2009_Book_FoodChemistry

15.4 Baked Products 717
oxidizing agents like bromate, no overdosage is
observed.
In dough making, the Asc added to the flour oxidizes
very rapidly to DHAsc (Fig. 15.32). Diastereomers
of Asc are converted at the same rate.
In contrast, the four diastereomers of Asc (stereochemistry
in Fig. 15.33) as well as the corresponding
DHAsc differ in their effect as flour improvers.
As shown in Table 15.42, L-threo-Asc
(vitamin C) has the highest dough strengthening
effect. The two erythro-Asc have a weaker effect
and D-threo-Asc is almost ineffective. Since these
Table 15.42. Effect of additives on the rheological
properties of wheat dough
Additive
Resistance to Extensi-
(0.15 µmol/g flour) extension (%) bility a (%)
Control (without additive) 100 100
Cysteine 63 106
Glutathione (reduced form) 56 105
L-threo-Ascorbic acid 147 58
D-erythro-Ascorbic acid 122 86
L-erythro-Ascorbic acid 118 93
D-threo-Ascorbic acid 94 88
L-threo-Dehydroascorbic
acid 145 56
a Relative values.
Fig. 15.32. Oxidation of ascorbic acid in dough making
from wheat flour (according to Nicolas et al., 1980)
◦—◦ ascorbic acid, •—• dehydroascorbic acid,
— sum of ascorbic and dehydroascorbic acids
differences correspond with the substrate specificity
of the GSH dehydrogenase found in flour
(cf. 15.2.2.7), it is assumed that this enzyme is
involved in flour improvement on the addition of
Asc as shown in Fig. 15.34.
The atmospheric oxygen kneaded into the
dough first oxidizes Asc to DHAsc (Reaction a
in Fig. 15.34). The reaction is accelerated by
ascorbic acid oxidase (cf. 15.2.2.9) and other
factors. Subsequently (Reaction b), the GSH
present in flour is oxidized to the disulfide.
This reaction proceeds very rapidly because it
Fig. 15.33. Chemical structures of stereoisomeric ascorbic
acids (Asc).
(a) L-threo-Asc (vitamin C), (b) D-threo-Asc,
(c) D-erythro-Asc (isoascorbic acid), (d) L-erythro-Asc
Fig. 15.34. Reactions involved in flour improvement by
ascorbic acid (according to Grosch and Wieser, 1999)
Asc, ascorbic acid; DHAsc, dehydroascorbic acid;
AO, ascorbic acid oxidase; GSH-DH, glutathione dehydrogenase;
GSH, reduced glutathione; GSSG, oxidized
glutathione; CSH, cysteine; CSSC, cystine;
PSSP, gluten proteins