Cellulose is a long linear polymer ranging from 1000 to 1000 000 d-glucose units. Glucose monomers are linked together with b-1,4-glycosidic bonds to form highly stable chains, and these chains further aggregates together via hydrogen bonds to form a rigid crystalline structure that is water-impermeable, water-insoluble, and resistant to enzymatic hydrolysis (Linko 1987). The boundary and sequence structure of the molecules determine the chemical properties of cellulose (Alen and Sjostrom 1985). On the other hand, hemicellulose, which is alkali-soluble, is composed of short, highly branched copolymer of both six-carbon and five-carbon sugars. The branched structure allows hemicellulose to exist in an amorphous form that is more susceptible to hydrolysis. Compared to cellulose, which is similar across all biomass sources, hemicellulose is quite diverse in structure and composition, depending on the source. The hydrolysis product of hemicellulose is typically a mixture of xylose, arabinose, galactose, mannose, glucuronic acid, and other sugars. The ratios of five-carbon and six-carbon sugars depend on the source species of the biomass. Compared to hardwoods and agricultural residues, softwoods generally contain more six-carbon sugars (d-glucose, d-galactose, and d-mannose) but less five-carbon sugars (d-xylose and l-arabinose) (Alen and Sjostrom 1985). With cellulose, a chief engineering challenge is degradation; whereas, with hemicellulose, a major limitation lies in microbial fermentation of some sugars. Although there exists significant variations in the composition among species, biomass roughly consists of 50% cellulose, 30% hemicellulose, and 20% lignin.
Cellulose material obtained from waste sources is typically inexpensive but difficult to degrade. Certain processes require that lignin and other components will first be removed from cellulose before the actual processing of the material can proceed. In order to remove this nonbio-degradable component and other impurities from cellulose feedstock, we must pretreat the feedstock with mechanical, thermal, or enzymatic means. Of the many possible products derived from cellulose, ethanol manufactured via microbial fermentation has long captured the attention of many researchers in the following areas of applications: (a) potable ethanol for beer, wine, and distilled beverages, (b) solvent ethanol for laboratory and pharmaceutical applications, and (c) as a fuel additive/blend to reduce harmful emissions or as a complete substitute for gasoline. Lactic acid, which is a specialty chemical utilized mainly in the food industry as an acidulent and preservative, finds use in the chemical processing industry as a metal deliming agent (Iyer and Lee 1999; Schmidt and Padukone 1997).
A number of processes have been developed to degrade cellulose and to produce ethanol, although successful commercial units to date remain few in number. The most well known is simultaneous saccharification and fermentation (SSF), where the bioreactor brings together cellulose, glucose, cellobiose, cellulase enzymes, yeast (but not fungi), and nutrients to produce ethanol (Philippidis 1992). This process involves a number of different steps all carried out in one vessel. To avoid the setback associated with SSF, another process known as mixed culture and fermentation was developed, where a consortium of fungi and bacteria, or fungi and yeast, rather than simply yeast alone as was in SSF, convert cellulose into ethanol in one combined step (Wilke et al. 1983). We will review cellulose pretreatment and focus on fungal conversion of cellulose into ethanol, and finally we will touch upon the economic aspects of ethanol production.
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