ABSTRACT
Biochemical conversion proceeds at lower temperatures and lower reaction rates than thermochemical conversion. [5] Biochemical conversion generally
first uses low-severity thermochemical pretreatment at temperatures between 100 and 300 °C to break down the lignocellulosic matter and improve enzymatic or chemical accessibility (Chapter 7). The pretreatment step is usually followed by hydrolysis of cellulose to fermentable sugars. There are two main types of hydrolysis processes: enzymatic/microbial (the most investigated route) and acid (either concentrated or dilute). The fermentable sugars obtained from hydrolysis can then be fermented into ethanol or chemicals using native or genetically modified microorganisms. Hexoses, such as glucose, galactose and mannose, can readily be fermented by many naturally occurring organisms, but the pentoses, such as xylose and arabinose, are fermented by few native strains, and usually at relatively low yields. [6] The development of recombinant strains resulted in bacteria and yeasts capable of co-fermenting hexoses and pentoses into ethanol and other value-added products at high yields. [6] Biomass processing schemes involving enzymatic or microbial hydrolysis involve four biologically mediated transformations: cellulase production, cellulose hydrolysis, hexose fermentation and pentose fermentation. [7] Four process configurations proposed for the biological transformations differ in their degree of integration: Separated Hydrolysis and Fermentation (SHF), Simultaneous Saccharification and Fermentation (SSF), Simultaneous Saccharification and Co-Fermentation (SSCF) and Consolidated Bioprocessing (CBP) – featuring cellulase production, hydrolysis and fermentation in one step.
