Ultra high fructose syrup is of increasing importance, because of its sweetening properties. Fructose is often obtained by glucose isomerization. However, this process has a relatively low conversion (around 45%), and it uses a complex purification scheme. An alternative process involves the hydrolysis of natural products, such as inulin, a polymer which contains between 5 and 65 molecules of fructose. The use of enzymes in such hydrolysis reactions would add the benefit of mild reaction conditions and a better product profile. In this work, the enzymatic hydrolysis of inulin has been studied in a membrane reactor (EMR). A first set of experiments aimed to find the optimum operating conditions, in terms of pH (a value of 4.5 was obtained as optimum), substrate concentration (a maximum of 100 g/L is recommended, because of the ease of inulin precipitation), and substrate/enzyme ratio. All experiments were conducted at 50 degrees C. Then, enzymes were immobilized in order to be able to reuse them, which would result in a more economically feasible process. As immobilization supports both hollow fibre membranes and particulates have been used and compared, and methods of improving the amount and/or the stability of the immobilized enzymes have also been proposed and tested. The results show that the EMR is capable to retaining more than 50% of the initial activity that the enzyme has in solution over at least five repeated reaction cycles. This work can provide a preliminary basis for the production of fructose syrups with a commercial inulinase preparation immobilized onto a membrane. (c) 2005 Elsevier B.V. All rights reserved.