Ethanol production from coffee mucilage fermentation by S. cerevisiae immobilized in calcium-alginate beads

Soluble sugars in coffee mucilage were directly fermented into ethanol using immobilized Saccharomyces cerevisiae cells in calcium alginate beads. Several tests were performed via alginate-entrapped cells in different bead sizes (3 or 7 mm) combined to alginate concentrations (2-4%). The highest yield of 0.33 g ethanol/g sugar was achieved at 18 h using alginate-entrapped cells in 3 mm diameter and 2% (w/v) alginate, which corresponded to 64% of the theoretically achievable yield of ethanol. Tests with 7 mm beads took 6 h longer to reach maximum ethanol productions. The reduction of bead size increased mass transfer of substrates from the liquid to the immobilized cells, accelerating sugar consumption and ethanol production. Entrapment enabled reuse of the cells for up to 3 consecutive batches with stable ethanol production (up to 72 h). This work confirms the reuse of alginate-entrapped cells is feasible for ethanol fermentation of mucilage without supplement of nutrients.