(2) altering the carbohydrate-lignin associations, or (3) cleaving the redeposited
xylan.
Hexenuronic acid which absorbs UV is formed during Kraft pulping from
4-methyl-glucuronic acid residues of xylan [160]. Xylan hydrolysis allows easy
removal of such groups and decreases the amount of bleaching chemicals that
would be needed to bleach those residues. Moreover, hexenuronic acid removal by
xylanases helps to prevent brightness reversion of the treated Kraft pulps [161].
After pulping, a considerable amount of xylan is present in the pulp, some of
which is reprecipitated on the surface of the fiber and decreases the accessibility of
lignin. Hydrolysis of this xylan is believed to render the fiber structure more
permeable and allows the alkali or bleaching agents to have better access to lignin.
Moreover, the increased permeability allows the passage of lignin or lignincarbohydrate molecules efficiently in subsequent processes [162], and this enables
the removal of lignin and achieves a better bleaching efficiency with reduced amount
of bleaching chemicals. On the other hand, there is evidence that xylan does not
necessarily reprecipitate on the surface of all pulps [163] and the demonstration of
xylan removal does not always correlate with bleaching efficacy and varies from
enzyme to enzyme [164]. This shows that the exact mechanism of xylanasemediated bleaching is not fully understood, and hence, at this point, it is difficult
to predict if bleaching will be enhanced by a given xylanase or not. However, some
properties of xylanases are considered desirable. Among these traits, the temperature
and pH profiles of the enzymes are known to be relevant to this application.
The Kraft process results in pulp that is alkaline and hot, and hence an ideal
bleaching enzyme should be operationally stable under these conditions and could
be used directly without any temperature or pH adjustment. However, the great
majority of xylanases are neither active nor stable at alkaline condition and high
temperature. In fact, the first-generation xylanases used in pulp treatment were not
active at alkaline conditions. Thus, pulp cooling and pH adjustment were necessary
prerequisites. The pulp pH is often adjusted to around neutral range with sulfuric
acid, and this has been associated with the accelerated corrosion of equipment used
in enzymatic treatment of pulp [165]. On the other hand, the use of alkaline active
and thermostable enzymes potentially avoids the cooling and pH adjustment steps,
which concomitantly minimizes not only equipment corrosion and the related
maintenance cost but also the time required to make the adjustments. This has led
to the continuous search and development of alkaline active thermostable enzymes
that are more suitable for direct Kraft pulp treatment, and xylanases such as Ecopulp
TX-200C that are operationally stable around pH 10 and 90
C [56] are the results of
this effort. In addition to activity and stability at high pH and temperature, the
molecular weight and specificity of the enzyme affect the biobleaching efficiency.
Compared to GH10 xylanases, those that belong to family GH11 are more effective
in biobleaching of pulp [166]. It is believed that this is due to smaller size which
allows GH11 xylanases diffuse effectively in the pulp and remove the xylan which
cannot be accessed by the bigger GH10 xylanases. Moreover, most GH10 xylanases
show catalytic promiscuity and degrade cellulose, a property that can deteriorate the
quality of the pulp.
272
G. Mamo
xylan.
Hexenuronic acid which absorbs UV is formed during Kraft pulping from
4-methyl-glucuronic acid residues of xylan [160]. Xylan hydrolysis allows easy
removal of such groups and decreases the amount of bleaching chemicals that
would be needed to bleach those residues. Moreover, hexenuronic acid removal by
xylanases helps to prevent brightness reversion of the treated Kraft pulps [161].
After pulping, a considerable amount of xylan is present in the pulp, some of
which is reprecipitated on the surface of the fiber and decreases the accessibility of
lignin. Hydrolysis of this xylan is believed to render the fiber structure more
permeable and allows the alkali or bleaching agents to have better access to lignin.
Moreover, the increased permeability allows the passage of lignin or lignincarbohydrate molecules efficiently in subsequent processes [162], and this enables
the removal of lignin and achieves a better bleaching efficiency with reduced amount
of bleaching chemicals. On the other hand, there is evidence that xylan does not
necessarily reprecipitate on the surface of all pulps [163] and the demonstration of
xylan removal does not always correlate with bleaching efficacy and varies from
enzyme to enzyme [164]. This shows that the exact mechanism of xylanasemediated bleaching is not fully understood, and hence, at this point, it is difficult
to predict if bleaching will be enhanced by a given xylanase or not. However, some
properties of xylanases are considered desirable. Among these traits, the temperature
and pH profiles of the enzymes are known to be relevant to this application.
The Kraft process results in pulp that is alkaline and hot, and hence an ideal
bleaching enzyme should be operationally stable under these conditions and could
be used directly without any temperature or pH adjustment. However, the great
majority of xylanases are neither active nor stable at alkaline condition and high
temperature. In fact, the first-generation xylanases used in pulp treatment were not
active at alkaline conditions. Thus, pulp cooling and pH adjustment were necessary
prerequisites. The pulp pH is often adjusted to around neutral range with sulfuric
acid, and this has been associated with the accelerated corrosion of equipment used
in enzymatic treatment of pulp [165]. On the other hand, the use of alkaline active
and thermostable enzymes potentially avoids the cooling and pH adjustment steps,
which concomitantly minimizes not only equipment corrosion and the related
maintenance cost but also the time required to make the adjustments. This has led
to the continuous search and development of alkaline active thermostable enzymes
that are more suitable for direct Kraft pulp treatment, and xylanases such as Ecopulp
TX-200C that are operationally stable around pH 10 and 90
C [56] are the results of
this effort. In addition to activity and stability at high pH and temperature, the
molecular weight and specificity of the enzyme affect the biobleaching efficiency.
Compared to GH10 xylanases, those that belong to family GH11 are more effective
in biobleaching of pulp [166]. It is believed that this is due to smaller size which
allows GH11 xylanases diffuse effectively in the pulp and remove the xylan which
cannot be accessed by the bigger GH10 xylanases. Moreover, most GH10 xylanases
show catalytic promiscuity and degrade cellulose, a property that can deteriorate the
quality of the pulp.
272
G. Mamo