Chapter 13: Ethanol Production from Biomass

The text details the biochemical engineering required to convert these substrates, beginning with Stage I pretreatments like milling, steam explosion, and enzymatic hydrolysis to break down complex polymers into fermentable monosaccharides. The discussion centers heavily on the metabolic pathways of the primary industrial organism, Saccharomyces cerevisiae, exploring the stoichiometry of the Gay-Lussac equation, the Embden-Meyerhof glycolytic pathway, and the physiological impact of ethanol toxicity on cell membrane integrity, which involves adjustments in fatty acid chain length and ergosterol content. The summary contrasts eukaryotic yeast fermentation with the prokaryotic bacterium Zymomonas mobilis, which utilizes the Entner-Doudoroff pathway to achieve higher specific productivity and ethanol yields despite generating metabolic by-products like sorbitol and levan. Significant attention is given to process optimization technologies, specifically Simultaneous Saccharification and Fermentation (SSF), which combines hydrolysis and fermentation to reduce product inhibition. The chapter also highlights advances in metabolic engineering, such as the creation of recombinant yeast strains capable of co-fermenting pentose sugars like xylose alongside glucose by overcoming catabolite repression and redox imbalances. Furthermore, the text evaluates the potential of thermophilic Clostridium species and their complex cellulosome systems for consolidated bioprocessing, ultimately framing the future of energy in the context of versatile biorefineries that produce fuels and chemicals from sustainable agricultural and forest resources.