until a cell density of 1.7 g l was reached, and then the medium was continuously pumped through the column and recirculated back to the reactor. The pH was controlled in the reactor at pH 6 by titration with NHOH. For this study, the volume of medium used was 700 ml and that of insoluble biocatalyst, 350 ml. Once the sugar was completely utilized, the reactor was emptied, washed by circulating 700 ml of 0.9% NaCl solution, and fresh medium (synthetic or WP) containing 100 g l sugar was pumped in.