Continuous cultivation of dilute-acid hydrolysates to ethanol by immobilized Saccharomyces cerevisiae

The continuous cultivation of immobilized Saccharomyces cerevisiae CBS 8066 on dilute-acid hydrolysates of forest residuals was investigated. The yeast cells were immobilized in 2-4% Ca-alginate beads. The 2% beads were not stable. However, the 3 and 4% beads were stable for at least 3 wk when an extra resource of calcium ions was available in the medium. The continuous cultivation of a dilute-acid hydrolysate by the immobilized cells at dilution rates of 0.3,0.5, and 0.6 h(-1) resulted in 86,83, and 79% sugar consumption, respectively, and an ethanol yield between 0.45 and 0.48 g/g. The hydrolysate was fermentable at a dilution rate of 0.1 h(-1) in a free-cell system but washed out at a dilution rate of 0.2 h(-1). The continuous cultivation of a more inhibiting hydrolysate was not successful by either free- or immobilized-cell systems even at a low dilution rate of 0.07 h(-1). However, when the hydrolysate was overlimed, it was fermentable by the immobilized cells at a dilution rate of 0.2 h(-1).