Microbial Degradation Of Crop Residues And Conversion And Utilization

Energy consumption has increased steadily over the last century as the world population has grown and more countries have become industrialized. Therefore, there is a great interest in exploring alternative energy sources. Lignocellulosic materials are the most abundant renewable organic resource present on Earth. Agricultural residues account for more than half of the total plants and could contribute to a large extent in the production of value-added products.It is a slow course for corn stover to be degraded by microorganisms directly because of the complex structures, and the efficiency and rate of degradation cannot be tolerated in industrialized production. Pretreatment is an important way for practical cellulose conversion processes. The aim of the present work was to investigate various pretreatment methods including physical, chemical and biological types as well as their effects on the cellulase-catalyzed reactions. The following utilization of pretreated lignocellulosics by several microorganisms were also discussed. The cellulose powder was swollen after absorbing the soil lixivium, which enhanced the enzymolysis rate of the cellulose powder, whereas the soil lixivium had no obvious cellulase activity. It indicated that maybe in the soil lixivium there were some factors that could cooperate with cellulase, and swelled the cellulose by sorption; or could directly break the ultrastructures of cellulose, reduced the intensity of hydrogen bonds, and then facilitated the enzymatic hydrolysis. All the above could be the important reasons why the cellulose could be degraded with a high efficiency in the natural soil. Supercritical CO)2 has distinct impacts on the ultrastructures of cellulose and the cellulase-catalyzed reaction. The cellulase activity did not change when it was treated with SC-CO₂ at 10 Mpa and at 50℃ for 30min. However, when the cellulose was pretreated with cellulase, the catalytic activity of the cellulase waslost and did not produce reducing sugar. The cellulose pretreated with and without cellulase under the same SC-CO2 condition was hydrolyzed with crude cellulase. The final yield of reducing sugar from hydrolysis of the cellulose SC-CO2 pretreated with cellulase was higher than that of the cellulose pretreated without cellulose. It was also found that the improvement of the enzymolysis rate had a direct relevance with the amount of cellulase during SC-CO2 pretreatment. The moisture content of cellulose before SC-CO2 pretreatment had an obvious influence on the later enzymolysis rate.The FT-IR spectra showed that the structure of the cellulose pretreated with cellulase changed compared with that of the cellulose pretreated without cellulase, including the reducing of the hydrogen bond and the content of le phase, which is stress-reduced crystalline form of cellulose. It is the change in the cellulose structure that leads to the higher enzymolysis rate in the further enzymatic hydrolysis.? The similar results of enzymatic hydrolysis were found when cellulose was pretreated by microwave. The crude cellulase contained most of its catalytic activity when expanded under the radiation of 150W microwave for 2h, while the sugar yield was improved when the hydrolysic reaction was processed under certain power of microwave. It may suggest that microwave can destroy the structure of cellulose and the microenvironment of cellulase was meliorated with the existence of microwave. It is also found that microwave had a synergetic effect on cellulase, and the cooperation of Dimethyl Sulphoxide (DMSO) and microwave could boost the enzymolysis rate of microcrystal cellulose with about 20%.? Cellulose solvent is a new-style pretreatment method of cellulose. Simple urea and sodium hydroxide that dissolved cellulose resulted in 90% conversion of cellulose to glucose. It showed enzymolysis could be greatly improved when the lignocellulosics structure was disrupted before hydrolysis. Both the dissolution and swelling of Urea/NaOH with different concentration to cellulose promoted the enzymolysis rate in a large extent. While this promotion was not due to the rupture of glycosidic bonds, but due to the destroy and the amorphism ofcellulose structure. The enzymolysis rate had a direct relevance with the solubility of cellulose, while not with the concentration of Urea/NaOH.? The main effect of dilute acid to lignocellulosics is the hydrolysis of hemicellulose, whereas the sensitivity of pretreated materials to enzyme did not be improved obviously.? The major changes to lignocellulosics by steam explosion are attributed to the removal of hemicellulose, meanwhile it could change the state of lignin and reduce the particle size. The enzymatic hydrolysis of different sizes of steam-exploded corn stalk indicated that steam explosion could not only improve the accessibility of the enzymes to the cellulose fibrils, but also change the structure of cellulose which would facilitate the further enzymatic hydrolysis.? The fermentation of different types of carbon resources showed that the cellulase activities of steam exploded corn stover changed compared with those of unpretreated stover, while the protein capacity was increased in a large extent with a higher consumption rate. The direct conversion of lignocellulosids to ethanol by TV. crctssa and F. oxysporum revealed that the ethanol yield of steam exploded corn stover were higher than that of those unpreatreated ones. Anyway, from the applied view of point, the most promising pretreatment method maybe the steam explosion technique within all the above pretreatment technologies.