Development of raw starch hydrolysing yeasts for industrial bioethanol production

The use of biomass for the production of bioethanol has received considerable interest in recent years. Starchy and cellulosic materials are the most abundant biomass resources. Fuel ethanol has already been produced from sugar cane and starch rich grains in Brazil and United States. Using starchy substrates, such as maize and industrial residues (cereal brans and potato peel by-products) for ethanol commercial process is a fairly mature technology, although the energy cost is still very high. As a consequence, the need to develop a more feasible process is evident. A raw starch hydrolysing and fermenting yeast could yield large cost reductions in Consolidated Bioprocessing (CBP), a one-step hydrolysis and fermentation technology. The aim of this study was to engineer an efficient amylolytic Saccharomyces cerevisiae strain capable for industrial ethanol production. A codon-optimised gene encoding a fungal glucoamylase was introduced into the chromosomal δ sequences of a wild type S. cerevisiae strain with promising fermentative traits. The obtained transformants, mitotically stable after 120 generations in non-selective broth, showed interesting amylolytic activities both on soluble and raw starch. During anaerobic cultivation, an ethanol yield of 0.44 gram ethanol per gram available sugar (g g−1) was produced from soluble starch (79% of theoretical maximum) at a specific ethanol productivity of 0.041 gram ethanol per gram dry cells per hour (g (g dw)−1 h−1). On raw starch, the engineered strains exhibited improvable fermentative performance with an ethanol yield of 0.41 (g g−1), corresponding to 72% of the theoretical maximum. On the basis of the preliminary fermentation studies, the constructed yeasts could be promising for the Consolidated Bioprocessing of different starchy industrial residues. The co-expression of the codon-optimised glucoamylase and other amylolytic genes in the recombinant strains is in progress to enhance and optimise their starch conversion efficiency