3. Results and discussion
3.1. Batch and repeated batch fermentation
Preliminary studies (data not shown) of 72-h batch fermentation have shown that an optimal ethanol concentration of 98.6 g/L with an ethanol yield (YP/S) of 0.47 (g/g) are obtained by TSC-immobilized cells (TSC-cell) at an initial reducing sugar concentration of 240 g/L. The ethanol concentration and conversion yield of the immobilized cells were 11.5% and 9.3% higher than those of the free cells. The inhibitory effect of high sugar concentration on ethanol fermentation by free cells has been previously reported [14]. It was found that, at high initial sugar concentrations, the system using TSC-cells was more efficient than when using free cells.
Ethanol production using TSC-cells was further examined by five-cycle repeated batch fermentation. The results of the ethanol production and reducing sugar consumption by the immobilized cells were compared to those using the suspended cells (SC). The duration of the experiments on each batch was 48 h, and the initial sugar concentration was 240 g/L. Table 1 summarizes the yields, end products and productivities of the repeated batch fermentations. In the first batch, the initial rates of ethanol production and sugar consumption in the TSC-cell culture were slightly lower than those in the SC culture. Nonetheless, the final end-product concentration, overall sugar consumption and ethanol yield (YP/S) were similar in both cultures. From the second to the fifth batch