An apparatus for dehydrating a liquid sample of biological material has a microwave waveguide that is open to the atmosphere. It has a microwave generator, means for introducing a container of the material into the waveguide, means for evacuating the container, means for rotating the container and means for removing the container from the waveguide. It can include means for moving the container through the waveguide and for sealing it. In a dehydration method, a container of the liquid sample is put into the open waveguide, evacuated, rotated at high speed and microwaved. The container of dehydrated material is then removed from the waveguide. The apparatus and method are particularly suitable for dehydrating vaccines.