584 Part V: Fruits, Vegetables, and Cereals
dry and firm crumb, resulting in a short shelf life
(Autio and Salmenkallio-Marttila 2001, Fabritius
et al. 1997). However, when rye flours with low
-amylase activity contain high levels of total and
water-extractable arabinoxylans, better dough and
bread characteristics can be observed (Weipert
1993a). It is believed that optimum bread-making
performance is reached at an optimal arabinoxylan
to starch ratio of 1:16 (Seibel and Brümmer 1991).
This ratio provides balanced water-binding capacity
and water distribution in the dough and the bread
crumb. A shift in favor of more arabinoxylan causes
stiffening of the dough (Kühn and Grosch 1989),
due to the high water-holding capacity of the arabi-
noxylans. It also results in breads of greater specific
volume and a firmer crumb (Kühn and Grosch
1989). A shift in favor of starch results in a low
dough yield, soft dough, a flat bread form, a split
and tight crumb, and a low shelf life (Vinkx and
Delcour 1996).
Arabinoxylans Although many studies have re-
ported conflicting results concerning the functional
role of arabinoxylans in the bread-making process
and during storage of breads, they are of consider-
able importance for rye bread-making quality.
Rye cultivars with high total arabinoxylan con-
tent, particularly when they also have a high water-
extractable arabinoxylan fraction, perform much
better in bread making than those with low ara-
binoxylan content. This is due to the high water-
holding capacity of the arabinoxylans, which results
in a delay in starch gelatinization and protects starch
from -amylase degradation, resulting in a higher
volume yield, a better crumb elasticity, and a longer
bread shelf life (Weipert 1993a, 1994, 1997). Small
differences in arabinoxylan content cause differ-
ences in water absorption and significant differences
in bread-making performance (Hansen et al. 2004,
Weipert 1997).
Besides the content of arabinoxylans, their struc-
ture also significantly influences rye bread-making
performance. Differences in arabinoxylan structure,
such as degree of branching and ferulic acid cross-
linking, as well as molecular size, may have influ-
ence on water absorption (Hansen et al. 2004). A
high degree of branching and a high degree of fer-
ulic acid cross-linking of arabinoxylans are related
to low water absorption in rye flour (Hansen et al.
2004).
The dough and bread-making properties of rye
flour also depend on the type of arabinoxylans pres-
ent. In general, dough consistency is negatively relat-
ed to dough water content. Because of their high (and
similar) water-holding capacities, both water-
extractable and water-unextractable arabinoxylans
affect dough consistency (Kühn and Grosch 1989).
However, the water-extractable and -unextractable
arabinoxylans affect the bread-making properties of
rye flour differently. The water-extractable arabi-
noxylans increase bread specific volume and
decrease crumb firmness, whereas the water-unex-
tractable arabinoxylans decrease bread specific vol-
ume and form ratio and slightly increase crumb
firmness (Kühn and Grosch 1989). Increased water-
extractable to water-unextractable arabinoxylan
ratios result in flatter breads with higher specific vol-
ume, softer crumb, and darker crust (Cyran and
Cygankiewicz 2004, Kühn and Grosch 1989).
Enzymic breakdown of the water-unextractable
arabinoxylans by endoxylanases to solubilized ara-
binoxylans with medium to high molecular weight
and with limited depolymerization of the water-
extractable fraction has been reported to improve
both rye (Kühn and Grosch 1988) and wheat (Cour-
tin et al. 1999) bread-making performance. The
increase in the level of high molecular weight arabi-
noxylans in the dough aqueous phase results in an
improved water distribution and facilitation of mo-
lecular interactions between the dough constituent
macromolecules. The subsequent increase in viscos-
ity of the dough aqueous phase is likely to play a
positive role in dough rheology and gas retention
(Courtin et al. 1999, Rouau et al. 1994). Kühn and
Grosch (1988) showed an improved crumb structure,
decreased crumb firmness, increased width to height
ratio, and increased loaf volume of rye bread after
enzymic treatment of the water-unextractable arabi-
noxylans. Shelf life is also increased, resulting from
an initial decrease in crumb firmness (Kühn and Gro-
sch 1988). The endogenous arabinoxylan-degrading
enzymes of rye might be active under the conditions
prevailing in rye dough (Hansen et al. 2002, Ras-
mussen et al. 2001) and in the first part of the oven
phase, thus prior to enzyme inactivation (Rasmussen
et al. 2001). Endogenous enzymes in flour may thus
affect crumb properties such as firmness, elasticity,
and stickiness (Nilsson et al. 1997b).
According to Figueroa-Espinoza et al. (2004), the
enzymic release of high molecular weight arabi-