Enhanced production of bioethanol and ultrastructural characteristics of reused Saccharomyces cerevisiae immobilized calcium alginate beads

► We examine optimal batch fermentation condition of immobilized yeast with Ca-alginate beads. ► Reused yeast immobilized beads give excellent results in converting glucose to ethanol. ► Short fermentation time and the high ethanol yield with the reused beads are due to a higher yeast population and porosity. ► We explain and confirm “egg-box” model of alginate matrix structure using TEM observation. Yeast immobilized on alginate beads produced a higher ethanol yield more rapidly than did free yeast cells under the same batch-fermentation conditions. The optimal fermentation conditions were 30°C, pH 5.0, and 10% initial glucose concentration with 2% sodium alginate beads. The fermentation time using reused alginate beads was 10–14h, whereas fresh beads took 24h, and free cells took 36h. All bead samples resulted in nearly a 100% ethanol yield, whereas the free cells resulted in an 88% yield. Transmission electron microscopy (TEM) showed that the shortened time and higher yield with the reused beads was due to a higher yeast population per bead as well as a higher porosity. The ultrastructure of calcium alginate beads and the alginate matrix structure known as the “egg-box” model were observed using TEM.