To develop a feasible enzymatic process for the concomitant D-tagatose and D-fructose production, the thermostable L-arabinose isomerase of Bacillus stearothermophilus US 100 (L-AI US 100) and the mutant D-glucose isomerase obtained from that of Streptomyces SK (SKGI-A103G) were successfully co-expressed in Escherichia coli HB101 strain. The recombinant cells were immobilized in alginate beads and showed, similarly to the free cells, optimal temperatures for D-galactose and D-glucose isomerisation of 80 and 85 degrees C, respectively. The two isomerases were optimally active at pH 7.5. Cell entrapment significantly enhanced the acidotolerance of the two isomerases, as well as their stability at high temperatures. To perform simultaneous isomerisation of D-galactose and D-glucose at 65 degrees C and pH 7.5 in packed-bed bioreactor, cells concentration, dilution rate, productivity and bioconversion rate were optimized to be 32 g/l, 2.6 h(-1), 3 g/l h and 30%, respectively. (C) 2006 Elsevier Inc. All rights reserved.