Chemistry Reference
In-Depth Information
to glycoside hydrolase family 8.
92-94
This enzyme is a typical psychrophilic enzyme and
presents a high catalytic activity at low temperatures. It is not homologous to family 10
or 11 xylanases, but has 20-30% identity with glycoside hydrolase family 8 members
(formerly family D), a family that comprises mainly endoglucanases, but also lichenases and
chitosanases.
6.3.2 Mechanism
The mechanism of action of xylanases in bread preparation is still not clearly elucidated.
Many types of hemicellulase preparations have been used for the applications mentioned
above, and are commercially available. They are produced by microbial fermentation using
various microorganisms as enzyme sources. Many of these enzymes are produced by ge-
netically modified microorganisms. All documented commercial uses of xylanases relate to
enzymes belonging to either glycoside hydrolase family 10 or family 11, as defined previ-
ously. Examples of commercial xylanases are the xylanases from
Bacillus
sp.,
Trichoderma
sp.,
Humicola
sp. and
Aspergillus
sp.
It has been assumed for a long time that, depending on the application, preferential attack of
WE-AX or WU-AX is needed, whereas activity towards the other fraction is not desired.
29
In
bread making, endoxylanases that have a preference towards WU-AX have been considered
beneficial.
29, 95
The difference in substrate selectivity is therefore an important parameter
in developing and selecting proper xylanases.
26
Comparison of selectivities and activities
of a Bacillus xylanase and an Aspergillus xylanase showed that the Bacillus enzyme has
a clear preference for WU-AX, whereas the Aspergillus xylanase more readily hydrolyzed
WE-AX.
96
Nevertheless, both enzymes have a certain (although not the same) positive effect
on bread making, confirming the findings of Wang
24
that both WE-AX and WU-AX affect
gluten network formation in a similar negative manner. This means that hydrolyzing either
one of these components can have a positive effect.
Figure 6.7 shows that the positive effect of xylanases is more pronounced when bread is
made with low protein flour.
97
Fig. 6.7
Effect of a xylanase, in terms of per cent volume increase, on flour types with varying protein
contents.
