Lignins carbohydrate content

Chemical pulps are produced in a digester where the wood is cooked in pressurized vessels using heat and chemicals to break the intercellular stmcture of the wood and extractives. The objective is to remove the lignin from the fibers without degrading the carbohydrate content of the wood. 

Table 6.2.3. Carbohydrate content, elemental composition, and C9 unit formula of some lignin preparations... 

Lignin preparation Carbohydrate content Percent of lignin Elemental composition11, % C H O S OCH, C<> Unit Formula... 

The phenolic hydroxyl and etherified benzyl alcohol group can be selectively oxidized by periodate [385] and 2,3-dichloro-5,6-dicyano-l, 4-benzoquinone (DDQ) [386], respectively. The periodate oxidation of guaiacyl and syringyl nuclei yields 0-quinones plus methanol and has been used to estimate the phenolic hydroxyl group content of lignin. DDQ oxidation has been applied to estimate the content of lignin-carbohydrate linkages [28]. 

Cabannes factor 500 carbohydrate content of lignin pepara-tions 73, 306, 307, 343, 413, 421 carbon-13 nuclear magnetic resonance spectroscopy, C NMR 43, 146, 250-273,... 

Ethanol/Benzene. The solubility of wood in EtOH/benzene (benzene is a known carcinogen toluene can be substituted) in a 1 2 volume ratio will give a measure of the extractives content. This procedure is Tappi Standard T 204 and ASTM Standard D 1107. The wood meal is refluxed 6-8 h in a Soxhlet flask, and the weight loss of the extracted, dried wood is measured. Sometimes the lignin, carbohydrate, and other components are determined on wood that has been extracted previously with EtOH/benzene (see Table XIII). 

Holocellulose is the total carbohydrate content of wood. The values here are 100 — (the sum of percent ash, EtOH/benzene solubles, hot-water solubles, and lignin). Values from Refs. 73 and 76 were experimentally determined. 

Structure of Wood Components. As described in Chapter 2, wood consists of cellulose, hemicelluloses, lignin, and extractives. The first three are polymeric and are intimately associated with each other at the molecular level to form the cell wall. Carbohydrate content (cellulose and hemicelluloses) may reach 75% of the wood substance, so the reactions of carbohydrates are especially important. Although the extractives are extraneous materials, their presence can often influence reactions with the cell wall materials, and some wood properties also may be affected by reactions involving extractives. 

To investigate the importance, not only of laccase mediators, but also of lacca-ses per se, several laccases were studied for the oxidation of the nonphenolic lignin dimer I. In the presence of the redox mediators 1-HBT or violuric acid, it was found that the oxidation rates of dimer I by the laccases differed considerably. In oxidation of dimer I, both 1-HBT and violuric acid were to some extent, consumed. The consumption rate followed the same order of laccases as the oxidation rates of dimer I. The oxidation rate of dimer I was found to be dependent on both k, jt and the stability of the laccase in question. Both 1-HBT and violuric acid inactivated the laccases— violuric acid to a greater extent then 1-HBT. The presence of dimer I in the reaction mixture slowed down this inactivation. Inactivation seems to be mainly due to the reaction of the redox mediator free-radical with the laccases. No relationship between the carbohydrate content of the laccases and their inactivation was found. When the redox potential of the laccases is in the range of 750-800 mV, i.e., about that of the redox mediator, a further increase in redox potential does not affect k(,jt and the oxidation rate of dimer I [147]. 

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