**1. Background**

For thousands of years in the human being history, production of alcohol from natural and agricultural products with high starch or sugar contents, such as fruits, grains, sugarcane, or corn, has been well known. Microorganisms are cultivated in these carbohydrate materials to convert sugars and starches to ethanol through metabolism. Since these polysaccharides are polymers of monosaccharides, also known as single sugars, such as glucose, fructose, xylose, etc., they can be first enzymatically hydrolyzed and then fermented to ethanol by the microorganisms. Through such bioprocesses, alcoholic mixtures are obtained [1].

Nowadays, biofuels attract intensive interests from over the world due to its environmental friendliness. Biofuels are renewable as a neutral carbon source, which does not break the balance in atmosphere's air contents leading to global warming. Biofuels are among the most feasible ways to free the human being from dependence on traditional fossil resources [2].

In the last 20 years, the crude oil crisis causes an increasingly high demand of renewable energy, especially biofuels. America and Brazil are the two leading countries in producing bioethanol from sugarcane and corn. A report in 2009 claimed that Brazil produced annually 12.5 billion liters of bioethanol from sugarcane as fuels for the industry, while America also produced 5 billion liters of bioethanol from corn with establishing 111 gas stations selling E85 gasoline (a gasoline mixture composed of 85 vol.% as bioethanol) [3]. However, the production of biofuels from starches and sugars vigorously violates against the world's food security for humanity.


#### **Table 1.**

*A comparison of starch vs. cellulose and glycogen.*

In an attempt to find an alternative approaches to bioethanol production, lignocellulosic biomass raises an intensive attention as cellulose is similar to starch and sugar because it is also a polymer of glucose. A comparison between three popular polysaccharides, starch, cellulose, and glycogen, is presented in **Table 1**; the most difference between cellulose and starch is their glycosidic linkages and the complex form of cellulose in plants, where it strongly incorporates with lignin and hemicellulose. In contrast, lignocellulose is the most abundant biomass in the world, which can be found as leaves, peels, bodies, branches, etc. of almost all the existing plants. Therefore, lignocellulosic bioethanol production is definitely a strategy of energy supply, especially suitable for countries with agricultural and forestry wastes to be utilized as the input materials.
