ENZYMATIC ACCESSIBILITY OF XYLANS IN LIGNOCELLULOSIC MATERIALS

The hydrolysis of fibre-bound and isolated xylans from both birch and pine wood and kraft pulps was studied using purified xylanolytic enzym es of Trichoderma reesei. Despite high enzyme loading, the degree of h ydrolysis of fibre-bound substrates did not exceed 20% of the theoreti cal value, apparently due to limited accessibility of the substrates. The fibre-bound xylans were as equally accessible in softwood as in ha rdwood pulps. The isolated xylans of wood and kraft pulps could be sol ubilized more extensively, with a hydrolysis yield of 50-65%. The subs titution degree of the isolated xylan substrates was reflected in the different hydrolysis yields obtained by the two xylanases, with isoele ctric point (pi) values of 9.0 and 5.5. On the more substituted substr ates, i.e. pine kraft xylan and pine wood xylan, the two enzymes acted almost similarly, whereas on the less substituted xylan substrates, s uch as isolated birch kraft xylan, the pI-9.0 enzyme was more efficien t. The side-group-cleaving enzymes increased only moderately the solub ilization of the substrates.