Effect of pretreatments and fermentation on pore size in cellulosic materials

Abstract

Surface area has been proposed as a major factor determining the extent of enzymatic hydrolysis of cellulose. We used cornstalk residue (CR) and Solka Floc BW-300 (SF) as substrates and NaOH (a cellulose swelling agent) and iron sodium tartrate (FeTNa, intercolates between cellulose microfibrils) as pretreatments to study the effect of surface area on extent of fermentation. Micropore sizes (8-130 A) were determined by a solute exclusion technique using glucose, cellobiose, and polyethylene glycols as molecular probes. The pore size distributions follow the logistic model function: I = a/[1+exp(b - cX)] where I is pore volume; X = log D; D is the molecular probe diameter; and a, b, and c are constants. The pore volumes of CR (1.9 mL/g) and SF (1.6 mL/g) are increased to 2.1 mL/g by pretreatment with NaOH. Pretreatment of SF with NaOH and cornstalk residue with FeTNa caused an upward shift in the pore size distribution. Fermentation of untreated CR by rumen microbes resulted in a 46% loss of dry matter while increasing the internal pore size and decreasing the pore volume to 0.9 mL/g. Fermentation of NaOH pretreated CR resulted in a 73% loss of dry matter with little change in pore size, total pore volume, or fiber composition. Fiber analysis indicated that selective utilization of hemicellulose over cellulose in both fermentations was small. The data show that: (1) removal of hemicellulose and lignin increases dry matter disappearance upon fermentation of the remaining material; (2) relative to the size of bacterial cellulases (40-160 A), the pretreatments have little effect on increasing accessibility of surface internal to the cellulose particles; and (3) the micropore changes caused by NaOH or FeTNa treatment do not explain the enchanced fermentation obtained for treated cornstalk residue. These observations infer that external or macropore surface properties may be a significant factor in determining the extent of utilization of the solid substrates by cellulolytic microorganisms.