574 Part V: Fruits, Vegetables, and Cereals
al. 1997, Nilsson et al. 2000) and temperature, pH,
and salt concentration (Girhammar and Nair 1992b)
also influence the viscosity of rye water-extractable
arabinoxylan solutions.
Water-extractable arabinoxylans undergo oxida-
tive gelation upon addition of hydrogen peroxide
and peroxidase, resulting in an increase in viscosity
and eventually the formation of a gel (Dervilly-Pinel
et al. 2001, Vinkx et al. 1991). The phenomenon
is caused by the formation of covalent linkages
through oxidative coupling of ferulic acid residues
esterified to arabinoxylan. The aromatic ring and not
the propenoic moiety of ferulic acid is involved in
the oxidative gelation (Vinkx et al. 1991). The oxi-
dative gelation capacity of arabinoxylans depends
on several parameters including the ferulic acid con-
tent, the molecular weight, and the structure of the
arabinoxylans. Arabinoxylan fractions with higher
ferulic acid content, higher molecular weights, and
fewer disubstituted xylose residues have a higher
gelation potential (Dervilly-Pinel et al. 2001, Vinkx
et al. 1993). Dervilly-Pinel et al. (2001) stated that
the intrinsic viscosity of arabinoxylans is the main
parameter governing gel rigidity.
PROTEINS
The total protein content in rye is influenced by both
environmental and genotypic factors and varies
from 8.0 to 17.7% (see Table 25.1). The protein con-
tent is higher in the outer layers than in the inner lay-
ers of the rye kernel (Glitso and Bach Knudsen
1999; Nilsson et al. 1996, 1997a).
Protein Classification
Rye proteins can be separated by the classical
Osborne fractionation procedure. The albumins are
extractable with water, the globulins with dilute salt
solutions, the prolamins with alcohol, and the glu-
telins with dilute acid or alkali. Chen and Bushuk
(1970) found that the albumin, globulin, prolamin,
and glutelin fractions accounted for 34, 11, 19, and
9%, respectively, of the total nitrogen content in a
rye flour, with 21% of the protein remaining in the
residue. Similar results were obtained by Preston
and Woodbury (1975) and Jahn-Deesbach and
Schipper (1980), except that the latter authors ex-
tracted a much higher proportion of glutelins.
Gellrich et al. (2003), however, extracted fewer
albumins and globulins and many more prolamins
from rye flour. In all cases, rye flour contained high-
er levels of albumins and globulins and lower levels
of prolamins and glutelins than wheat (Chen and
Bushuk 1970, Gellrich et al. 2003, Jahn-Deesbach
and Schipper 1980).
From a physiological point of view, rye proteins
can be divided into two main groups: the storage
proteins and the nonstorage proteins. The storage
proteins are deposited in protein bodies in the devel-
oping starchy endosperm and function solely as a
supply of amino acids for use during germination
and seedling growth. They include the prolamins,
glutelins, and the proteins in the Osborne residue.
The nonstorage proteins include, amongst others,
enzymes and (in some cases) their inhibitors and
correspond with the Osborne albumin and globulin
fractions.
Major differences are found in the amino acid
compositions of the proteins in the two groups. The
nonstorage proteins contain high proportions of
lysine and arginine and low proportions of gluta-
mine/glutamate and proline, whereas the opposite
was observed for the storage proteins (Dexter and
Dronzek 1975, Gellrich et al. 2003, Preston and
Woodbury 1975). No significant differences exist in
the amino acid composition of the proteins from dif-
ferent rye varieties (Dembinski and Bany 1991,
Morey and Evans 1983). The higher proportions of
lysine make rye proteins nutritionally superior to
wheat proteins (Chen and Bushuk 1970, Dexter and
Dronzek 1975, Jahn-Deesbach and Schipper 1980,
Morey and Evans 1983).
Table 25.3 gives an overview of the classification
of the rye proteins, which are of technological
importance and are further discussed in detail below.Storage ProteinsSecalins The rye alcohol-soluble prolamins, also
called “secalins,” form the major storage proteins in
rye (Gellrich et al. 2003). They have unusual amino
acid compositions with high glutamine/glutamate
and proline contents and low levels of basic and
acidic amino acids (Dexter and Dronzek 1975, Gel-
lrich et al. 2003, Preston and Woodbury 1975). The
secalins are highly polymorphic and can be classi-
fied into three groups on the basis of their amino
acid compositions: S-rich, S-poor, and high molecu-
lar weight (HMW) secalins (Fig. 25.1). Some