2009_Book_FoodChemistry

15.2 Individual Constituents 697
Table 15.20. Phytate content in wheat flour
Degree of
grinding
70% 53
85% 451
92% 759
a Based on solids.
Phytate
(mg/kg) a
Table 15.21. Phytase activity and phytate content in
cereals
Type of Phytase Phytate
cereal activity a content b
Wheat 180 12.4
Triticale 650 12.9
Rye 2800 11.8
Barley 350 11.9
Oats 48 11.3
Corn 9 9.2
a Activity: units/g cereal.
b Content: 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.
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 lipoxygenase
with lipoperoxidase activity. This activity reduces
the hydroperoxides initially formed, in the
presence of phenolic compounds as H-donors, to
the corresponding hydroxy acids:
(15.2)
(15.1)
Partial hydolysis of phytate to myoinositoltetrakis-
and -triphosphate is desirable
from a nutritional physiological point of view. In
comparison with phytate, these less phosphorylated
myo-inositols do not form such stable complexes
with cations. Consequently, the absorption
of zinc, iron, calcium and magnesium ions is not
Fig. 15.15. Formation of bitter tasting compounds in
oats (taste threshold values cf. 3.7.2.4.1)