25 Rye Constituents and Their Impact on Rye Processing 573Ferulic acid and ferulic acid dehydrodimer con-
tents in rye vary from 0.90 to 1.17% and from 0.24
to 0.41%, respectively, and are significantly influ-
enced by both rye genotype and harvest year (An-
dreasen et al. 2000b). These phenolic compounds
are concentrated in the bran of the rye kernel (An-
dreasen et al. 2000a, Glitso and Bach Knudsen
1999).
The water-extractable arabinoxylans from rye
flour contain low levels of ferulic acid (0.03–0.15%)
(Figueroa-Espinoza et al. 2002, Vinkx et al. 1993)
and ferulic acid dehydrodimers (0.03 10 -2%)
(Dervilly-Pinel et al. 2001, Figueroa-Espinoza et al.
2002).
The water-unextractable arabinoxylans from rye
flour contain higher amounts of ferulic acid (0.20–
0.36%) and ferulic acid dehydrodimers (0.35%)
than their water-extractable counterparts (Figueroa-
Espinoza et al. 2002). After alkaline extraction with
saturated barium hydroxide and 1.0 M sodium
hydroxide, rye flour arabinoxylans still contain a
substantial level of ferulic acid. Further fractiona-
tion of these arabinoxylans by ammonium sulfate
precipitation reveals increasing levels of ferulic acid
with increasing ammonium sulfate concentration
(Cyran et al. 2004). This observation might corrobo-
rate the above-mentioned hypothesis that the differ-
ent arabinoxylan fractions with varying structures
obtained from rye flour may reflect contamination of
the rye flour with bran fractions.
Arabinoxylan Physicochemical Properties
Solubility of a large part of the individual arabi-
noxylan molecules in water is mainly associated
with the presence of arabinose substituents attached
to the xylose residues, which prevent intermolecular
aggregation of unsubstituted xylose residues. High
molecular weight arabinoxylans with arabinose:
xylose ratios below 0.3 tend to be insoluble.
Rye arabinoxylans are said to have high water-
holding capacities, with water-extractable and water-
unextractable arabinoxylans able to absorb 11 and
10 times their weight of water, respectively
(Girhammar and Nair 1992a). These values were
typically determined by addition of arabinoxylan to
flour and measurement of the consecutive rise in
Farinograph absorption. However, it is not clear
whether the results obtained by this technique may
be simply defined as “water-holding capacity.” The
mechanism behind water binding of discrete cell
wall fragments that contain water-unextractable ara-
binoxylans is bound to be different from that of
water-extractable arabinoxylans in solution. It is
also unclear whether the increased Farinograph
absorption is solely caused by the arabinoxylans.
That water-holding values obtained with the Far-
inograph method have to be interpreted with care
is clearly demonstrated by Girhammer and Nair
(1992b), who reported a water-holding capacity of
0.47 g/g dry matter for rye water-extractable arabi-
noxylan, when determined by measuring the level
of unfreezable water associated with the water-
extractable arabinoxylan using differential scanning
calorimetry. These authors stated that the water-
holding capacity was not related to the molecular
weight of the arabinoxylan. Different milling frac-
tions of rye showed different water absorption
capacities in relation to this total arabinoxylan con-
tent (Härkönen et al. 1997).
Even at relatively low concentrations, water-
extractable arabinoxylans are able to form highly
viscous solutions in water (Girhammar and Nair
1992b). Highly positive correlations were found be-
tween the viscosity of a rye extract and its content of
water-extractable arabinoxylans (Boros et al. 1993;
Fengler and Marquardt 1988a; Härkönen et al. 1995,
1997; Ragaee et al. 2001). For different rye milling
fractions, extract viscosity correlates with the con-
tent of water-extractable arabinoxylans (Fengler and
Marquardt 1988a, Glitso and Bach Knudsen 1999,
Härkönen et al. 1997). Water extracts from rye are
more viscous than those from other cereals (Boros et
al. 1993, Fengler and Marquardt 1988a), which can
be ascribed to the higher concentration of water-
extractable arabinoxylans in rye than in other cere-
als. Differences in viscosity are related to differ-
ences in the structural features and molecular weight
of water-extractable arabinoxylans. Although the
impact of un-, mono-, and disubstitution is unclear
(Bengtsson et al. 1992b, Dervilly-Pinel et al. 2001,
Ragaee et al. 2001), molecular weight seems a logi-
cal determining parameter (Girhammar and Nair
1992b, Nilsson et al. 2000, Ragaee et al. 2001,
Vinkx et al. 1993). Arabinoxylan conformation was
also found to have a bearing on viscosity, with a
higher viscosity being associated with a larger ra-
dius of gyration (Dervilly-Pinel et al. 2001, Ragaee
et al. 2001). Viscosity measurement conditions such
as shear rate (Bengtsson et al. 1992b, Härkönen et