LC biomass has great potential for biofuel production. In particular, second-generation bioethanol can contribute to a cleaner environment and a carbon-neutral cycle. Thermotolerant and ethanologenic yeast strains can be used for bioethanol production by SSF at elevated temperatures, as they reduce the costs of the overall process. Thermotolerant yeasts can be developed by mutation, genetic engineering, metabolic engineering, and physiological adaptation. Fermentation at high temperatures with thermotolerant yeasts and saccharification simultaneously has several advantages such as reduced cooling cost, reduced need for utilities, higher saccharification efficiency, and no feedback inhibition of cellulolytic enzymes. Compared with commercial enzymes, cold-active cellulases and hemicellulases from organisms typically inhabiting temperate regions hydrolyze the biomass at low temperature but with lower efficiency. Therefore, thermotolerant yeast and cold-active hydrolytic enzymes must be developed for a more cost-effective SSF process.
Traditional methods are limited by various technological gaps. Thus, modern methods of genetic engineering such as SDM or genome shuffling along with high-throughput screening techniques can be used to develop improved yeast strains. These methods can also be used to enhance the expression of hydrolytic enzymes to suit the SSF process. Functional genomics together with metabolic engineering can be used to develop robust yeast strains capable of fully utilizing the sugar component of LC biomass. However, construction of recombinant strains has been limited to a few species such as K. marxianus and Pichia kudriavzevii because effective genetic tools are lacking. Comparing the metabolic profiles of thermotolerant yeast and well-established mesophilic S. cerevisiae may further elucidate the thermotolerance mechanism of yeast. A combination of cold-active cellulolytic enzymes and thermotolerant yeasts can overcome the problem of different temperature optima in the SSF process. However, further research into the sugar uptake mechanism, effects of inhibitors on yeast growth, and metabolic engineering for generating co-fermenting yeasts is needed.