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Freeze-drying frozen maltodextrin solutions with solid contents of 5% and 30% (w/w) was experimentally investigated using neutron imaging at PSI Villigen/Switzerland. Different solid contents, as well as annealing at −5 °C for 11 h, were used to modify the porous structure of the samples, which was quantified using X-ray computed tomography. Annealing of the 5% (w/w) sample, with a pore size distribution (PSD) of 23.7 ± 11.1 µm, yielded a very open pore space with high porosity (ε = 0.96) and a PSD of 33.0 ± 27.0 µm. In contrast, the higher solid content resulted in small, lamellar, narrow pores with high anisotropy and a porosity of ε = 0.65, as well as a PSD of 13.5 ± 4 µm. In operando neutron imaging was used to show the impact of the structure of frozen maltodextrin on the overall drying kinetics and shape of the sublimation front during freeze-drying. For this purpose, a freeze-drying stage was employed, which allowed a novel approach to time- and space-resolved monitoring of the ice phase. The sublimation front propagation was quantitatively analyzed based on ice saturation profiles and sublimation rates. The dependence of drying velocity on structure is nicely demonstrated by the data. In addition, it is shown that the sublimation front widened during freeze-drying, resulting in either rather concave or convex shape depending on morphological parameters.

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Freeze-drying is a gentle drying technique to dry high value products, such as pharmaceuticals, without impacting the quality of the product. However, this method is very time and cost intensive. It is known that larger pores reduce the duration of primary drying due to facilitated mass transport. However, next to the pore size, other structural parameters exist whose influence on drying kinetics is still unknown. Therefore, the aim of this article is to investigate the influence of the microstructure (pore size, shape and orientation) on local primary drying kinetics. In the study, freeze-drying experiments on maltodextrin and sucrose solutions (c1 = 0.05 and c2 = 0.15 w/w) were carried out in a lyomicroscope. Two-dimensional images were recorded during the whole drying process and in the dry state and analyzed on the movement of the sublimation front, pore size, orientation and shape. Different microstructures were created by using different freezing parameters, namely two different cooling rates and solid concentrations. It could be shown that for pores with a high aspect ratio, the pore orientation was more important for the drying kinetics than the pore size, while for pores with a lower aspect ratio the pore size was the decisive parameter.

This study shows that changing solid content and particle size leads to different inner and outer porosities and thus, leads to different drying properties. These relationships were investigated with neutron imaging by using maltodextrin as the model substance. The experimental results are assessed based on analyses made with GeoDict (Math2Market). It is shown that particle beds with larger particles (x = 3550 µm) have generally better mass transfer conditions, which are additionally positively affected by lower solid concentrations (c = 0.05 w/w). In contrast heat transfer is generally enhanced in the case of fine particles (x = 70 µm) but impairs with increasing porosity.