Partial decolorization of Kraft pulp at high temperature and at high pH values with an extracellular xylanase from Bacillus stearothermophilus

A thermophilic, alkaline-tolerant bacterium that grows on xylan as its sole carbon source, was isolated by enrichment culture. The bacterium was identified as a strain of Bacillus stearothermophilus. When grown on xylan, or preferably xylose, the bacterium produced an extracellular xylanase. The enzyme was concentrated and partially purified by ammonium sulfate precipitation. The partially purified xylanase was inhibited by 1 mM CdCl₂ or ZnSO₄ and activated by 1 mM AgNO₃, CaCl₂ or KCl. The enzyme was active and stable over a broad range of pH (6.0-10.0) and temperature (58-85°C). Maximum xylanase activity at pH 7 and 8 was at 75°C, whereas maximum activity at pH 9 was at 65°C. The enzyme partially delignified softwood pulp at pH 9.0 and 65°C. The enzymatically catalyzed release of lignin from the pulp was both time and enzyme concentration-dependent. Maximum solubilization of lignin obtained (26%) occurred with 10 units ml^-1 enzyme for 18 h at pH 9 and 65°C.