Abstract
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.
Highlights
To increase the shelf life of biogenic substances of relevance in the food and pharmaceutical industries such as proteins, antibodies, immunoglobulins or microorganisms and to improve their handling, biomolecules are often preserved by freeze-drying (FD), which mainly uses sugar as protectants [1,2]
Due to the layer(Figure of sample, it was assumed thatfast the cooling heating rates, of theafreeze-drying microscope is connected to the stage
The freezing rate was calculated by the mean of the temperature slopes until nucleation occurred and used for the setup in the freeze-drying microscope (Table 1)
Summary
To increase the shelf life of biogenic substances of relevance in the food and pharmaceutical industries such as proteins, antibodies, immunoglobulins or microorganisms and to improve their handling, biomolecules are often preserved by freeze-drying (FD), which mainly uses sugar as protectants [1,2]. To accelerate the drying process, energy can be introduced using microwave technology, which allows a more effective and more uniform volumetric energy input [5], and by foaming the product before drying This will increase the surface area of the product and lower the water vapor transfer resistance [6]. For the microwave-supported freeze-drying (MWFD) of aerated milk [17] These authors stated that the water vapor leaves the frozen foam through air voids, creating an open porous structure where gas can pass through the structure without resistance. Different concentrations of the polysaccharide maltodextrin were used in combination with surfactant polysorbate 80 This process resulted in different foam properties such as volume increase (overrun) and bubble size and gave the possibility of examining the water vapor pathway of different product structures. The results were compared with those of the pilot-plant experiments to validate the findings from the microstructural analysis of freeze-drying
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