(342c) Ethanol Fermentation From Hot-Water Wood Extracts With E. Coli Fbr 5
There are four ?components? in plant biomass: extractives, lignin, hemicellulose, and cellulose. About 2~8% of dry wood mass falls into the category of extractives. Extractives are easily separable fraction of plant biomass that can be extracted by water or organic liquid under normal (pH and temperature) conditions. Lignin is an amorphous ?polymer? of various aromatic substances. About 21% of hardwood dry mass and 25% softwood dry mass are made of lignin. At least two common units are found in lignin: syrangyl and guaiacyl units. It acts as a glue to keep the plant in shape, and provide a barrier for the plant biomass to resist / repel biological attack. Lignin has a 3-D structure and is most energy dense component in plant biomass. Cellulose is a linear polymer of glucose. It makes up approximately 45% of the dry wood mass. Cellulose provides the strength to plant. It is one of the strongest substances in nature. Cellulose is also the most stable component in plant biomass: resistant to biological, caustic, and acidic agents. Therefore, there are many applications for cellulose as a natural polymer: paper, paper board, and particle/fiber board. Besides its utility as a biopolymer, cellulose can be depolymerized to glucose. Glucose is the preferred substrate for micro-organisms and is a good platform sugar for biological conversion to desired biofuel, biochemical, and bioplastics products. Between the two extremes of cellulose and lignin, there is the Hemicellulose. Hemicellulose is a 2-D ?polymer? of various sugar units with various degree of acetylation. Both five- and six- carbon sugar units are found in hemicellulose. Purified hemicellulose can be used as bio-polymer. Short fragments or xylo-oligomers can also be utilized as nutritional supplements that are known to have special health and bio enhancing functions. Still, large quantities of hemicellulose in woody biomass need to be converted to biofuels in order for a wood based biorefinery to be economically feasible. Hemicellulose can be depolymerized (or hydrolyzed) to produce 5- and 6- carbon sugars, as well as acetic acid. All of which are platform chemicals. Acetic acid has the highest commodity value at present time. Six carbon sugars are the substrate of choice for microorganism. Therefore, the bottleneck in a wood based biorefinery is the biological conversion of five carbon sugars. Adapting microorganisms to utilize five carbon sugars to produce ethanol or other biofuels is the key for a successful biorefinery. Successful utilization of xylose would help in driving the biomass-ethanol fermentation process favorably even for a purposely build green cellulosic ethanol plant. During hot-water extraction process, pentose and hexose sugars can degrade to furfural and hydroxymethylfurfural. Both are toxic compounds that can inhibit cell activity and affect the specific growth rate. Detoxification of hydrolyzate involving chemical, physical and biological methods is effective in reducing toxicity. But in the detoxification process, some fermentable sugars are removed. To avoid sugar loss, seeking strain that can tolerate higher concentrations of toxins becomes imperative. The present study focuses on the adaptation of a recombinant strain of Escherichia coli FBR5 which is characterized for ethanol production from xylose. It has a high salt tolerance of 40 g/L and can tolerate a maximum xylose concentration of 250 g/L. The maximum ethanol tolerance is 50 g/L. Adaptation of the strain was performed by sequentially transferring and growing cells in media containing increasing concentrations of crude hydrolyzate supplemented with other nutrient ingredients. The subculturing was carried out to obtain an ?adapted strain'. The ethanol productivity from wood extract is characterized after the sequential adaptation.