Ethanol production from starchy materials by conventional fermentation requires saccharification with amylolytic enzymes and subsequent fermentation using the yeast Saccharomyces cerevisiae, because this yeast cannot utilize starchy materials. The two-step process results in high production costs and low productivity of ethanol. Many reports have been published on the development of S. cerevisiae strains capable of secreting amylolytic enzymes [2]. Although yeast strains capable of utilizing starch have been developed, their ethanol-producing ability remains unsatisfactory. Recently, yeast strains displaying various proteins on their cell surface have been developed using genetic engineering [3]. Laboratory yeast strains displaying Rhizopus oryzae glucoamylase (EC 3.2.1.3) [4], which cleaves glucose from α-1,4-linked and α-1,6-linked polysaccharides, have produced ethanol directly from soluble and cooked corn starch [5] and [6]. However, these surface-engineered yeast stains, because of the nature of the parent laboratory strain, cannot produce and accumulate high concentrations of ethanol at a rapid rate. Therefore, it is necessary to apply the surface-engineering technology to a high-ethanol-yielding and ethanol-tolerant industrial strain to produce a practically useful yeast strain for ethanol production.
This report describes the development of a surface-engineered yeast strain displaying R. oryzae glucoamylase using an industrial ethanol-producing yeast, S. cerevisiae Kyokai no. 7, and its fermentation properties on cassava pulp.