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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

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