Screening And Optimizting Of Bacteria Coverting Lignocellose To Ethanol

With the rapidly development of economy, there is a lack of mineral resource of sorts day by day. Energy sources scarcity threastened the continuable development of society. The importance of alternative energy sources gave birth to initiatives. Fuel ethanol is regarded as one of the most promising environmentally friendly renewable resource. Expansion of production of bioethanol utilizing lower-value lignocellulose offers a great potential for not only reducing the production cost but also protecting the environment as it produces far less air pollutants than gasoline. D-Xylose is the most abundant monosaccharide in lignocellulose hydrolysate after glucose. Consequently, ethanol fermentation of xylose is of key factor for the efficient utilization of lignocellulosic hydrolysates to produce fuel ethanol. Therefore, to obtain a bacterial strain which can largely use xylose to produce ethanol is always a hot topic within this area.This sutdy was based on microbiology theory. According to the habitation of xylose zymogen, a bacterium which can ferment xylose to produce ethanol was screened from natural environment. It was identified by analyzing its physiological and biochemical characteristics and applying molecule biological means. Then the technical parameter of fermenting xylose to produce ethanol were optimized, to lay a foundation of the cellulase to the alcohol production. The main content of this research is as follows.1. Study on the isolation and screening and identification of bacteria fermenting xylose for producing alcohol.With plate cultivation of xylose-screening medium, in which the xylose is the only source of carbon, we screened 22 bacteria which can produce alcohol from the soil which contains ample rotted cellulose. By the fermentation xylose for production alcohol, in which the xylose is the only source of carbon, and based on the volume of production of alcohol, we screened a strain out of the 22s, which was named strain 18th. According to the morphologic specificity and physiological-biochemical characteristic of strain 18th and its sequence of 16s rDNA, the strain 18th is 100% match with Bacillus cereus, so it is Bacillus cereus.2. Study on the physiological and biochemical characteristics analysis of bacteria fermenting xylose for producing alcoholThe tolerance of strain 18th showed that it suit a wide pH value range, it can grow in the medium pH value from 4.5 to 10.0, and in the medium pH value 6.0～8.5 it was relatively good growth; its NaCl concentration range of adaptation is relatively wide, and this strains can grow within medium NaCl concentration of 5～40g/L.the strain is best growth in the NaCl concentration of 15g/L. The oxidase, catalase and nitrate reduction experiment were positive.3. Study on the optimization of conditions of fermentationThe strain 18th fermented xylose and glucose for production alcohol. Under anaerobic conditions, the maximum yield of the xylose-fermentation was 6.36954g alcohol/90g xylose, which was the 20.11 percent of theoretical yield; the maximum yield of glucose-fermentation was 7.14627g alcohol/90g glucose, which was the 21.19 percent of theoretical yield. The characteristics has theoretical significance for the fermentation of glucose and xylose which was hydrolysis products of wood cellulose, while for the application of production fuel ethanol from wood cellulose, also has some value. Based on the xylose fermentation medium, the fermentation parameters were optimized. Strain 18th with xylose as sole carbon source, at the temperature 32℃, xylose 30g/L, culture 72h under anaerobic conditions, has the highest yield of alcohol, up to 32.24 percent theoretical yield of fermentation xylose for production alcohol, at this time accompanied by a certain amount of acid generated, the pH value of 3.32.