Role of Hemicelluloses in the Formation of Chromophores During Heat Treatment of Bleached Chemical Pulps

Abstract

The heat-induced colour formation of TCF-and ECF-bleached chemical pulps has been investigated. Partial hydrolysis of the pulps and analysis of the carboxyl content of the pulps and hydrolysed matter were carried out. It was found that the yellowing tendency of the pulps corresponds to the content of hydrolysable substances. The hydrolysis leads to a considerable removal of the carboxyl groups in the case of hardwood pulps. Differences between softwood and hardwood pulps are discussed in terms of their hemicellulose content. The yellowed pulps were extracted and the extracts were analysed by different NMR and mass spectrometric techniques. Structures of low molecular products were elucidated and the routes of their formation were proposed. The results suggest that the yellowing proceeds in two main steps, i.e. decay of unstable polysaccharide chains such as hemicelluloses to low molecular carbohydrate compounds that subsequently undergo dehydration and condensation reactions to form coloured products.

Brightness reversion of eucalyptus kraft pulp: Effect of carbonyl groups generated by hypochlorous acid oxidation

“…A lower hemicellulose content may, however, lead to a less extensive oxidation of the pulp (Figure 2c) and this fact can indirectly affect the brightness stability. This result correlates quite well with that obtained in previous studies investigating the role of hemicelluloses on pulp yellowing (Vuorinen et al 1999;Beyer et al 2006).…”

“…Other factors, which are often considered to have a less pronounced effect on pulp brightness reversion, include metal ions, pH, etc. (Gullichsen and Söderhjelm 1984;Chirat and Chapelle 1999;Grandström et al 2001;Beyer et al 2006).…”

Xylanase Treatment Suppresses Light- and Heat-Induced Yellowing of Pulp

“…A likely cause of the phenomenon is attributed to more hemicellulose and HexA contents in wheat straw CP and more lignin in poplar CTMP. Hemicellulose polysaccharides, because of their relatively low degree of polymerization, were easily hydrolyzed to low-molecular carbohydrate compounds, which subsequently underwent dehydration and condensation reactions to form chromogenic substances2728; HexA is also easily degraded during heat- and light-induced yellowing to generate colored compounds27. Increased xylanase dosages led to enhanced hemicellulose removal, for example, hemicellulose content of wheat straw CP is 13.6% and 12.8% when xylanase dosages are 20 and 80 U/g, respectively, thus decreases pulp PCN.…”

Causes of color changes in wood during drying

“…Beyer et al (2006) found heat induces thermal processes which usually lead to non-beneficial effects such as yellowing, discoloration, or unpleasant odor. In earlier studies discoloration of birch had been investigated using colorimetric methods, with results indicating a notable color change occurring during kiln drying, of which temperature appeared to be the most important parameter (Kreber and Haslett, 1997;Luostarinen and Verkasalo, 2000;Kreber et al, 2001;Luostarinen and Luostarinen, 2001;Mononen et al, 2002;Sundqvist, 2002;Mononen et al, 2004).…”