In modern society, industrial factories and automobiles feed on coal and petroleum to generate power. However, these fossil fuels are not only unsustainable, which are predicted to be used up in the next 40-50 years, but also are harmful to the environment. When fossil fuels are combusted, they emit huge amounts of carbon dioxide and other harmful gases, such as sulfur dioxide. Sulfur dioxide then mixes with water vapor and clouds in the air, forming the notorious acid rain that causes vast amount of plants and animal to die, and disturbance in equilibrium of many ecosystems. Wars are still going on to fight over the limited oil today, however, finding and exploiting alternative energy resources is vital in the 21st-century society.

BIOENERGY

Biomass is the earth’s most attractive alternative among fuel sources and most sustainable energy resource and is reproduced by the bioconversion of carbon dioxide. Ethanol produced from biomass is today the most widely used biofuel when blended with gasoline (e.g., E10 [gasoline containing 10% ethanol]). As the carbon dioxide released by combustion is recycled into biomass, the use of biofuels can significantly reduce the accumulation of greenhouse gas. Of the biomass materials, cellulose, a major component of the cell wall of plants, is the most abundant and renewable carbohydrate. In recent years, it has been proposed that waste cellulosic biomass could be used as a cheap and readily available sugar to replace starchy materials in fermentation. Many researchers have previously tried to develop an efficient and inexpensive process for ethanol production from such waste by using recombinant bacteria and yeast (e.g., Saccharomyces cerevisiae) (1, 2, 10, 12, 37), but so far without success. A process of this kind is needed to solve environmental problems such as global warming and to construct a society independent of fossil fuels.

LIGNOCELLULOSIC BIOMASS

Lignocellulosic biomass refers to plant biomass that is composed of cellulose, hemicellulose, and lignin. The carbohydrate polymers (cellulose and hemicelluloses) are tightly bound to the lignin. the fermentation of lignocellulosic biomass to ethanol[1] is an attractive route to fuels that supplements the fossil fuels. Biomass is a carbon-neutral source of energy: Since it comes from plants, the combustion of lignocellulosic ethanol produces no net carbon dioxide into the earth’s atmosphere. One barrier to the production of ethanol from biomass is that the sugars necessary for fermentation are trapped inside the lignocellulose. Lignocellulose has evolved to resist degradation and to confer hydrolytic stability and structural robustness to the cell walls of the plants. This robustness or "recalcitrance" is attributable to the crosslinking between the polysaccharides (cellulose and hemicellulose) and the lignin via ester and ether linkages.[3] Ester linkages arise between oxidized sugars, the uronic acids, and the phenols and phenylpropanols functionalities of the lignin. To extract the fermentable sugars, one must first disconnect the celluloses from the lignin, and then acid-hydrolyze the newly freed celluloses to break them down into simple monosaccharides. Another challenge to biomass fermentation is the high percentage of pentoses in the hemicellulose, such as xylose, or wood sugar. Unlike hexoses, like glucose, pentoses are difficult to ferment. The problems presented by the lignin and hemicellulose fractions are the foci of much contemporary research.