2009_Book_FoodChemistry

718 15 Cereals and Cereal Products
is catalyzed by enzyme GSH-DH (cf. 15.2.2.7)
which requires DHAsc as a cofactor. Ascorbic
acid is reformed which explains that relatively
small amounts of Asc are sufficient for flour
improvement. The GSSG formed in Reaction (b)
can undergo an SH/SS exchange with gluten
proteins (Reaction c), which has been shown to
proceed especially rapidly on addition of Asc.
Thus, GSH is incorporated into gluten proteins
as a terminator of polymerization reactions via
the intermediate GSSG. On the other hand, GSH
formed in Reaction (c) is immediately oxidized.
The reaction sequence (a→b→c) stops when
all the GSH is present as GSSG or incorporated
into gluten proteins. Consequently, GSH is
largely withdrawn from the dough before it
can depolymerize the gluten proteins by SH/SS
interchange. If it is not withdrawn from the
dough, Reactions (d)–(f) proceed without any
interference. In comparison with Reaction (c),
Reaction (d) can result in the softening of the
Table 15.43. Influence of L-threo- and D-erythroascorbic
acid (Asc) on the concentration of cysteine
(CSH) and glutathione in wheat dough
Sample a Additive (30 mg/kg) CSH GSH
(nmol/g) (nmol/g)
Flour Without 13 100
Dough Without 42 44
Dough L-threo-Asc 28 20
Dough D-erythro-Asc 41 39
a Production of the dough: DNS flour (0.78% ash, 10 g)
and water (6.5 ml) are kneaded at 30 ◦ C for 2 min; liquid
nitrogen is poured over the dough which is then
freeze dried; CSH and GSH are determined by isotopic
dilution analysis.
dough because GSH very specifically cleaves
intermolecular disulfide bonds of the gluten
proteins (cf. 15.2.3).
The results summarized in Tables 15.43 and 15.44
show that free cysteine increases rapidly in dough
making. This is explained by Reaction (e) in
Fig. 15.34, which shows that GSH reacts with
the cystine present in flour. Cysteine increases
and can, in turn, depolymerize gluten proteins
(Reaction f). If, however, L-threo-Asc is added
to the dough, GSH is so rapidly oxidized (Reaction
b) that the comparatively slow Reaction (e)
is strongly inhibited and cysteine increases only
slightly (Tables 15.43 and 15.44). Corresponding
to the substrate specificity of GSH-DH
(cf. 15.2.2.7), D-erythro-Asc is almost ineffective;
cysteine increases and GSH decreases as in
the experiment without an additive (Table 15.43).
The reaction scheme shown in Fig. 15.34 also
explains the baker’s experience that the consistency
of dough can be reduced by the addition
of cysteine almost independently of the presence
of L-threo-Asc. Reaction (f) is not prevented due
to the substrate specificity of GSH-DH which is
only directed at GSH. Reaction (g) is promoted
by the addition of L-threo-Asc because the
reduction of cystine (Reaction e) is inhibited by
the trapping of GSH. Consequently, cystine can
undergo SS/SH interchange with gluten proteins
according to Reaction (g) so that PSSC increases,
as demonstrated in model experiments.
The reaction scheme in Fig. 15.34 also explains
the observation that Asc cannot be overdosed.
The effect of Asc stops at the moment when all
the GSH is bound as GSSP and GSSG. An increase
in Asc has no further effect.
Table 15.44. Changes in concentration of glutathione (GSH) and cysteine (CSH) in dough on addition of L-threoascorbic
acid or bromate
Additive a GSH (nmol/g) CSH (nmol/g)
Kneading time (min) at 30 ◦ C
3 9 9+ 20 b 3 9 9+ 20 b
Without 57 22 17 68 28 31
L-threo-Asc (30 mg/kg) 11 6 2 26 17 19
KBrO 3 (50 mg/kg) 40 20 11 52 26 27
a The DNS flour used contained 124 nmol/g GSHand22nmol/g cysteine.
b The dough was allowed to rest for 20 min.