Saccharification is the critical step for bioethanol production where complex carbohydrates are converted to simple monomers.
Compared to acid hydrolysis, enzymatic hydrolysis requires less energy and mild environment conditions [58]. The optimum conditions for cellulase have been reported as temperature of
40e50 C and pH 4e5 [39]. Assay conditions for xylanase have also been reported to be 50 C temperature and pH 4e5 [93]. Therefore, enzymatic hydrolysis is advantageous because of its low toxicity, low utility cost and low corrosion compared to acid or alkaline hydrolysis [28,59]. Moreover, no inhibitory by-product is formed in enzymatic hydrolysis [58]. However, enzymatic hydrolysis is carried out by cellulase enzymes that are highly substrate specific
[23,59]. Here cellulase and hemicellulase enzymes cleave the bonds of cellulose and hemicellulose respectively. Cellulose contains glucan and hemicellulose contains different sugar units such as mannan, xylan, glucan, galactan and arabinan. Cellulase enzymes involve endo and exoglucanase and b-glucosidases. Endoglucanase
(endo 1,4-D glucanhydrolase or E.C. 3.2.1.4) attacks the low crystallinity regions of the cellulose fiber, exoglucanase (1,4-b-D glucan cellobiohydrolase or E.C. 3.2.1.91) removes the cellobiase units from the free chain ends and finally cellobiose units are hydrolysed to glucose by b-glucosidase (E.C. 3.2.1.21) [23,59]. Hemicellulolytic enzymes are more complex and are a mixture of at least eight enzymes such as endo-1,4-b-D-xylanases, exo-1,4-b-D xylocuronidases, a-L-arabinofuranosidases, endo-1,4-b-D mannanases, b-mannosidases, acetyl xylan esterases, a-glucoronidases and a-galactosidases [60]. Cellulose is hydrolysed to glucose whereas hemicellulose gives rise to several pentoses and hexoses. Several species of Clostridium, Cellulomonas, Thermonospora, Bacillus, Bacteriodes,
Ruminococcus, Erwinia, Acetovibrio, Microbispora, Streptomyces are able to produce cellulase enzyme. Many fungi such as
Trichoderma, Penicillium, Fusarium, Phanerochaete, Humicola,
Schizophillum sp. also have been reported for cellulase production
Table 6
Summary of some bio-delignification processes.
Substrate Microorganism