696 15 Cereals and Cereal Products
Table 15.19.Amylases in wheat
Properties α-Amylase Iα-Amylase IIβ-Amylase
pH optimum 3.6–5.75 5.5–5. 75 .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
agel chromatography,bultracentrifugation.
Twoα-amylases,α-AI andα-AII, have been iso-
lated from wheat by affinity chromatography and
chromatofocussing. These two enzymes produce
a series of multiple forms on SDS-PAGE elec-
trophoresis. The ratio of the concentrations of the
twoα-amylases depends on the stage of develop-
ment. After flowering,α-AI appears first in the
outer layers of the kernel, then decreases with in-
creasing ripeness. Low activities ofα-AII are de-
tectable even before dormancy, but they greatly
increase during germination. The twoα-amylases
differ in their pH optimum, molar mass, and iso-
electric point (Table 15.19).α-AII is more tem-
perature resistant.
The pH optimum ofα-amylase in germinating rye
lies in a range similar to that ofα-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 wheatβ-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 cleav-
age 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 iso-
lated from wheat germ, is not considered a lipase
but an esterase (cf. 3.7.1.1). The activity in dor-
mant seeds is low, but increases greatly on ger-
mination and can be detected with great sensitiv-
ity with a fluorochrome substrate, e. g., fluores-
cein 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 ob-
servable 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 lipoxyge-
nase 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 ex-
traction that the phospholipase D activities are
relatively high in ripe cereals and this enzyme
transfers the phosphatidyl residue of phospho-
lipids 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 (“phos-
pholipase 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 prod-
ucts, phopholipases B and D can lower the phos-
pholipid content.15.2.2.4 PhytaseCereals contain about 1% of phytate [myoinos-
itol (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