The performance of steady-state flow ultrafiltration membrane reactors can be considerably increased by improving their hydrodynamic characteristics. A tubule recycle membrane reactor was designed and constructed. This type of reactor approximates plug flow behavior and uses few movable parts in addn. to a small membrane area. The performance of a pilot scale tubular recycle membrane reactor was verified for the hydrolysis of lactose using lactase [9031-11-2] from Aspergillus niger. The operational stability of the lactase during a long-term operation of the reactor was estd. The substrate employed for this purpose was a deproteinized, fermented lactic acid whey, the base for a non-alc. drink. [on SciFinder (R)]