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This study aimed to identify the water-vapor transport mechanisms through an aerated matrix during microwave freeze-drying. Due to the larger surface area and lower water vapor transport resistance of an aerated product compared to the solution, foam structures dry faster. Different foam structures were produced with different maltodextrin (MD) concentrations (10–40%) as a foam-stabilizing agent. Depending on the initial viscosity of the solution prior to foaming, the samples differed in overrun (41–1671%) and pore size (d50 = 58–553 µm). Experiments were partially performed in a freeze-drying chamber of a light microscope to visualize structural changes in-situ. Different mechanisms were identified explaining the accelerated drying of foams, depending on the MD concentration, above or below 30%. At lower MD concentration, high overruns could be produced prior to freezing with big bubbles and thin lamellae with short diffusion pathway length. At 40% MD concentration, the viscosity was too high to integrate much air into the product. Therefore, the foam overrun was low and the bubble size small. Under these conditions, the water vapor generates high pressure, resulting in the formation of channels between bubbles, thus creating the pathways with low resistance for a very fast water vapor mass transfer. In addition, microwave freeze-drying experiments using a pilot plant unit were conducted to validate the findings of the freeze-drying microscope. A reduction of the drying time from 150 min (10% MD) to 78 min (40% MD) was achieved.

Water Vapor Pathways during Freeze-Drying of Foamed Product Matrices Stabilized by Maltodextrin at Different Concentrations