Abstract
Lignocellulosic biomass is the most abundant renewable source of energy that has been widely explored as second-generation biofuel feedstock. Despite more than four decades of research, the process of ethanol production from lignocellulosic (LC) biomass remains economically unfeasible. This is due to the high cost of enzymes, end-product inhibition of enzymes, and the need for cost-intensive inputs associated with a separate hydrolysis and fermentation (SHF) process. Thermotolerant yeast strains that can undergo fermentation at temperatures above 40°C are suitable alternatives for developing the simultaneous saccharification and fermentation (SSF) process to overcome the limitations of SHF. This review describes the various approaches to screen and develop thermotolerant yeasts via genetic and metabolic engineering. The advantages and limitations of SSF at high temperatures are also discussed. A critical insight into the effect of high temperatures on yeast morphology and physiology is also included. This can improve our understanding of the development of thermotolerant yeast amenable to the SSF process to make LC ethanol production commercially viable.
Highlights
Driven primarily by the global increase in energy consumption, depletion of fossil fuel reserves and concerns about climate change, new renewable and environment-friendly sources of energy are being explored
Ylitervo et al [84] reported that encapsulated S. cerevisiae CBS8066 successfully fermented 30 g/L of glucose with high ethanol yield in five consecutive batches of 12-h duration at 42°C, compared to freely suspended yeast, which was completely inactivated after the third batch
simultaneous saccharification and fermentation (SSF) is preferred over separate hydrolysis and fermentation (SHF), because it reduces the need for separate fermenters, in turn reducing the overall cost of ethanol production
Summary
Driven primarily by the global increase in energy consumption, depletion of fossil fuel reserves and concerns about climate change, new renewable and environment-friendly sources of energy are being explored. Due to the high cost of the biomass-hydrolyzing enzymes (cellulases and hemicellulases) and pretreatment methods, the production of LC ethanol by SHF is not economically viable. The hydrolytic enzymes are subject to feedback inhibition due to the accumulation of sugar monomers and cellobiose in the medium. This in turn reduces the efficiency of these enzymes. This limitation can be overcome by a process known as simultaneous saccharification and fermentation (SSF). Saccharification and fermentation are performed simultaneously; the hydrolyzed sugars are continuously converted into ethanol, thereby enhancing the efficiency of enzymatic saccharification in the absence of feedback inhibition. Reduction in cooling cost, as chiller unit is not required Continuous ethanol evaporation from broth under reduced pressure Suitability for use in tropical regions with high temperatures
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