A spruce wood (40 80 mesh meal extracted with ethanol benzne) was digested to various stages of delignification by liquor of SO
2-MgSO
4 bicomponent, or of Mg-base sulfite at temperatures of 127°, 137° and 147°C, respectively.
In the two systems, change of the selectivity of delignification with temperature was evaluated. The rate of delignification in Mg-base cooking was larger than that in SO
2-MgSO
4 cooking at given temperatures, but there was little different degree of dissolution of carbohydrates at same delignification level between them. As the temperature was lowered, the amount of dissolving carbohydrates decreased at each delignification, i. e., it means an increase in the selectivity of delignification reaction.
Next, pore size distribution of cell wall of the digested meal and molecular weight distribution of dissolved lignin were measured by the “Solute exclusion technique” for the former, and gel-filtration method using Sephadex columns for the latter. It was shown that increases of total pore volume (Fiber Saturation Point) and median pore width (MPW) during digestion were in good agreement with change of average molecular weight of dissolved lignin, which was calculated from the distribution pattern, in the both systems. The value of MPW was found to be linearly increased with degree of dissolution of carbohydrates, regardless of the temperature.
These results suggests that dissolution of carbohydrates would greatly contribute the development of the porous structure of cell wall, and thereby would affect the rate of delignification.
Furthermore, this point of view indicates the validity for the previous kinetic studies on the cooking reactions that dissolution of carbohydrates are closely associated with delignification reaction, particularly in an initial stage of cooking, and concurrently supports the “Back diffusion mechanism” on delignification process.
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