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 657other cell wall constituents. 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 genetic factors but also by the conditions un-
der which extraction occurs (Cyran et al. 2003, H ̈arkonen et al. ̈
1995). While the rye bran and shorts are high in arabinoxy-
lan (14.9–39.5% and 13.5–18.7%, respectively), the endosperm
contain lower levels (2.1–4.9%) (Fengler and Marquardt 1988a,
Glitso and Bach Knudsen 1999, H ̈ark ̈onen et al. 1997, Nilsson
et al. 1997a).
A considerable variation in substitution degree and pattern of
xylose residues, arabinoxylan molecular weight, and ferulic acid
content has been reported for both water-extractable and water-
unextractable arabinoxylan. This variability in structure can, in
part, account for the differences in extractability and solubility
of the arabinoxylans.Substitution Degree and Pattern of Xylose Residues
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, arabinoxylan I and arabinoxylan II. Arabinoxy-
lan I contains mainly un- and monosubstituted xylose residues
with on average 50% of the xylose residues substituted at O-3
with an arabinose residue. Only 2% of the xylose residues are
disubstituted at O-2 and O-3 with arabinose residues (Bengts-
son and Aman 1990). The xylose residues carrying an arabi-
nose side chain occur predominantly as isolated residues (36%)
or small blocks of two residues (62%) (Aman and Bengtsson
1991). Arabinoxylan II contains mainly un- and disubstituted
xylose residues with on average 60–70% of the xylose residues
substituted at O-2 and O-3 with arabinose residues (Bengtsson
et al. 1992a). The levels of arabinoxylan I and II in different rye
varieties ranges from 1.4 to 1.7% and from 0.6 to 1.0%, respec-
tively (Bengtsson et al. 1992b). In contrast to the two classes
of arabinoxylans described by Bengtsson et al. (1992a), Vinkx
et al. (1993) concluded that there were a range of rye water-
extractable arabinoxylans. The latter authors fractionated rye
water-extractable arabinoxylans by graded ammonium sulfate
precipitation into several fractions with arabinose:xylose (A/X)
ratios of 0.5–1.4, with among them a major fraction containing
almost purely O-3 monosubstituted arabinoxylans and a minor
fraction consisting of almost purely disubstituted arabinoxylans.
All arabinoxylan fractions contained a small amount of xylose
residues substituted at O-2 with arabinose (Vinkx et al. 1995a).
Cyran et al. (2003) obtained rye flour arabinoxylans with
different structural features after sequential extraction with wa-
ter at different temperatures. A gradual increase in the degree
of substitution in general and disubstitution in particular and a
decrease in O-3 monosubstitution were observed from cold to
hot water-extractable fractions. Within a water-extractable frac-
tion, subfractions obtained after ammonium sulfate precipitation
showed structures analogous to those reported by Bengtsson
et al. (1992a) and Vinkx et al. (1993).
Rye water-unextractable arabinoxylan molecules also consist
of a range of structures, which can only be studied after alka-
line solubilization of the arabinoxylans. This treatment results
in the saponification of the ester bonds linking ferulic acid toarabinose, releasing individual arabinoxylan molecules from the
cell wall structure. Based on the studies of Hromadkova et al.
(1987), Nilsson et al. (1996, 1999) and Vinkx et al. (1995b)
(Table 34.2), three different groups of alkali-extractable arabi-
noxylans can be distinguished in rye bran. A first group shows
an intermediate arabinose:xylose ratio (0.54–0.65) and contains
mainly unsubstituted (57–64%) and monosubstituted (24–29%)
xylose residues. A second group consists of almost pure un-
substituted (89%) arabinoxylans with a low arabinose:xylose
ratio (0.20–0.27). A third group is characterized by a high arabi-
nose:xylose ratio (1.08–1.10) and contains approximately 46%
monosubstituted and 33% di-substituted xylose residues. In con-
trast to rye bran water-unextractable arabinoxylans, the rye flour
alkali-solubilized arabinoxylans all show similar xylose substi-
tution levels (Cyran et al. 2004, Vinkx 1994) (Table 34.2). How-
ever, further fractionation of the alkali-extracted arabinoxylans
from rye flour by ammonium sulfate precipitation yields sub-
fractions that differ in structure (Cyran et al. 2004). The arabi-
noxylans sequentially extracted with alkali from rye bran show
lower degrees of branching than those extracted from rye flour
by the same extraction solvent (Table 34.2).
In general, for rye bran as well as for rye flour, low yields
of alkali-extractable arabinoxylan fractions with low (0.2) and
high (1.1) arabinose:xylose ratios are obtained. Glitso and Bach
Knudsen (1999) found lower substitution degrees for water-
unextractable arabinoxylans in the aleurone layer (0.35) than
for those in the starchy endosperm (0.83) and pericarp/testa
fraction (1.02). From these results and because in rye milling
the endosperm can be contaminated with bran, one can assume
that the arabinoxylan fractions with high substitution degrees
isolated from rye flour originate from contamination with bran
fractions. Similarly, the arabinoxylan fractions isolated from rye
bran might be contaminated with endosperm fractions.
Water-unextractable arabinoxylans in bran have a lower de-
gree of branching than their water-extractable counterparts
(Glitso and Bach Knudsen 1999), whereas the opposite is the
case in the endosperm (Cyran and Cygankiewicz 2004, Glitso
and Bach Knudsen 1999).Molecular Weight Large differences in molecular weight of
rye arabinoxylans exist. For rye, whole meal water-extractable
arabinoxylans, average molecular weights of 770 k (Girhammar
and Nair 1992a) and more than 1000 k (Hark ̈ onen et al. 1995), ̈
as determined by gel permeation chromatography, have been
reported. Whether the water-extractable arabinoxylans from rye
bran have a higher (Meuser et al. 1986) or lower (H ̈arkonen et al. ̈
1997) mean molecular weight than those from rye flour is not
clear. It is also not clear whether extraction temperature affects
the molecular size distribution of rye flour water-extractable
arabinoxylans (Cyran et al. 2003) or not (H ̈arkonen et al. 1995, ̈
Meuser et al. 1986). For water-extractable arabinoxylans iso-
lated from rye varieties differing in extract viscosity, increasing
molecular weights were correlated with increasing rye extract
viscosity (Ragaee et al. 2001). Rye water-extractable arabinoxy-
lans have higher average molecular weights than those of wheat
(Dervilly-Pinel et al. 2001, Girhammar and Nair 1992a).
The water-unextractable arabinoxylans from rye flour
have a higher molecular weight (1500–3000 k) than their