BLBS102-c34 BLBS102-Simpson March 21, 2012 14:7 Trim: 276mm X 219mm Printer Name: Yet to Come
34 Rye Constituents and Their Impact on Rye Processing 655Europe and Asia, but of less importance in America. Production
has remained fairly constant, but with a downward trend, during
the past 10 years.THE RYE KERNEL
The main components of the rye kernel are, from the outside to
the inside, the hull, pericarp, testa, nucellar epidermis, aleurone
and starchy endosperm, and the germ. As the hull threshes free,
the pericarp is the actual outer part of the grain, surrounding the
seed and consisting of different layers, ranging from the epider-
mis, hypodermis, and cross cells to tube cells. The latter adhere
to the testa or seed coat, which is in turn lined with the nucellar
epidermis. The aleurone cells are located beneath the nucellar
epidermis, forming the outermost layer of the endosperm tis-
sue, and merge with the scutellum of the germ. The starchy
endosperm is composed of three main cell types, that is, the sub-
aleurone or peripheral cells, the prismatic cells, and the central
endosperm cells, and contains two major storage reserves. The
subaleurone cells contain high levels of storage protein, while
the prismatic cells contain high levels of starch granules (Shewry
and Bechtel 2001). The pericarp, testa, and nucellar epidermis
form the botanical bran and are separated from the starchy en-
dosperm together with the aleurone layer and the germ in the
milling process, forming the technical or miller’s bran.RYE CONSTITUENTS
The major rye constituents are starch and nonstarch polysaccha-
rides and protein. Its lipids, vitamins, and minerals are minor
constituents (Table 34.1).StarchStarch CompositionStarch granules are mainly composed of two types of polymers,
amylose, and amylopectin. In general, amylose is an essentially
linear polymer consisting ofα-d-glucopyranose residues linked
byα-1,4 bonds with few (<1%; Ball et al. 1996)α-1,6 bonds.
Amylose occurs in both free and lipid-complexed forms. Lipid-complexed amylose may be present in native starch, but is pos-
sibly also formed during gelatinization of starch (Andreev et al.
1999, Morrison 1995). Amylose contents in rye starch vary from
12 to 30% (Andreev et al. 1999, Fredriksson et al. 1998, Mo-
hammadkhani et al. 1998, Wasserman et al. 2001). Berry et al.
(1971) reported the rye amylose fraction to have a molecular
weight of about 220 k.
Amylopectin is a highly branched polymer consisting ofα-
d-glucopyranose residues linked byα-1,4 linkages with 4–5%
(according to Keetels et al. 1996)α-1,6 linkages. Berry et al.
(1971) showed that rye amylopectin has 4.8% branching, with
branch points about every 21 glucose units on average. The
chains of amylopectin can be classified into three types: A-, B-,
and C-chains. The A-chains are unbranched outer chains and are
attached to the inner B-chains through their potential reducing
end. In their turn, the B-chains are attached to other B-chains
or to the single C-chain through their potential reducing end.
The C-chain is the only chain carrying a reducing end group.
Lii and Lineback (1977) reported an A-chain to B-chain ratio
of 1.71–1.81 and an average unit chain length of 26 for rye
amylopectin.
Minor components of starch granules are lipids, proteins,
phosphorus, and ash.Starch StructureRye starch has a bimodal particle size distribution comprising a
major population of large, lenticular A-type granules (> 10 μm,
85%), and a minor population of small, spherical B-type granules
(> 10 μm, 15%) (Schierbaum et al. 1991, Verwimp et al. 2004).
Rye A-type starch granules have larger average particle sizes
and broader particle size distribution profiles than their wheat
counterparts (Fredriksson et al. 1998, Verwimp et al. 2004).
In starch granules, the amylose and amylopectin molecules are
radially ordered, with their single reducing end groups towards
the center, or hilum. Ordering of crystallites within the starch
granule causes optical birefringence, and a Maltese cross can
be observed under polarized light. Starch granules are made up
of alternating semicrystalline and amorphous growth rings, ar-
ranged concentrically around the hilum. The amorphous growthTable 34.1.Composition of the Rye KernelConstituent Content (%) ReferencesStarch 57.0–65.6 Aman et al. 1997, Hansen et al. 2004
Nonstarch polysaccharide 13.0–15.0 Hark ̈ onen et al. 1997 ̈
Arabinoxylan 6.5–12.2 Aman et al. 1997, Hansen et al. 2003, 2004, Henry 1987,
Vinkx and Delcour 1996
Mixed-linkageβ-glucan 1.5–2.6 Aman et al. 1997, Hansen et al. 2003, Henry 1987
Cellulose 2.1–2.6 Aman et al. 1997, Vinkx and Delcour 1996
Arabinogalactan peptide 0.2 Van den Bulck et al. 2004
Fructan 4.6–6.6 Hansen et al. 2003, Karpinnen et al. 2003
Protein 8.0–17.7 Fowler et al. 1990, Hansen et al. 2004
Lipid 2.0–2.5 Hark ̈ onen et al. 1997, Vinkx and Delcour 1996 ̈
Mineral 1.7–2.2 Aman et al. 1997, Hansen et al. 2004