2009_Book_FoodChemistry

692 15 Cereals and Cereal Products
Fig. 15.10. Schematic representation of the disulfide structures of α-gliadins, γ-gliadins and LMW subunits (according
to Köhler et al., 1993). Segments I–V (cf. Table 15.18)
HMW subsunits of the y-type as well as to LMW
subunits could be detected (Fig. 15.12).
It is noticeable that small amounts of α-, γ-andωgliadins
are not extractable from flour with aqueous
alcohol, but remain with the glutenins. It is
assumed that these proteins contain an odd number
of cysteine residues due to point mutations,
one residue being available for intermolecular
disulfide bonding. In fact, it has been observed
that LMW subunits are linked to γ-gliadins
which have 9 instead of 8 cysteine residues, via
a bridge from C x to C b∗ (Fig. 15.10).
15.2.1.4.2 Contribution of Gluten Proteins
to the Baking Quality
Investigations of the structure and amount of
gluten proteins in wheat cultivars with varying
dough and baking properties allow an estimation
of the contributions of individual gluten proteins
to quality. An important feature is the suitability
for forming, as far a possible, high molecular
protein aggregates. The x-type of the HMW
subunits appears to be especially predestined for
this because, e. g., it can form linear polymers via
the cysteine residues C d and C z (Fig. 15.12). This
is expressed in the close relationship (correlation
coefficient r > 0.8) between its amount and the
strength of the dough (Fig. 15.13).
The considerably lower coefficient in the case of
y-type (r < 0.3; Fig. 15.13) indicates that cross
linkage via C c1 and C c2 or of C y with LMW
subunits (Fig. 15.12) does not have an especially
positive effect on the consistency of the dough.
Apart from the HMW subunits of the x-type,
the LMW subunits also make a positive contribution
to the strength of dough and gluten
(r = 0.58−0.85). It is to be assumed that the tendency
of C b∗ and C x to polymerize is responsible
for this effect (Fig. 15.10). However, about twice
the amount of LMW subunits in flour is required
for the same effect. The reason for this could be
that the bonds of C b∗ and C x are not firmly directed
but variable. Thus, C x also binds to γ-gliadin
with an odd number of cysteine residues
(cf. 15.2.1.4.1) which would lead to chain breakage
in the polymerization of gluten proteins,
which possibly occurs during dough making.
Monomeric gliadins (cysteine-free ω-gliadins,
α- and γ-gliadins with an even number of
cysteine residues) are regarded as “solvent” or