2009_Book_FoodChemistry

15.4 Baked Products 719
15.4.1.4.2 Bromate, Azodicarbonamide
Addition of alkali bromates to flour also prevents
excessive softening of gluten during dough
making. The reaction involves oxidation of
endogenous glutathione to its disulfide. Bromate
reacts slower than ascorbic acid (Table 15.44).
After a kneading time of 3 minutes, GSH decreases
from 124 nmol/g to a concentration
of 40 nmol/g and cysteine increases from
22 nmol/g to 52nmol/g. These values are
relatively close to the corresponding values in
dough without additives. On the other hand,
only 11 nmol/g of GSH remain and cysteine
increases only slightly after the addition of Asc.
The reactions of bromate in dough have not yet
been elucidated. Model experiments indicate that
it can link gluten proteins by the formation of intermolecular
disulfide bonds. Then the oxidation
of GSH would not be the decisive step in flour
improvement. In comparison with Asc, bromate
can be overdosed, which also shows that another
mechanism must be involved here. During
baking, bromates are completely reduced to bromides
with no bromination of flour constituents.
Azodicarbonamide is of interest as a flour improver
H 2 N CO N N CO NH 2 (15.7)
since it improves not only the dough properties
of weak gluten flour, but also lowers the energy
input in dough mixing (cf. Fig. 15.40). Details of
the reactions involved are unknown.
15.4.1.4.3 Lipoxygenase
The addition of a small amount of enzymeactive
soy flour to a wheat dough increases the mixing
tolerance, improves the rheological properties
and may increase the bread volume. The effect on
dough rheology is shown only with high-power
mixing in the presence of air. The carotenoid pigments
of wheat flour are bleached by the addition
of enzyme-active soy flour. This is desirable
in the production of white bread. The amount of
enzyme-active soy flour is restricted to approximately
1% since higher levels may generate offflavors.
It was demonstrated that nonspecific lipoxygenase
(cf. 3.7.2.2) is responsible for the
Fig. 15.35. Wheat flour quality improvement by the
nonspecific lipoxygenase enzyme of soybean a (according
to Kieffer and Grosch, 1979). Additions: 1 control
(no addition, bread volume 31 ml), 2 extract of defatted
soya meal in which lipoxygenase was thermally inactivated
(31 ml), 3 extract of a defatted soya meal with
290 units of lipoxygenase b (35 ml), 4 purified type-II
enzyme with 285 activity units (37 ml).
a Results in small-scale baking, 10 g flour cv. Clement.
b One enzyme unit = 1µmole· min −1 oxygen uptake
with linoleic acid as substrate
improver action (Fig. 15.35) and the bleaching
effect caused by the enzyme-active soy flour.
This enzyme, in contrast to endogenous wheat
flour lipoxygenase, releases peroxy radicals
which cooxidize carotenoids and other flour
constituents.
15.4.1.4.4 Cysteine
Cysteine, in its hydrochloride form, softens
gluten due to a SH/SS interchange with the
glutenin fraction. The resistance to extension