25 Rye Constituents and Their Impact on Rye Processing 577glutamine/glutamate, proline, and phenylalanine
and the absence of cysteine and methionine (Tatham
and Shewry 1991). The -secalins consist almost
entirely of repetitive sequences (Tatham and Shewry
1991) forming a stiff, wormlike coil (Tatham and
Shewry 1995). They are present predominantly as
monomers (Field et al. 1983, Gellrich et al. 2003)
and are homologous to the -gliadins of wheat and
to the C hordeins of barley (Field et al. 1983, Gel-
lrich et al. 2003, Tatham and Shewry 1991) (Fig.
25.1).
HMW secalins are also quantitatively minor com-
ponents, accounting for about 5% of the total secalin
fraction (Gellrich et al. 2003). SDS-PAGE molecu-
lar weights above 100 k were observed (Field et al.
1982, Gellrich et al. 2003, Kipp et al. 1996, Shewry et
al. 1983), whereas sedimentation equilibrium ultra-
centrifugation showed a molecular weight of 68 k
(Field et al. 1982). The HMW secalins have a high
content of glycine and glutamine/glutamate and
contain lower levels of proline than the other secalin
groups (Field et al. 1982, Tatham and Shewry 1991).
The conformation of HMW secalins has not been
reported in detail, but based on studies with HMW
glutenin subunits (HMW-GS) of wheat, it was sug-
gested that they contain central repetitive sequences
with a rodlike shape, flanked by nonrepetitive
domains having a globular conformation rich in -
helical structure. The nonrepetitive domains contain
most of the cysteine residues involved in intermole-
cular cross-linking (Shewry et al. 1995). The HMW
secalins are present only in an aggregated state sta-
bilized by disulphide bonds (Field et al. 1983,
Gellrich et al. 2003, Shewry et al. 1983). They are
homologous to the HMW-GS of wheat and the D
hordeins of barley (Field et al. 1982, 1983; Gellrich
et al. 2003; Shewry et al. 1982, 1988; Fig. 25.1).
Glutelins In contrast to the wheat glutelins, which
are called “glutenins,” no alternative name has been
given to the rye glutelin fraction. The glutelins are
not soluble in aqueous alcohols. They form HMW
polymers stabilized by interchain disulphide bonds.
However, the individual subunits of the polymers
obtained in the presence of a reducing agent are sol-
uble in alcohol media and rich in proline and gluta-
mine (Field et al. 1982, 1983; Gellrich et al. 2003;
Kipp et al. 1996; Shewry et al. 1983). The building
blocks of the glutelins are therefore considered to be
secalins. The glutelins soluble in alcohol after re-
duction contain mainly HMW-secalins (26%) and
75k -secalins (52%) and small levels of 40k -
secalins (12%) (Gellrich et al. 2003; Fig. 25.1).
When the distribution of the 75k-, 40k-,- and
HMW secalins over the storage proteins (secalins
glutelins) is considered, it can be concluded that the
major portion of 75k -secalins is present in the pro-
lamin fraction (40%) and only 7% in the glutelin
fraction. The 40k -secalins (23%) and -secalins
(16%) appear mainly in the prolamin fraction due to
their monomeric state. A larger portion of HMW
secalins is present in the prolamin (4%) than in the
glutelin fraction (3%) (Gellrich et al. 2003).Gluten Formation The wheat gliadin and glu-
tenin fractions can form a polymeric gluten network
by intermolecular noncovalent interactions and di-
sulphide bonds. In contrast to wheat, rye storage
proteins do not form gluten. Despite partial homolo-
gy, storage proteins of rye differ significantly from
those of wheat with respect to quantitative and struc-
tural parameters that are important for the formation
and properties of gluten. Typical for rye are the low
content of storage proteins and the high ratio of
alcohol-soluble proteins to insoluble proteins (Chen
and Bushuk 1970, Gellrich et al. 2003, Preston and
Woodbury 1975). The 75k -secalins and HMW
secalins differ in aggregation behavior from the
LMW-GS and HMW-GS of wheat, respectively, due
to the absence (Gellrich et al. 2004b) or presence
(Köhler and Wieser 2000) of cysteine residues in the
C-terminal domain in positions favorable for forma-
tion of intramolecular disulphide bonds and unfa-
vorable for formation of intermolecular disulphide
bonds. It is likely that these structural differences
contribute to the lack of ability of rye to form gluten.
Another factor hampering the formation of a protein
network from rye secalin and glutelin fractions may
be a higher degree of glycosylation in rye subunits
than in wheat subunits (Kipp et al. 1996).Functional ProteinsAmong the many enzymes and structural proteins in
cereals in general, the most important enzymes pres-
ent in rye grain that break down its major con-
stituents (starch, arabinoxylan, and protein) are the
-amylases, endoxylanases, and proteases, respec-
tively. Such enzymes can be of major importance in
rye processing. Rye also contains specific proteins