An apparatus for dehydrating organic material such as food products and biologically-active materials such as vaccines, antibiotics, proteins and microorganism cultures using microwave vacuum-drying. The apparatus has (a) a vacuum chamber (34)； (b) a set of two or more microwave generators (50)； (c) a microwave-transparent window (36) for transmission of microwave radiation from the microwave generators (50) into the vacuum chamber (34)； (d) a microwave chamber (52) between the microwave generators (50) and the microwave-transparent window (36), the two or more microwave generators (50) in the set being positioned such that a stream of microwaves from anyone generator (50) overlaps in the microwave chamber (52) with a stream of microwaves from any other generator (50) of the set； and (e) means for controlling the operation of the microwave generators (50) to repeatedly actuate and de-actuate the microwave generators (50) to cause interference between the stream of microwaves from any one generator (50) and the stream of microwaves from any other generator (50) of the set. The invention aims to permit enhanced control of the drying process by utilising the interference of microwaves so that the organic materials are treated uniformly.
