2009_Book_FoodChemistry

696 15 Cereals and Cereal Products
Table 15.19. Amylases in wheat
Properties
α-Amylase I α-Amylase II β-Amylase
pH optimum 3.6–5.75 5.5–5.7 5.4–6.2
Molar mass 37,000 a 21,000 a 64,200 b
Isoelectric
point
4.65–5.11 6.05–6.20 4.1–4.9
a gel chromatography, b ultracentrifugation.
Two α-amylases, α-AI and α-AII, have been isolated
from wheat by affinity chromatography and
chromatofocussing. These two enzymes produce
a series of multiple forms on SDS-PAGE electrophoresis.
The ratio of the concentrations of the
two α-amylases depends on the stage of development.
After flowering, α-AI appears first in the
outer layers of the kernel, then decreases with increasing
ripeness. Low activities of α-AII are detectable
even before dormancy, but they greatly
increase during germination. The two α-amylases
differ in their pH optimum, molar mass, and isoelectric
point (Table 15.19). α-AII is more temperature
resistant.
The pH optimum of α-amylase in germinating rye
lies in a range similar to that of α-AII of wheat.
Therefore, α-amylase is partially inhibited by the
decrease in pH in sour dough (cf. 15.4.2.2).
The properties of wheat β-amylase are shown in
Table 15.19.
15.2.2.2 Proteinases
Acid endopeptidases with pH optima of 4–5
occur in wheat, rye and barley. Their substrate
specificity has been determined. The possibility
that the wheat proteinases are involved in cleavage
of gluten bonds, thereby affecting softening
or mellowing of gluten during baking, is still
disputed.
15.2.2.3 Lipases
These enzymes occur in various concentrations in
all cereals. Carboxylester hydrolase, readily isolated
from wheat germ, is not considered a lipase
but an esterase (cf. 3.7.1.1). The activity in dormant
seeds is low, but increases greatly on germination
and can be detected with great sensitivity
with a fluorochrome substrate, e. g., fluorescein
dibutyrate. Therefore, this forms the basis of
a method for the quick detection of “sprouting” in
wheat and rye.
In addition to the esterase, a wheat lipase occurs
enriched in the bran. A rise in free fatty acids observable
during flour storage also involves lipases
from metabolism of microorganisms present in
flour.
In comparison to other cereals, oats contain
a significant level of lipase. Its high activity is
released once the oat kernel is disintegrated,
crushed or squeezed. Linoleic acid is released
from the acyl lipids that are present. It is then
converted into hydroxy fatty acids by lipoxygenase
and hydroperoxidase enzymes, giving rise
to off-flavors (Fig. 15.11). All these enzymes are
inactivated by heat treatment and thus quality
deterioration can be avoided (cf. 15.3.2.2.2).
It should be taken into account in lipid extraction
that the phospholipase D activities are
relatively high in ripe cereals and this enzyme
transfers the phosphatidyl residue of phospholipids
to alcohols, which are used to extract
lipids (cf. 3.7.1.2.1). The enzyme is inactivated
during extraction with boiling water-saturated
butanol. A phospholipase that hydrolyzes both
acyl residues in the lecithin molecule (“phospholipase
B”) has been found in germinating
cereal. It influences the foam stability in beer
(cf. 20.1.7.9).
In the production and storage of egg dough products,
phopholipases B and D can lower the phospholipid
content.
15.2.2.4 Phytase
Cereals contain about 1% of phytate [myoinositol
(1,2,3,4,5,6) hexakisphosphate], which binds
about 70% of the phosphorus in the grain. Since it
occurs mainly in the aleurone layer, the content of
phytate in flour depends on the extent of grinding
(Table 15.20). A part of it is hydrolyzed in stages
to myo-inositol during dough making.
The phytases originate in cereals (Table 15.21),
but are also synthesized by microorganisms,
e. g., yeast. Therefore, if the baking process