A thermostable alpha-amylase produced from Bacillus subtilis was immobilized by entrapment in calcium al-inate gel capsules and it was used repeatedly in batch processes of starch hydrolysis. The degree of starch degradation and operational stability of the immobilized system were increased by tailoring the characteristics of the capsules. Capsules prepared from 2% (w/v) sodium alginate and 5% (w/v) CaCl2 were suitable for up to 20 repeated uses, losing only 30% of their initial efficiency. Even higher operational stability was achieved when silica gel 2% (w/v) was added into the biopolymer solution prior to Ca2+-induced gelation. These alginate/silica capsules carrying alpha-amylase retained 90% of their initial efficiency after 20 starch hydrolysis batches and released more than 10,700 mg of reducing sugars during a processing period of 160 h. Increase of the reaction temperature from 50 to 70 degrees C resulted in higher starch hydrolysis rates by the immobilized a-amylase without affecting the capsules stability. Among the various starchy substrates tested, higher amounts of reducing sugars were released from potato starch hydrolysis. (c) 2005 Elsevier Ltd. All rights reserved.