Non-conventional yeast strains: Unexploited resources for effective commercialization of second generation bioethanol

The conventional yeast (Saccharomyces cerevisiae) is the most studied yeast and has been used in many important industrial productions, especially in bioethanol production from first generation feedstock (sugar and starchy biomass). However, for reduced cost and to avoid competition with food, second generation bioethanol, which is produced from lignocellulosic feedstock, is now being investigated. Production of second generation bioethanol involves pre-treatment and hydrolysis of lignocellulosic biomass to sugar monomers containing, amongst others, D-glucose and D-xylose. Intrinsically, S. cerevisiae strains lack the ability to ferment pentose sugars and genetic engineering of S. cerevisiae to inculcate the ability to ferment pentose sugars is ongoing to develop recombinant strains with the required stability and robustness for commercial second generation bioethanol production. Furthermore, pre-treatment of these lignocellulosic wastes leads to the release of inhibitory compounds which adversely affect the growth and fermentation by S. cerevisae. S. cerevisiae also lacks the ability to grow at high temperatures which favour Simultaneous Saccharification and Fermentation of substrates to bioethanol. There is, therefore, a need for robust yeast species which can co-ferment hexose and pentose sugars and can tolerate high temperatures and the inhibitory substances produced during pre-treatment and hydrolysis of lignocellulosic materials. Non-conventional yeast strains are potential solutions to these problems due to their abilities to ferment both hexose and pentose sugars, and tolerate high temperature and stress conditions encountered during ethanol production from lignocellulosic hydrolysate. This review highlights the limitations of the conventional yeast species and the potentials of non-conventional yeast strains in commercialization of second generation bioethanol.

Automated summary

The most studied yeast, Saccharomyces cerevisiae, has been widely used in industrial production of bioethanol from first generation feedstock. However, for cost reduction and to avoid competition with food, researchers are now exploring second generation bioethanol production from lignocellulosic feedstock. Genetic engineering is being employed to develop recombinant S. cerevisiae strains that are capable of fermenting pentose sugars and with the necessary stability and robustness for commercial second generation bioethanol production. Additionally, non-conventional yeast strains are being investigated as potential solutions due to their ability to ferment both hexose and pentose sugars, withstand high temperatures, and tolerate inhibitory substances encountered during lignocellulosic hydrolysis. This review highlights the limitations of conventional yeast species and the potential of non-conventional yeast strains in the commercialization of second generation bioethanol.