692 15 Cereals and Cereal Products
Fig. 15.10.Schematic representation of the disulfide structures ofα-gliadins,γ-gliadins and LMW subunits (ac-
cording toKöhleret 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 ofα-,γ-andω-
gliadins are not extractable from flour with aque-
ous alcohol, but remain with the glutenins. It is
assumed that these proteins contain an odd num-
ber of cysteine residues due to point mutations,
one residue being available for intermolecu-
lar disulfide bonding. In fact, it has been ob-
served that LMW subunits are linked toγ-gliadins
which have 9 instead of 8 cysteine residues, via
a bridge from Cxto Cb∗(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 Cdand Cz(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 Cc1and Cc2or of Cywith 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 con-
tribution to the strength of dough and gluten
(r= 0. 58 − 0 .85). It is to be assumed that the ten-
dency of Cb∗and Cxto 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 Cb∗and Cxare not firmly di-
rected but variable. Thus, Cxalso binds toγ-glia-
din with an odd number of cysteine residues
(cf. 15.2.1.4.1) which would lead to chain break-
age in the polymerization of gluten proteins,
which possibly occurs during dough making.
Monomeric gliadins (cysteine-free ω-gliadins,
α-andγ-gliadins with an even number of
cysteine residues) are regarded as “solvent” or