Capability of sweet sorghum stalks as supporting materials for yeast immobilization to produce ethanol under various fermentation processes

Unpeeled sweet sorghum stalk (SSS) pieces sizing 6- to 20-mm diameter and 6-mm thick were used as carriers for Saccharomyces cerevisiae NP 01 immobilization to produce ethanol. The diluted sweet sorghum juice containing 100 g/l of total sugar without nutrient supplementation was a suitable medium for yeast cell immobilization, and 18 h incubation time was sufficient for the immobilization process. In repeated-batch ethanol fermentation from sweet sorghum juice (240 g/l of total sugar) at 30 °C, the average ethanol concentration (P), productivity (QP) and yield (YP/S) by the immobilized yeast cells were 93.4 g/l, 1.30 g/l h and 0.47 g/g, respectively. When the continuous system was operated in a double-tubular packed-bed bioreactor with 50% bed height, the average P, QP and YP/S were comparable to those of the repeated-batch fermentation. Supplementation the juice with 6 g/l of yeast extract resulted in significant increases in P, QP and YP/S to 105.7 g/l, 2.43 g/l h and 0.48 g/g, respectively at the dilution rate of 0.023/h. These results demonstrated that the unpeeled SSS pieces were successfully used as low-cost carriers for yeast cell immobilization to produce ethanol under both repeated-batch and continuous systems.