Food Chemistry

15.4 Baked Products 717

oxidizing agents like bromate, no overdosage is
observed.
In dough making, the Asc added to the flour oxi-
dizes very rapidly to DHAsc (Fig. 15.32). Dia-
stereomers of Asc are converted at the same rate.
In contrast, the four diastereomers of Asc (stereo-
chemistry in Fig. 15.33) as well as the corre-
sponding DHAsc differ in their effect as flour im-
provers. As shown in Table 15.42,L-threo-Asc
(vitamin C) has the highest dough strengthening
effect. The twoerythro-Asc have a weaker effect
andD-threo-Asc is almost ineffective. Since these

Fig. 15.32.Oxidation of ascorbic acid in dough making
from wheat flour (according toNicolaset al., 1980)
◦—◦ascorbic acid,•—•dehydroascorbic acid,
— sum of ascorbic and dehydroascorbic acids

Fig. 15.33.Chemical structures of stereoisomeric as-
corbic acids (Asc).
(a)L-threo-Asc (vitamin C), (b)D-threo-Asc,
(c)D-erythro-Asc (isoascorbic acid), (d)L-erythro-Asc

Table 15.42.Effect of additives on the rheological

properties of wheat dough

Additive Resistance to Extensi-

(0.15 μmol/g flour) extension (%) bilitya(%)

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

aRelative values.

differences correspond with the substrate speci-

ficity 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. 15.34.Reactions involved in flour improvement by

ascorbic acid (according toGroschandWieser, 1999)

Asc, ascorbic acid; DHAsc, dehydroascorbic acid;

AO, ascorbic acid oxidase; GSH-DH, glutathione de-

hydrogenase; GSH, reduced glutathione; GSSG, oxi-

dized glutathione; CSH, cysteine; CSSC, cystine;

PSSP, gluten proteins