state (produced either by autoxidation or enzyme-oxidation) to doughs mixed from fatextracted flour under nitrogen resulted only in a small rheological improvement, greater for the autoxidised than the enzyme-oxidised lipid, but in no way comparable with the large rheological effect of lipoxygenase action during dough mixing in air. Furthermore, the presence of an antioxidant, nordihydroguaiaretic acid (NDGA), during dough development, although greatly inhibiting peroxide formation, only marginally impaired the rheological improvement due to lipoxygenase action. Additional evidence is therefore provided for a coupled oxidation mechanism being responsible for the rheological effect, since lipoxygenase-catalysed oxidation actively occurring in the dough during mixing appears to be the fundamental requirement, irrespective of whether the primary oxidation products lead to lipid peroxides or oxidised NDGA. IntroductionIn the first paper1 of this series soya lipoxygenase enzymes were shown to influence the mechanical development of wheat flour doughs in air. Dough relaxation times were increased, consistent with an oxidative improvement of the gluten proteins, and higher levels of mechanical work were tolerated before dough breakdown occurred. These effects were found to depend in magnitude on the rate of mechanical work input and were paralleled by an increase in the proportion of readily extractable ("free") lipid in the dough. When the lipoxygenase was inactivated, either by heat denaturation or by mixing under nitrogen, such effects were largely or completely absent. Addition of enzymically pre-peroxidised flour lipid to nitrogen-mixed doughs was also without effect. Support was therefore provided for the mechanism of Daniels et a1.2 in which it was proposed that lipid release resulted from a lipoxygenase mediated coupled oxidation of -SH groups and consequent structural changes in the dough protein.An effect of rate of work input has since been noted by Drapron et al.,3 who found oxidation of linoleic acid to increase with high speed mixing in doughs containing horse bean lipoxygenase. Most recently, Morrison and Panpaprai4 have demonstrated substantial oxidation of linoleic and linolenic acids in all the major glycerolipids when soya flour is added to dough.Two questions regarding the mechanism of the lipoxygenase effect were raised by the original investigation1 and formed the major objectives of the work described here: one, to examine whether lipoxygenase act...

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