Food Chemistry

15.2 Individual Constituents 697

Table 15.20.Phytate content in wheat flour

Degree of Phytate
grinding (mg/kg)a

70% 53
85% 451
92% 759

aBased on solids.

Table 15.21.Phytase activity and phytate content in
cereals

Type of Phytase Phytate
cereal activitya contentb

Wheat 180 12. 4
Triticale 650 12. 9
Rye 2800 11. 8
Barley 350 11. 9
Oats 48 11. 3
Corn 9 9. 2

aActivity: units/g cereal.
bContent: mg/g cereal.

takes 1 hour, 85–90% of the phytate is degraded
in white bread made of flour with 1.2g of
phytate/kg. In rye whole grain bread (10 g of
phytate/kg), 25–35% is degraded and 40–50% if
the baking process is extended to 4 hours.

(15.1)

Partial hydolysis of phytate to myo-
inositoltetrakis- and -triphosphate is desirable
from a nutritional physiological point of view. In
comparison with phytate, these less phosphory-
lated myo-inositols do not form such stable com-
plexes with cations. Consequently, the absorption
of zinc, iron, calcium and magnesium ions is not

impeded. On the other hand, they still possess the

positive nutritive properties of phytate.

15.2.2.5 Lipoxygenases

Cereals contain lipoxygenases (cf. 3.7.2.2)

which, with the exception of the enzyme from

rye, oxidize linoleic acid preferentially to

9-hydroperoxy acids. The rye lipoxygenase

forms mainly the 13-hydroperoxide isomer.

Though the enzyme from wheat belongs to the

specifically reacting LOX (cf. Table 3.33) and

thus cooxidizes carotenoids at a slow rate, it can

still bring about a loss of yellow color in pasta

products. This is the reason for the inactivation

of wheat lipoxygenase during the preparation of

pasta products (cf. 15.5).

The involvement of endogenous lipoxygenase in

the baking of wheat flour is not clear. However,

by addition of lipoxygenase-active soy flour,

a significant improvement of the flour quality is

achieved (cf. 15.4.1.4.3).

As shown in Fig. 15.11, oats contain a lipoxyge-

nase with lipoperoxidase activity. This activity re-

duces the hydroperoxides initially formed, in the

presence of phenolic compounds as H-donors, to

the corresponding hydroxy acids:

(15.2)

Fig. 15.15.Formation of bitter tasting compounds in

oats (taste threshold values cf. 3.7.2.4.1)