Extracellular Ca2+ plays an important role in flocculation because calcium-bridges link flocculent cells (Speers et al., 1992). Thus, the supplementation of feedstock with CaCl2·2H2O was opti- mized. Fig. 1b shows that the sugars were almost completely consumed and the ethanol yield was at the same level of approx- imately 83% with the addition of various amounts of CaCl2·2H2O, while the number of yeast and viability in the reactor increased with increasing amounts of CaCl2·2H2O added (Fig. 1a). It could be concluded that the increase in the number of yeast cells was not caused by over-reproduction but rather by the addition of CaCl2·2H2O.