Production of dextransucrase and dextran by Leuconostoc mesenteroides immobilized in calcium-alginate beads: II. Semicontinuous fed-batch fermentations

Cells of Leuconostoc mesenteroides immobilized in calcium alginate beads were used to produce dextransucrase (DS) in three sequential cycles of semicontinuous fed-batch fermentations. Each cycle consisted of a fed-batch DS production period of 24 h followed by a batch dextran production period for another 24 h. Free, suspended cells were used in only one cycle of fed-batch DS production followed by a dextran production period. It was impractically tedious to separate and reuse free cells. Increasing sucrose feed rate from 5 to 10 g/L h led to increases of the total enzymatic activity by about 88% with immobilized cells and by about 100% with free cells. In DS fed-batch semicontinuous fermentation, total enzymatic activity produced by immobilized cells was 1.35 and 1.56 times greater than that produced by free cells with respective sucrose feeding rates of 10 and 5 g/L h. These increases in enzyme productivity with immobilized cells, however, required total overall operating times three times longer (three cycles) than with free cells (one cycle). Growing the microorganism at optimum conditions for DS production also increased the dextran yield and shortened the time of conversion of sucrose to dextran, regardless of whether the cells were free or immobilized. Moreover, during three cycles of semicontinuous operation (144 h) immobilized cells produced more than three times as much dextran as free cells during one cycle (24 h).