Galacto-oligosaccharide (Gal-OS) was produced from lactose by a yeast, Sirobasidium magnum CBS6803. With toluene-treated resting cells, 136 mg ml-1 of Gal-OS was produced from 360 mg ml-1 of lactose at 50 degrees C for 42 h. Then, the yield of Gal-OS was increased by a culture method in which cell growth followed the enzymatic reaction: 224 mg ml-1 of Gal-OS was produced at 30 degrees C for 60 h. Finally, combination of the toluene-treated resting cells and glucose oxidase plus catalase was applied to improve productivity by the removal of a by-product, glucose, which inhibits the Gal-OS production, from the reaction mixture. In this case, 242 mg ml-1 of Gal-OS was produced at 50 degrees C for 42 h without cell growth. The structure of the major product was identified as 4'-galactosyl-lactose.

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