25 Rye Constituents and Their Impact on Rye Processing 571arabinoxylan I and arabinoxylan II. Arabinoxylan 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 (Bengtsson 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 res-
idues with on average 60–70% of the xylose residues
substituted at O-2 and O-3 with arabinose resi-
dues (Bengtsson et al. 1992a). The levels of arabi-
noxylan I and II in different rye varieties ranges
from 1.4 to 1.7% and from 0.6 to 1.0%, respectively
(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 precipi-
tation 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 monosubsti-
tuted 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 arabinoxy-
lans with different structural features after sequen-
tial extraction with water 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 fraction, subfractions obtained
after ammonium sulfate precipitation showed struc-
tures 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 alkaline solubilization of the arabi-
noxylans. This treatment results in the saponifica-
tion of the ester bonds linking ferulic acid to arabi-
nose, 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 25.2), three
different groups of alkali-extractable arabinoxylans
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 sec-
ond group consists of almost pure unsubstituted
(89%) arabinoxylans with a low arabinose:xylose
ratio (0.20–0.27). A third group is characterized by a
high arabinose:xylose ratio (1.08–1.10) and contains
approximately 46% monosubstituted and 33% di-
substituted xylose residues. In contrast to rye bran
water-unextractable arabinoxylans, the rye flour
alkali-solubilized arabinoxylans all show similar
xylose substitution levels (Cyran et al. 2004, Vinkx
1994) (Table 25.2). However, further fractionation
of the alkali-extracted arabinoxylans from rye flour
by ammonium sulfate precipitation yields subfrac-
tions that differ in structure (Cyran et al. 2004). The
arabinoxylans sequentially extracted with alkali
from rye bran show lower degrees of branching than
those extracted from rye flour by the same extraction
solvent (Table 25.2).
In general, for rye bran as well as for rye flour,
low yields of alkali-extractable arabinoxylan frac-
tions 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 frac-
tions isolated from rye bran might be contaminated
with endosperm fractions.
Water-unextractable arabinoxylans in bran have
a lower degree 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 (Härkönen et al. 1995), as deter-
mined by gel permeation chromatography, have been