Progress and challenges for microbial fermentation processes within the biorefinery context

Conversion of biomass components to fuels, chemicals, and other products by microbial cell factories is a key component of the biorefinery model. Metabolic engineering has enabled the design of microbial strains that can consume a range of biomass-associated monomeric units, such as sugars, to produce a vast array of fermentation products, ranging from relatively low value fuels and chemicals, such as ethanol and lactic acid, to amino acids, high-value secondary metabolites, and to bioprivileged molecules. These microbial cell factories have fairly stringent requirements for biological activity, and thus substrate and product toxicity and sensitivity to high temperatures, and extreme pH values increase the cost of the fermentation product. This chapter provides a historical perspective on fermentation processes, highlights the successes in terms of strain development for new fermentation products, and summarizes some of the other factors impacting the process cost.