Comparison of digestibility of wood pulps produced by the sulfate and TMP methods and woodchips of various botanical origins and sizes

Poplar and pine cellulosic pulps derived by the sulfate process (Kappa numbers of 15.4 and 31.4, respectively) and a poplar thermomechanical pulp (TMP, Kappa number of 124.7) as well as 0.43-0.8 mm woodchips of various botanical origins (poplar, birch, lime, oak, beech, pine, and spruce) and poplar chips of five different average particle sizes, ranging from 1.6-2.0 to <0.43 mm, were digested by a commercial preparation of cellulases and xylanases (NS-22086 from Novozymes). Yields of reducing sugars derived by enzymatic hydrolysis of the poplar and pine pulps that were obtained by the sulfate method achieved 100 and 89 % on a dry weight basis (52.3 and 35.9 % d. w. wood, respectively) while glucose yields reached 79 and 70.7 % d. w. pulp (41.3 and 28.6 % d.w. wood), respectively. Enzymatic hydrolysis of the pine TMP resulted in glucose and reducing sugars yields of only 14.0 and 36.7 % d.w. pulp (around 13 and 34 % d.w. wood, respectively). Yields of glucose and soluble reducing sugars released from the poplar chips were inversely proportional to their size and varied between around 5.3-7.8, and 15.7-22.4 % d.w., respectively. Reducing sugars yields from beech, birch and lime chips were slightly higher than from the poplar chips of the same size while pine, spruce and oak chips were less susceptible to enzymatic digestion. The dominating sugars in the hydrolysates of woodchips were glucose and cellobiose (70-90 % w/w of the sum of 5 detected sugars) while mannose, xylose and arabinose concentrations were relatively low.