Experimental Study of the Impact of Pore Structure on Drying Kinetics and Sublimation Front Patterns

Pore shape matters - In-situ investigation of freeze-drying kinetics by 4D XCT methods

Freeze-drying is a commonly employed method in the food industry to extend shelf life of products. However, this process remains time and energy consuming. While higher shelf temperatures accelerate the process, they also pose the risk of product damage. The microstructure of the product, influencing heat and mass transport, is a critical factor. This study aims to understand the impact of 3-dimensional (3D) structural parameters (pore size, shape and orientation) on local primary freeze-drying kinetics. Freeze-drying experiments were conducted with maltodextrin solutions (c1 = 0.05, c2 = 0.15 and c3 = 0.3 w/w) at different shelf temperatures (T1 = -11, T2 = -15 and T3 = -33 °C) with the use of a freeze-drying stage that allows in-situ visualization of the process inside a 4D-X-Ray computed tomography (XCT). The findings show the importance of understanding the microstructure in detail to optimize the sublimation time during the freeze-drying process. It is shown that for longitudinal pores, the orientation is a crucial parameter.

A Non-Isothermal Pore Network Model of Primary Freeze Drying

In this work, a non-isothermal pore network (PN) model with quasi-steady vapor transport and transient heat transfer is presented for the first time for the application of primary freeze drying. The pore-scale resolved model is physically based and allows for the investigation of correlations between spatially distributed structure and transport conditions. The studied examples were regular PN lattices with a significantly different structure, namely a spatially homogeneous PN, also denoted as monomodal PN, and a PN with significant structure variation, referred to as bimodal PN because of its bimodal pore size distribution. The material properties selected for the solid skeleton in this study are equivalent to those of maltodextrin. The temperature ranges applied here were -28 °C to -18 °C in the PN and -42 °C in the surrounding environment. The environmental vapor pressure was 10 Pa. The PNs were dried with constant temperature boundary conditions, and heat was transferred at the top side by the vapor leaving the PN. It is shown how the structural peculiarities affect the local heat and mass transfer conditions and result in a significant widening of the sublimation front in the case of the bimodal PN. The possibility of spatially and temporally resolved front structures is a unique feature of the PN model and allows the study of situations that are not yet described by classical continuum approaches, namely heterogeneous frozen porous materials. As demonstrated by the thin layers studied here, the pore-scale simulations are of particular interest for such situations, such as in lyomicroscopes or collagen scaffolds, where a length-scale separation between dry and ice-saturated regions is not possible.

New Trends in Freeze-Drying of Pharmaceutical Products

Freeze-drying, also known as lyophilization, is a process that facilitates the removal of water through sublimation from a frozen product (primary drying) [...].

The Influence of Local Microstructure Inhomogeneities on Local Drying Kinetics during Freeze-Drying

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.