We have built and tested a flat-sheet membrane evaporator for removing water from dilute feed streams like milk and orange juice. The energy for the water's evaporation comes from steam channels next to the feed channels, so that the operation differs sharply from other forms of "membrane distillation". The membrane evaporator retains flavors effectively. Because it has an overall vapor phase mass transfer coefficient of about 1 cm/s, it is only 68% efficient: only about 0.68 kg water is evaporated per kg steam condensed. This efficiency should be over 95% for a membrane which is 10 times more permeable.