ABSTRACT
Microwave-vacuum drying (MVD) offers an attractive, cost-effective alternative to freeze-drying (F-D) in the production of high-quality heatsensitive products. In most freeze-dryers, heat is transferred to the surface of the frozen material by conduction from heating plates. In the course of drying, heat is transferred from the material surface to the sublimation front by conduction through the layer of already dry material. To perform the process at reasonably high drying rates, a suffi cient temperature difference should be maintained between the material surface and its core. Thus, the surface layers can easily be overheated, especially for materials of low thermal conductivity. A solution to this problem could be microwave (MW) heating, where the material core is usually at a higher temperature because of internal heat generation. In fact, MW F-D has been extensively studied in the past 20 years and a number of laboratory and pilot tests have documented the technical feasibility and substantial advantages over conventional F-D (Kudra and Strumillo, 1998). MW F-D is, however, diffi cult to control because of a much higher loss factor for liquid water than that for ice. Thus, any localized melting would result in runaway overheating (Liapis and Bruttini, 1995). MW drying under vacuum appears to be the solution of choice as it offers all the advantages of dielectric heating but at a reduced processing temperature, which is a function of the operating pressure (Figure 26.1). In addition, a volumetric heat-transfer mechanism coupled with drying in the absence of oxygen (vacuum) provides an ideal low-temperature drying technique.
