Abstract: A process (51) for producing alternan-oligosaccharide (8), comprising contacting in a reactor (11) sucrose (9) with a catalytically effective amount of alternansucrase enzyme (13) and acceptor molecules (12), wherein the alternansucrase enzyme (13) and acceptor molecules (12) are present in the reactor (11) in an aqueous liquid (4) and the sucrose (9) is continuously or half-continuously fed to the reactor (11), and wherein the sucrose (9) and the acceptor molecules (12) are converted to alternan-oligosaccharide (8), and fructose (6) is formed as a by-product, continuously or half-continuously removing at least a part of the fructose (6) from the reactor (11) by membrane filtration (17).