Redox reactions in wheat dough as affected by ascorbic acid

Ascorbic acid (AA) is used as bread improver, as its addition to dough caus es an increase in loaf volume and an improvement in crumb structure. To exp lain these effects we review the stereospecificity of the improver action a nd the properties of ascorbate oxidase and glutathione dehydrogenase and th e occurrence of low molecular thiols in flour and their concentration chang es during dough mixing in the presence and absence of AA. On rile basis of thr results the improver action of AA is explained by a reaction sequence l eading to a rapid removal of endogenous GSH, which otherwise would cause do ugh weakening by sulphhydryl/disulphide interchange reactions with gluten p roteins. To rest this hypothesis the binding sites of endogenous GSH in glu ten proteins have been determined by the addition of S-35-labelled GSH as a tracer to flour before dough mixing. The distribution of radioactivity in the gliadin and glutenin fractions of gluten obtained from dough indicates that the major portion of GSH is bound to glutenins. The isolation and sequ ence analysis of radioactive cystine peptides from an enzymatic digest of g lutenins demonstrates that GSH is almost exclusively linked to those cystei ne residues of LMW subunits that hale been proposed to form intermolecular disulphide bonds. (C) 1999 Academic Press.