The three major methods of pretreatment that allow the recovery of solid cellulose are based on physical, biological, and chemical technology (Hsu et al. 1996).

• Physical pretreatments. Physical pretreatment involves reducing the size of the biomass particles so cellulases have access to the cell wall materials with reduced interference from the lignin and hemicellulose. An example of this is dry ball milling of biomass, which is very energy intensive and not likely to become practical at large scale (Zhu et al. 2008). In physical pretreatment, most of the lignin, cellulose, and xylan remain in the solid phase.

• Biological pretreatments. Biological pretreatment uses enzymes or microorganisms to produce simple sugars from the carbohydrate polymers with minimal mechanical milling of the biomass. Although biological pretreatment is usually less energy intensive than chemical or mechanical pretreatment, it is still in the early stages of development (Sawada et al. 1995) . Bio-pulping, not yet economically competitive with traditional pulping methods, is one example of biological pretreatment. Depending on the enzymes or microorganisms used, cellulose and/or hemicellulose may be hydrolyzed and even metabolized by the pretreatment strain.

• Chemical pretreatment. Chemical pretreatment is the most widespread method currently in use. Biomass is pretreated using acid or base, usually in combination with high heat and/or pressure. Many pretreatment conditions and catalysts have been used in biomass conversion, including concentrated and dilute acid, steam explosion, alkali, organic sol­vents, and ammonia. Biomass pretreatment has also been reviewed earlier by Dale (1985).

Acid hydrolysis is the most common method, with sulfuric, hydrochloric, and phosphoric acid all being used (Hsu et al. 1996). Nitric and peracetic acid have also been used, but their method of action is not polysaccharide hydrolysis, but rather by fiber matrix degradation through lignin oxidation. Acid hydrolysis is typically carried out under conditions that maxi­mize hemicellulose hydrolysis and minimize cellulose degradation. The most widespread process is the use of dilute (less than 1% w/v) sulfuric acid in combination with heat and pressure. Dilute sulfuric acid is inexpensive and hydrolyzes the hemicellulose almost com­pletely, while degrading little of the cellulose. The drawbacks of this method are the demand for corrosion-resistant equipment and disposal of large amounts of gypsum generated during neutralization. Sulfuric acid pretreatment leaches toxic metal ions from the equipment and converts small amounts of glucose to hydroxymethyl furfural and xylose to furfural. Other inhibitors produced include oxidized phenolics from lignin degradation and acetic acid from xylan hydrolysis (van Walsum et al. 1996). Phosphoric acid is a weaker acid, causes few waste disposal problems, and can be used as a nutrient by yeast after neutralization with ammonia. However, phosphoric acid is about eight times as expensive as sulfuric acid.

Pretreatment with lime at elevated temperatures has recently been developed as an alterna­tive pretreatment. Chang and coworkers demonstrated that pretreating switchgrass with 0.1 g Ca(OH)2)g dry biomass at 100°C or 120°C for 2 hours removed 29% of the lignin while hydrolyzing only 10% of the cellulose and about 27% of the xylan (Chang et al. 1996). The xylan and cellulose contents of the original biomass were about 21% and 37% (w/w), respec­tively. Accounting for the loss of xylan in the pretreatment, the xylan hydrolysis was near 100% theoretical after treatment with commercial cellulase preparations. Xylan hydrolysis was probably enhanced through the alkaline deacetylation of the hemicellulose. Removing the ester-linked groups greatly enhances the digestibility of the xylan by exposing the xylan backbone to enzyme hydrolysis. Although lime pretreatment gave high yields and excellent digestibility, the sugar loss during the pretreatment process was significant, with 10% of the cellulose and 27% of the xylan lost to the liquid stream.

In addition to the already mentioned methods, other pretreatment methods have been used, including steam explosion, acid catalyzed steam explosion, ammonia fiber explosion, organic solvents, supercritical fluid, irradiation, oxidizing agents, alkali, liquid hot water, ammonia recycled percolation, and ammonia-hydrogen peroxide percolation (Iyer et al. 1996; Kim and Lee 1996).