Micro-CT visualization of structure development during freeze-drying processes

Of all drying processes, freeze drying (FD) generally produces products of the best quality. Major drawbacks are high costs and process times. In order to overcome the disadvantages, serial combinations with other drying processes are discussed. FD as a predrying process can have a beneficial influence on subsequent drying processes, such as an improved mass transport and a better structure preservation. This study aims to clarify the mechanisms that lead to these phenomena by investigation of the development of 3-D pore structures with a cone-beam micro-CT. Carrot discs were dried with FD, microwave-vacuum drying (MVD) and a serial combination with different changeover-points. All combination-dried products had porous central parts, while the outer parts were dense and collapsed. This indicates a radial, rather than the expected upward progress of the drying front inside the carrot discs during FD. CT-measurements of partly dried samples could prove that, in fact, the center of carrot discs dries first during FD.