Experimental and Simulation Studies of the Primary and Secondary Vacuum Freeze Drying at Microwave Heating

Abstract

Freeze drying (FD) is a dehydration method used to obtain high-quality products, mainly foodstuffs, biomaterials, pharmaceuticals and other thermolabile materials. Compared with standard methods, the freeze drying technique ensures retaining original shape, color and texture of the product as well as the preservation of its flavor, nutritive content and biological activity. However, FD is an expensive and lengthy dehydration process carried out in three stages: pre-freezing, primary drying when ice sublimation takes place under vacuum, followed by desorption of residual, unfreezable bounded water during the secondary stage. Material must be first cooled below its triple point to obtain frozen state. In the case of food and biomaterials freezing is done rapidly at temperature between -50°C and -80°C, below eutectic point to avoid destruction of cell walls by ice crystals. During the primary FD stage, the frozen water in dried material pores sublimates from the ice front, diffuses throughout the dried layer to the sample surface and next deposits on the condenser surface. The sublimation of water takes place in the range of temperature and pressure below the triple point (for water 273.16 K and 611.73 Pa, respectively). After primary drying, residual moisture content may be as high as 7%. Secondary drying is intended to reduce this to an optimum value for material stability – usually with moisture content between 0.5 and 2.0%. The typical freeze-dried products have a porous, nonshrunken structure resulted from structural rigidity achieved by frozen water and can be therefore easily rehydrated. The use of conventional FD on industrial scale is restricted to rather high-value products. Recent research is being focused on reducing operating costs of FD by intensifying heat and mass transfer in dried material. Proposed various heating methods, cycled pressure strategies and formulated optimal control policies are limited by temperature constraints (Liapis & Bruttini, 2006). They must be set to avoid ice melting and scorching of exposed dried material layer. The major difficulty of process optimization results from the fact that imposed thermal gradient has direction opposite to vapor concentration gradient. Moreover, dried layer acts as a thermal insulation for heat fluxes being transferred toward frozen layer. New FD method that overcomes these disadvantages is microwave freeze drying (MFD). Microwaves penetrate very well into ice and supply energy for sublimation volumetrically and selectively, bypassing the problem of heat transport through the dried layer of the