570 Part V: Fruits, Vegetables, and Cereals
et al. (2004) of 49.2–51.4°C. Rye starch has some-
what lower gelatinization temperatures and enthal-
pies than wheat starch (Gudmundsson and Eliasson
1991, Radosta et al. 1992, Verwimp et al. 2004).
Lower dissociation enthalpies of the amylose-lipid
complexes for rye starch than for wheat starch were
observed by Gudmundsson and Eliasson (1991) and
Verwimp et al. (2004), indicating lower levels of
lipids in rye starch.
When starch suspensions are heated beyond their
gelatinization temperature, granule swelling and
amylose and/or amylopectin leaching continue.
Eventually, total disruption of the granules occurs,
and a starch paste with viscoelastic properties is
formed. This process is called pasting. Upon cool-
ing, gelation occurs. During gelation, at sufficiently
high starch concentrations (6%, w/w), the starch
paste is converted into a gel. The gel consists of
an amylose matrix enriched with swollen granules.
During gelation and storage of starch gels, retrogra-
dation occurs. This is a process whereby the starch
molecules begin to reassociate in an ordered struc-
ture, without regaining the original molecular order.
Initially, starch retrogradation is dominated by gela-
tion and crystallization of the amylose molecules. In
a longer time frame, amylopectin molecules recrys-
tallize. Rye starch exhibits a unique, typical swell-
ing behavior with a high pasting temperature (79–
87°C according to Schierbaum and Kettlitz 1994;
75°C according to Verwimp et al. 2004), a stable
consistency during cooking, and a high viscosity
increase during cooling (Schierbaum and Kettlitz
1994, Verwimp et al. 2004). It has lower pasting
temperatures than wheat starch (Schierbaum and
Kettlitz 1994, Verwimp et al. 2004). Fredriksson et
al. (1998) and Gudmundsson and Eliasson (1991)
showed that rye starches retrograde to a lesser extent
than starches from wheat or corn and ascribed this to
differences in the structure of the amylopectin poly-
mers.
NONSTARCHPOLYSACCHARIDES
Rye contains on average 13.0–15.0% nonstarch
polysaccharides, which serve mainly as cell wall
components and the majority of which are arabi-
noxylans (Table 25.1). Other nonstarch polysaccha-
ride constituents of rye are mixed-linked -glucan,
cellulose, arabinogalactan peptides, and fructans
(Table 25.1). The variation in nonstarch polysaccha-
ride content and composition is significantly influ-
enced by both harvest year and rye genotype (Han-
sen et al. 2003, 2004). Most of the nonstarch poly-
saccharides are classified as dietary fiber because
of their resistance to digestion by human digestive
enzymes. In view of the major importance of arabi-
noxylans in rye, these components will be further
discussed in detail.Arabinoxylan Occurrence and StructureArabinoxylans consist of a backbone of 1,4-linked
-D-xylopyranose residues, unsubstituted or mono-
or disubstituted with single -L-arabinofuranose
residues at the O-2- and/or O-3-position. To some of
these arabinose residues, a ferulic acid moiety is
ester bound at the O-5 position. In rye, as in other
cereals, two types of arabinoxylans can be distin-
guished: water extractable and water unextractable.
Water-unextractable arabinoxylans, representing ap-
proximately 75% of the total arabinoxylan popula-
tion of the rye kernel, are unextractable because they
are retained in the cell walls by covalent and nonco-
valent interactions among arabinoxylans and be-
tween arabinoxylans and other cell wall constit-
uents. Water-extractable arabinoxylans are thought
to be loosely bound at the cell wall surface. Their
content in the rye kernel ranges from 1.5 to 3.0%
(Figueroa-Espinoza et al. 2002; Hansen et al. 2003,
2004) and is influenced by environmental and genet-
ic factors but also by the conditions under which
extraction occurs (Cyran et al. 2003, Härkönen et al.
1995). While the rye bran and shorts are high in ara-
binoxylan (14.9–39.5% and 13.5–18.7%, respec-
tively), the endosperm contain lower levels (2.1–
4.9%) (Fengler and Marquardt 1988a, Glitso and
Bach Knudsen 1999, Härkönen et al. 1997, Nilsson
et al. 1997a).
A considerable variation in substitution degree
and pattern of xylose residues, arabinoxylan mo-
lecular weight, and ferulic acid content has been
reported for both water-extractable and water-
unextractable arabinoxylan. This variability in struc-
ture can, in part, account for the differences in ex-
tractability and solubility of the arabinoxylans.Substitution Degree and Pattern of Xylose Res-
idues Bengtsson et al. (1992a) reported on what
could either be two different types of polymers or
two different regions in the same molecule, that is,