Direct contact membrane distillation (DCMD) process was chosen to produce a highly concentrated apple juice using hollow fiber modules. A high 64 degrees Brix concentration was achieved. The trans-membrane driving force decreased with increasing extra-fiber temperature but increased with higher feed and distillate flow rates in the intra- and extra-fiber volumes, respectively. Flux inversion was sensitive to small differences in temperature between the intra- and extra-fiber volumes and could be prevented by increasing the intra-fiber feed temperature by 2-4 degrees C. Flux rates were dependent upon the temperature polarisation coefficient (TPC) and the effect of the concentration polarisation coefficient (CPC) was negligible. Trans-membrane flux was also significantly increased by thermal-osmotic distillation (TOD) using a high concentration of CaCl2 as the permeate solution. A new model describing the fluid dynamics and membrane behaviour within the DCMD system is presented. In particular, the influence of various properties of membrane morphology, such as the distribution of pores of different diameters and elastic and other mechanical properties upon flux were taken into account in this model. (C)2000 Elsevier Science B.V. All rights reserved.