Bioethanol production of the first generation from sugar cane and from wheat or corn is well established in Brazil as well as in the US and Europe. The world’s ethanol production in more than 75 countries amounted in 2008 to more than 77 billion litres of ethanol (Sucrogen bioethanol, 2011).

Bioethanol production of the second generation can use lignocelluloses from non-food crops (not counted in the animal or the human food chain), including waste and remnant biomass e. g. wheat straw, corn stover, wood, and grass. These feedstocks are composed mainly of lignocellulose (cellulose, hemicelluloses and lignin).

The process of bioethanol production of the first generation is well established and shown in Fig. 1.

The process of bioethanol production from wheat normally consists of five major process steps:

1. Milling of the grain

2. Liquefication at high temperatures

3. Saccharification (enzymatic degradation of starch)

4. Fermentation with yeast

5. Distillation (rectification) of ethanol

The production of bioethanol from lignocelluloses follows more or less the same principle and is composed of the following sub-steps: milling, thermophysical pretreatment hydrolysis, fermentation, distillation and product separation/processing (Fig. 2).

The cellulose in the lignocellulose is not accessible to enzymes. Therefore, lignin and/or hemicelluloses have to be removed in order to make the enzymatic degradation of the cellulose possible. Ideal pretreatment should lead to better performance during bioethanol production from lignocelluloses.

The pretreament should cause the hydrolysis of hemicelluloses, high recovery of all carbohydrates, and high digestibility of the cellulose in enzymatic hydrolysis. No sugars should either be degraded or converted into inhibitory compounds. A high solid matter content and high concentration of sugars should be possible. The process should have low energy demands and require low capital and operational cost.

The pretreament methods can be classified roughly into three types: thermophysical methods, acid-based methods and alkaline methods. Thermophysical methods like steam pretreament, steam explosion or hydrothermolysis solubilise most of the cellulose and hemicelluloses. There is only a low level of sugar conversion. Cellulose and hemicelluloses have to be converted enzymatically into C6 sugars (mainly glucose) and to C5 sugars (mainly xylose). Acid-based methods use mineral acids like sulphuric acid and phosphoric acid. Hemicelluloses are degraded to sugar monomers, cellulose has to be converted to glucose enzymatically. Alkaline methods like ammonia fibre explosion leave some of the hydrocarbons in the solid fraction. Hemicellulases acting both on solid and dissolved hemicelluloses are required as well as the celluloytic enzymes.

Lignocellulose containing substrates are mainly composed of cellulose (40-50%), hemicellulose (25-35%) and lignin (15-20%). Cellulose is a glucose polymer, hemicellulose is a heteropolymer of mainly xylose and arabinose, and lignin is a complex poly-aromatic compound. The different pretreatment methods are necessary to loosen the close bonding between cellulose, hemicellulose and lignin. Wheat (Triticum aestivum L.) straw is composed of 45% cellulose, 26% hemicellulose and 19% lignin. Maize (Zea mays) straw is composed of 39% cellulose, 30% hemicellulose and 17% lignin.

The high percentage of hemicelluloses and the resulting pentoses, e. g. xylose from the hydrolysis of the polymer, are a further challenge to a cost-competitive bioethanol process with lignocelluloses as carbon source.

Yeasts used for the conversion of sugars into ethanol (mostly Saccharomyces spec.) usually only convert glucose into ethanol. C5 sugars like xylose are only converted into ethanol at low rates by very few yeast (Pichia spec.) strains. Research programs are underway either to adapt yeasts for the use of both C5 and C6 sugars or to modify Saccharomyces genetically to obtain yeast that produces ethanol simultaneously from C5 and C6 sugars.

Nevertheless, because of its ready availability and low costs, lignocellulosic biomass is the most promising feedstock for the production of fuel bioethanol. Large-scale commercial production of bioethanol from lignocellulose containing materials has still not been implemented.