Freeze drying of food and biological materials also has the advantage of little loss of flavor and aroma. Freeze drying requires very low pressures or high vacuum in order to produce a satisfactory drying rate. The freezing stage represents the first separation step in the freeze drying process, and the performance of the overall freeze drying process depends significantly on this stage. In industrial freeze dryers the freezing of the product is usually made in the same plant where the drying also occurs. The structural stability of a material relates to its ability to go through the freeze drying process without change in size, porous structure, and shape. The limitations on heat transfer rates in conventionally conducted freeze drying operations have led early to the attempt to provide internal heat generation through the use of microwave power.