| Abstract
|
:
|
Bioconversion of corn stover on an industrial scale requires a safe and long-term storage method for large quantities of corn stover in order to supply biorefineries continuously year round. The ensilage process, which is a traditional crop storage method for ruminants in winter, is proposed as a preservation method for corn stover. Since cell wall saccharification and sugar fermentation occur naturally during the ensiling process, preserving corn stover as silage is expected to benefically pretreat the feedstock for downstream bioconversion. In this study, cell wall degrading enzymes and chemicals were added to stover silage in order to reduce pH and/or encourage lactic acid fermentation. There are numerous industrial enzyme mixtures available commercially, derived from different microbial sources and containing different enzyme components. The impacts of seven commercial enzyme mixtures were examined on corn stover preservation and pretreatment, with different combinations of microbial source and enzyme components. These enzymes were produced by Aspergillus niger, Trichoderma reesei, and Trichoderma longimrachiatum . Treatments included three size grades of corn stover, two enzyme levels (1.67 IU g-1 DM and 5 IU g-1 DM based in hemicellulase), and various ratios of cellulose to hemicellulose (C:H) in products derived from each microbial source. Higher lactic acid content and lower pH were obtained with increasing C:H ratios, especially with Trichoderma reesei enzymes. The highest C:H ratio tested, 2.38, resulted in the most effective fermentation, with lactic acid the dominant product. Significant cellulose and hemicellulose degradation was observed in these high CA ratio enzyme mixtures derived from Trichoderma reesei, indicating the additive rates could be reduced if preservation is the primary goal. Compared with enzymes, chemical additives are currently more ecomonical. The effect of five chemicals, including sulfuric acid, formic acid, formaldehyde, ammonia, and urea, was evaluated for corn stover preservation and pretreatment. Treatments included 2, 4, and 8 g kg-1 DM of sulfuric acid, formic acid, and formaldehyde and 4, 8, and 16 g kg-1 DM of ammonia and urea, with each of these 15 chemical treatments applied to three different particle sizes of stover. Sulfuric acid, formic acid, and formaldehyde increased lactic acid concentration and decreased acetic acid concentration. Clostridia activities were inhibited at the 16 g kg-1 DM level of urea and ammonia. Sulfuric acid, commonly considered as an economical and effective pretreatment reagent, was reexamined as a preservation and pretreatment additive over a 63 day period. The long term trial of sulfuric acid-treated silage showed that WSC increased over 63 days at the 16 g kg-1 DM level, although fermentation was almost inhibited at this level. Sulfuric acid, formic acid, formaldehyde, and ammonia increased sugar yield in enzymatic hydrolysis when compared to fresh stover and to control samples ensiled without chemical additives. (Abstract shortened by UMI.)
|