Specific Reactions of Glucomannans

Glucomannans are less stable than xylans, and undergo a rapid endwise peeling.

One reason for the higher reactivity is the lack of substituents [93]. Galactose residues

in galactoglucomannans are believed to have a similar stabilizing effect as

the side chains in xylans [84,94]. However, Timell [95] considers the link between

Gal and the backbone as a rather weak one, which will be cleaved readily under

alkaline extraction conditions.

Xylan and Fiber Morphology

During pulping, the xylan is dissolved and also modified to a large extent. Towards

the end of the cook [96,97], part of the xylan – which is still of oligomeric nature –

180 4 Chemical Pulping Processes

is re-precipitated at the cellulose fibers [98–102], thereby increasing pulp yield

[103], changing the mechanical characteristics (increasing strength) of kraft pulps

[104,105], and affecting the fiber quality. Hereby, the structure of the xylan affects

the adsorption characteristics to a large extent, and increasing the removal of carboxyl

groups thereby favors the retake of xylan [106].

The interaction of cellulose and hemicellulose in kraft pulps can be addressed

by a number of modern analytical approaches. Differences in hemicellulose concentrations

are observed between the surface and the inner layer of a kraft pulp

fiber. In the pulps investigated (softwood and hardwood), the amount of hemicelluloses

is generally larger at the surface as compared to the inner layer. The MWD

and sugar composition of the hemicellulose deposits can be studied with MALDI

and CE [107,108]. Analysis of the solid material involves CPMAS-13C-NMR studies

[109–112], association and localization of hemicellulose on pulps are studied by

GPC [113–116], and the use of enzymes in combination with other analytical techniques

provides insights into bonding types [117–119], as well as LCC and hemicellulose

structures [120,121].

Carbohydrate-derived Chromophores

Low molecular-weight carbohydrates can undergo further reactions under the

alkaline conditions of a kraft cook. Besides a series of isomerizations and further

fragmentations, condensation to cyclic compounds and phenols can also occur

(cf. Scheme 4.21). A number of catechols, phenols and acetophenones could be

detected in model studies with glucose and xylose [122]. Some of these can also

form stable radicals [123], which can also be detected in alkaline solutions from

hot caustic extractions of cellulose [124]. Chromophores are also produced by oxidation

reactions under alkaline conditions [125].

OH

H

H OH

H OH

H O

H

HO

CH2OH

OH

H

H O

H

HO

H

O

H

H

OH

HO

H

H

OH

H

H O

H

OH

H

H O

HO

H

H

O

H

H O

H

H

O

CH3

O

H

O

H C 3

O

H

O

O

OH

R1 OH

R2

O

R1 O

R2

+

_

OH

OH

+

Scheme 4.21 Formation of chromophores from carbohydrate monomers.

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