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Enzymatic Saccharification of Lignocelluloses Should be Conducted at Elevated pH 5.2–6.2

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This study revealed that cellulose enzymatic saccharification response curves of lignocellulosic substrates were very different from those of pure cellulosic substrates in terms of optimal pH and pH operating window. The maximal enzymatic cellulose saccharification of lignocellulosic substrates occurs at substrate suspension pH 5.26.2, not between pH 4.8 and 5.0 as exclusively used in literature using T. reesi cellulase. Two commercial cellulase enzyme cocktails, Celluclast 1.5L and CTec2 both from Novozymes, were evaluated over a wide range of pH. The optimal ranges of measured suspension pH of 5.2–5.7 for Celluclast 1.5L and 5.5–6.2 for CTec2 were obtained using six lignocellulosic substrates produced by dilute acid, alkaline, and two sulfite pretreatments to overcome recalcitrance of lignocelluloses (SPORL) pretreatments using both a softwood and a hardwood. Furthermore, cellulose saccharification efficiency of a SPORL-pretreated lodgepole pine substrate showed a very steep increase between pH 4.7 and 5.2. Saccharification efficiency can be increased by 80 % at cellulase loading of 11.3 FPU/g glucan, i.e., from approximately 43 to 78 % simply by increasing the substrate suspension pH from 4.7 to 5.2 (buffer solution pH from 4.8 to 5.5) using Celluclast 1.5L, or by 70 % from approximately 51 to 87 % when substrate suspension pH is increased from 4.9 to 6.2 (buffer solution pH from 5.0 to 6.5) using CTec2. The enzymatic cellulose saccharification response to pH is correlated to the degree of substrate lignin sulfonation. The difference in pH-induced lignin surface charge, and therefore surface hydrophilicity and lignin–cellulase electrostatic interactions, among different substrates with different lignin content and structure is responsible for the reported different enhancements in lignocellulose saccharification at elevated pH.

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This work was partially supported by a USDA Small Business Innovative Research (SBIR) Phase II project (contract number 2010-33610-21589) to Biopulping International, Inc. The financial support from this project made the visiting appointment of Lan at the US Forest Service (USFS), Forest Products Laboratory (FPL) possible. We acknowledge Fred Matt and Kolby Hirth (both USFS-FPL) for carrying out the carbohydrate and sulfur content analyses, respectively.

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Correspondence to J. Y. Zhu.

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This work was conducted while Lan and Lou are visiting scientists at the US Forest Service, Forest Products Laboratory and on official government time of Zhu.

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Lan, T.Q., Lou, H. & Zhu, J.Y. Enzymatic Saccharification of Lignocelluloses Should be Conducted at Elevated pH 5.2–6.2. Bioenerg. Res. 6, 476–485 (2013).

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