Starch-converting enzymes, alpha-amylase and glucoamylase, were immobilized on surface-modified carriers using a co-immobilized as well as a single system. Hydrophilic silica gel and DEAE-cellulose entrapped in alginate beads were selected as the most effective carriers. The hydrophilic and the hydrophobic characteristics of carriers were made by polyethylimine-coating and silanization, respectively, and confirmed by contact angle measurements. All co-enzymes immobilized on hydrophilic carriers exhibited the highest activities in each modulation. The lower K-m values (0.102 mg/ml for hydrophilic silica gel and 0.099 mg/ml for DEAE-cellulose entrapped in beads) than that of native enzyme (0.199 mg/ml) represented the higher affinity of immobilized enzymes to substrates. When performing the hydrolysis of starch, the shifts of optimum temperature from 50 to 60 degreesC (or activation energy) and pH from 5.0 to 5.5 after immobilization were due to covalent bond formation and polyionic characteristics of carriers, respectively. The co-enzymes immobilized on hydrophilic silica gel and DEAE-cellulose entrapped in alginate beads exhibited 92.3 and 88.9% of the remaining activity even after 10 times of reuse. (C) 2003 Elsevier Ltd. All rights reserved.