2009_Book_FoodChemistry

698 15 Cereals and Cereal Products
15.2.2.6 Peroxidase, Catalase
Both enzymes are widely distributed among
cereals. The pH-activity curves of the enzymes
from wheat show that at the normal pH values
of a dough, about 6.3, catalase still has 40–50%
and peroxidase less than 10% of its activity at
the pH optimum (peroxidase pH 4.5; catalase
pH 7.5). Therefore, it is unlikely that the oxidative
cross-linkage of pentosans (Fig. 15.19),
which is catalyzed by peroxidase, plays an
essential role in dough.
As heme catalysts they accelerate the nonenzymatic
oxidation of ascorbic acid to the dehydro
form. The involvement of both enzymes in the action
of ascorbic acid as an improver will be discussed
(cf. 15.4.1.4.1).
Table 15.22. Activity of glutathione dehydrogenase
(GSH-DH) in wheat flour
Wheat cultivar
Kranich 17.3
Kolibri 13.2
Benno 15.4
Mephisto 16.1
Diplomat 13.2
Jubilar 16.1
Caribo 12.5
GSH-DH a
a Activity at pH 6.5 (25 ◦ C): µmol of L-threo-ascorbic
acid per minute and g of flour.
the diastereomeric dehydroascorbic acids in flour
improvement (cf. 15.4.1.4.1).
15.2.2.7 Glutathione Dehydrogenase
This enzyme catalyzes the oxidation of glutathione
(GSH) in the presence of dehydroascorbic
acid as an H-acceptor:
15.2.2.8 Polyphenoloxidases
In cereals, polyphenoloxidases preferably occur
in the outer layers of the kernels. Wheat enzymes
that exhibited cresol activity only (cf. 2.3.3.2) and
were known as tyrosinases have been separated
from polyphenoloxidases by chromatography and
preparative gel electrophoresis.
Polyphenoloxidases can cause browning in
whole-meal flours.
15.2.2.9 Ascorbic Acid Oxidase
(15.3)
It has been purified from wheat flour in which
its activity is relatively high (Table 15.22). The
enzyme is specific for the H-donor (Table 15.23)
because it oxidizes only GSH, and with a much
lower velocity also γ-glutamyl cysteine, but
neither cysteinyl glycine nor cysteine, which also
occur in wheat flour (Table 15.24). The specificity
for the H-acceptor is not so pronounced.
As shown in Table 15.23, all four diastereomeric
forms of dehydroascorbic acid are converted,
but with different velocities. The substrate
specificity corresponds to the varying activity of
An ascorbic acid oxidase (AO) occurs in wheat
flour (Table 15.25), which oxidizes L-threo- and
D-erythro-ascorbic acid at comparable rates. In
addition, a substance has been found in flour
extracts which oxidizes L-threo-ascorbic acid at
pH 10 at a maximal rate. In comparison with AO,
this activity does not decrease on incubation with
proteases nor is it inhibited by the addition of the
AO inhibitors KCN and NaF. It obviously catalyzes
a nonenzymatic oxidation of ascorbic
acid.
15.2.2.10 Arabinoxylan Hydrolases
In aqueous extracts of wheat flour, arabinoxylan
hydrolases have been detected with the synthetic