KINETICS OF FIBER DIGESTION BY RUMEN MICROBES
Abstract
Surface area has been proposed as a major factor determining the extent of enzymatic hydrolysis of cellulose. We used cornstalk residue and Solka Floc BW-300 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 to 130 (ANGSTROM)) were determined by a solute exclusion technique using glucose, cellobiose, and polyethylene glycols as molecular probes. The pore size distributions follow a logistic model function. 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 to 160 (ANGSTROM)), 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 enhanced fermentation obtained for treated cornstalk residue. These observations indicate that external or macropore surface properties may be a significant factor in determining the extent of utilization of the solid substrates by cellulolytic microorganisms. The effect of surface area was applied to a semi-continuous rumen-like fermentation system. Mathematical models were developed to describe the fiber and/or dry matter disappearance using surface area and weight of the sample as the substrate concentration, respectively. The predicted fiber disappearance using the model with surface area was found to agree with the observed fiber disappearance for NaOH-treated cornstalk residue. In comparison, the model with weight of the sample seems to underestimate the fiber disappearance by 8-10% for untreated and NaOH-treated cornstalk residue. These data show that fiber disappearance based on surface area explains the actual degradation of cellulosic materials by cellulolytic rumen microbes.
Degree
Ph.D.
Subject Area
Agricultural engineering
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