Food Chemistry

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 oxi-
dative cross-linkage of pentosans (Fig. 15.19),
which is catalyzed by peroxidase, plays an
essential role in dough.
As heme catalysts they accelerate the nonenzym-
atic oxidation of ascorbic acid to the dehydro
form. The involvement of both enzymes in the ac-
tion of ascorbic acid as an improver will be dis-
cussed (cf. 15.4.1.4.1).

15.2.2.7 Glutathione Dehydrogenase

This enzyme catalyzes the oxidation of glu-
tathione (GSH) in the presence of dehydroascor-
bic acid as an H-acceptor:

(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 speci-
ficity for the H-acceptor is not so pronounced.
As shown in Table 15.23, all four diastereo-
meric forms of dehydroascorbic acid are con-
verted, but with different velocities. The substrate
specificity corresponds to the varying activity of

Table 15.22.Activity of glutathione dehydrogenase

(GSH-DH) in wheat flour

Wheat cultivar GSH-DHa

Kranich 17. 3

Kolibri 13. 2

Benno 15. 4

Mephisto 16. 1

Diplomat 13. 2

Jubilar 16. 1

Caribo 12. 5

aActivity at pH 6.5(25◦C): μmol ofL-threo-ascorbic

acid per minute and g of flour.

the diastereomeric dehydroascorbic acids in flour

improvement (cf. 15.4.1.4.1).

15.2.2.8 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

An ascorbic acid oxidase (AO) occurs in wheat

flour (Table 15.25), which oxidizesL-threo-and

D-erythro-ascorbic acid at comparable rates. In

addition, a substance has been found in flour

extracts which oxidizesL-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 cata-

lyzes 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