Membrane distillation for milk concentration

Membrane distillation is an emerging technology to concentrate liquid products while producing high quality water as permeate. Application for desalination has been studied extensively the past years, but membrane distillation has also potential to produce concentrated food products like concentrated milk. Water vapour migrates from the milk feed side to the permeate side by the vapour pressure difference between the two sides of a hydrophobic membrane. Unlike pressure driven membrane filtration high solid concentrations are achievable.
Experimental results show that concentrations of 50% total solids can be achieved for milk, which makes membrane distillation a competitor to evaporation. In experiments for skimmed milk, ranging from 40 to 60°C, the flux was found to increase with higher feed temperatures. For skimmed milk with 25% total solids, at 60°C an initial flux of around 16 kg/m2 per hour was achieved. Due to gradual fouling built the flux declined to 8 kg/m2 per hour after 15 hours. At concentrations of 50% total solids the flux declined to 3 kg/m2 per hour.
Confocal laser scanning microscopy (CLSM) was used to investigate the size and morphology of the fouling layer. Images indicated homogeneous fouling layers with a thickness depending on the process temperature; lower temperature resulted in thinner fouling layer. Most plausible reason is the lower driving force and transmembrane flux. These lab scale experiments showed promising fluxes, which still can be improved, that are a good starting point for the development of large scale production units.