Alkali treatment reduces the lignin and hemicellulose content and can be carried out at lower temperatures and pressures than acid treatment. The treatment also increases the surface area of the biomass.

Ozonolysis

Ozone can be used to degrade lignin and hemicellulose (Sun and Cheng, 2002). The advantages are it removes lignin, produces no toxic residues and it is carried out at room temperature and pressure.

Enzymes

Fungal enzymes from white and brown rot fungi, such as Sporotrichum pulverulentum and Pleurotus osteatus, can be used to pretreat lignocellulose. The brown rot fungal enzymes degrade cellulose whereas white rot fungal enzymes degrade lignocellulose.

Cellulose breakdown

After pretreatment the cellulose is suitable for hydrolysis to glucose and a number of methods can be used (Fig. 6.10).

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Concentrated acid

Concentrated acid hydrolysis of cellulose gives a rapid and complete conversion to glucose using 70% sulfuric acid for 2-4 h. The problems are associated with the dif­ficulties of handling concentrated sulfuric acid and the cost of the acid, which requires recovery and reuse to be economical.

Dilute acid hydrolysis

The cellulose is broken down by dilute acid in complex reaction at a higher tempera­ture than the hemicellulose reaction and has a sugar recovery of around 50%.

Cellulase enzymes

Once the structure of the cellulose has been opened up by the pretreatment, enzym­atic hydrolysis can proceed. The crude cellulase enzyme is a consortium of enzymes, which operate under mild conditions, pH 4.8 and 45-50°C. Although cellulase is commercially available it is usually obtained from fungi such as Trichoderma reesei, and the yields are better than acid hydrolysis. Cellulases can be produced by both fungi and bacteria which can be grown both aerobically, and anaerobically. The bac­teria include Clostridium, Cellulomonas, Bacillus, Thermomonospora, Bacteroides, Erwinia, Acetovibrio, Microbispora and Streptomyces. Three enzymes are involved in the hydrolysis, endo-1,4-P-glucanases (endoglucanases), cellobiohydrolyases (exoglucanases), and P-glucosidases. The endoglucanases cleave the cellulose chain randomly and the exoglucanases hydrolyse the cellulose chain, releasing glucose and cellobiose. The P-glucosidases catalyse the conversion of cellobiose to glucose.

The fungi produce all three types of cellulase but the exoglucanases are the major enzymes with T. reesei. The Trichoderma sp. are considered the best of cellulase enzyme producers. Cellulose on hydrolysis liberates cellobiose which is cleaved into two molecules of glucose by the enzyme P-glucosidase. The disadvantage of the enzyme process is that both products glucose and cellobiose act as inhibitors of cel — lulase and P-glucosidase enzymes.