2009_Book_FoodChemistry

15.2 Individual Constituents 695
Fig. 15.14. Tensile tests of glutens with varying content
of gliadin (gluten K from retail wheat flour was
extracted with 70% aqueous ethanol. The extracted
gliadin and the remaining glutenin were freeze dried,
remixed in different proportions, and then hydrated.
Gliadin content of the glutens: K) 33.9%, 1) 55.9%,
14) 22.6%; the gliadin contents of the other samples
are in between, according to Kim et al., 1988)
amount of polymerizable gluten proteins (HMW
subunits of the x-type, LMW subunits) is required
with the lowest possible amounts of terminators
(low molecular thiol compounds, gliadins with
an odd number of cysteine residues, possibly also
HMW subunits of the y-type).
15.2.1.5 Puroindolins
The wheat endosperm contains two basic,
cysteine-rich proteins, puroindolin a and b (PINa
and -b). The name is derived from the presence
of tryprophan-rich segments in the amino aicd
sequences: Trp-Arg-Trp-Trp-Lys-Trp-Trp-Lys in
PIN-a and Trp-Pro-Thr-Trp-Trp-Lys in PIN-b.
PIN-a consists of a peptide chain with
115 residues (M r 12,479) and five disulfide
bridges. The peptide chains of PIN-a and PIN-b
are homologous to an extent of 60%. It has been
shown that the puroindolins are identical to the
basic friabilins which have been discovered on
the surface of starch granules. PIN-a and -b are
positively charged and bind negatively charged
phospholipids with a high affinity. PIN-a also
forms stable complexes with glycolipids while
PIN-b is less suitable. It is assumed that the
indolyl residues of the tryptophan-rich segments
are involved in the stabilization of the complexes,
hydrogen bridges being formed between the
indole-NH and the OH-groups of the glycolipids.
Thus, the higher stability of PIN-a compared
with PIN-b complexes is based on the longer
tryprophan-rich segment.
Foams of PIN-a and, to a smaller extent, of PIN-b
are stabilized especially in the presence of polar
wheat lipids. In this respect, PIN-a is clearly superior
to egg white proteins as shown by the following
comparison. After a drip off time of 5 min,
a foam density of 0.028 was obtained with 0.3mg
of PIN-a/ml, while 1.25 mg/ml of egg white proteins
were required for this purpose.
For the baking process, it is expected that the
puroindolins protect the foam-like texture of the
dough from destabilization by lipids.
15.2.2 Enzymes
Of the enzymes present in cereal kernels, those
which play a role in processing or are involved in
the reactions which are decisive for the quality of
a cereal product will be described.
15.2.2.1 Amylases
α- and β-amylases (for their reactions, see
4.4.5.1) are present in all cereals. Wheat and rye
amylases are of particular interest; their optimum
activities are desirable in dough making in the
presence of yeast (cf. 15.4.1.4.8). In mature
kernels, α-amylase activity is minimal, while it
increases abruptly during sprouting or germination.
Unlike the situation with wheat, dormancy
in rye is not very pronounced. Unfavorable harvest
conditions (high moisture and temperature)
favor premature germination (“sprouting”), not
visible externally. During this time, α-amylase
activity rises, resulting in extensive starch degradation
during the baking process. Bread faults
appear, as mentioned under 15.4.1.2.