Challenge and opportunity of direct contact membrane distillation in liquid food concentration

Direct contact membrane distillation (DCMD) was operated at low temperatures from 25 to 40°C to suit the purpose of concentrating thermally sensitive liquid foods, especially fruit juices to high solid content concentrate with most of the quality attributes preserved. A DCMD lab scale unit has been set up at the Centre for Plant and Food Science - UWS. Hollow fibre modules using five types of fibres of PVDF and Halar material, with mass transfer areas ranging from 281 to 573 cm2 were employed. Experiments for concentration of glucose solutions from 30 to 60% (w/w) were carried out. Results indicated that not only the operating conditions were important, but also the membrane properties. It was found that Halar fibres were 2 to 3 times better performing than PVDF fibres in term of removing water from the feed, and 3 to 4 times better in terms of energy saving. Results showed that an increase of the feed inlet temperature from 25 to 40°C improved the mass flux up to 6 times and energy efficiency up to 2.5 times depending on the feed concentration. With flux rate up to 2.88 kgm-2h-I for PVDF and 5.83 kgm-2h-I for Halar fibres when concentrating 30% glucose solution at 40°C, DCMD appears to be a competitive concentration technique, when product quality is the priority. However, with energy efficiency from as low as 2.1% to 14.9%, PVDF fibres employed in the study seems not to be very suitable for liquid food concentration under low temperature condition. Halar fibres, however, with higher energy efficiency from 7.8 to 45.6% could overcome the challenge of energy and be competitive