A freeze-drying microscopy study of the kinetics of sublimation in a model lactose system

Abstract

Freeze drying microscopy has been used to probe the lyophilisation kinetics of lactose solutions of various concentrations, at temperatures ranging from -50°C to -30°C and under a constant pressure of 1 Pa. Sublimation front velocities were determined by recording a sequence of video images of the sublimation and analysing the frontal progression using MATLAB. Initial experiments showed poor reproducibility. To combat this, silver iodide (AgI) was added as an ice nucleator, which raised nucleation temperatures and improved reproducibility when compared to non-AgI experiments. The lower supercooling on nucleation when AgI was used also produced larger ice crystals, which enabled the crystal microstructure of the more dilute samples to be more clearly observed. This showed long thin crystals, and the orientation of these crystals with respect to the direction of the frontal movement strongly affected frontal progression rates, which explained the earlier reproducibility problems. A twin resistance mass transfer model, comprising a fixed edge resistance and a resistance which increased with frontal depth, was able to describe the sublimation kinetics. The edge resistance first increased and then decreased with solids content. The resistance per unit depth increased exponentially with solids content, so much so that there is an optimal solids content in relation to the rate of production of dried material.Resistances were also much higher when crystals were oriented with their major axis perpendicular to the direction of frontal movement. Freeze drying rates were approximately proportional to the saturation vapour pressure of water, however the long-held belief that water vapour pressure is the main driving force for mass transfer in freeze-drying systems may be an oversimplification as this only reflects driving forces in the vapour phase (pores) rather than within the solid.

Characterization of Lyophilization of Frozen Bulky Solids

“…Therefore, it is the aim of the paper to present two different methods to study freeze-drying behavior in situ and to visualize sublimation fronts of particle beds, namely, by lyomicroscopy and neutron imaging. The lyomicroscopy is a light microscopic technique where the sublimation front can be assessed twodimensionally under defined conditions [10][11][12]. Neutron imaging is a powerful technique to visualize drying fronts in convective drying since neutrons are very sensitive to water [13,14].…”

Latest advances in imaging techniques for characterizing soft, multiphasic food materials

“…Hence, a variety of dynamic conditions can be applied to observe changes in the appearance of the specimen by coupling the microscope with a stage that controls temperature, pressure, or can apply shear. Examples of these include Hot-Stage Microscopy (HSM) (Harrison et al, 2016;Stapley et al, 2009;Stewart et al, 2017) and Freeze-Drying Microscopy (FDM) (Ray et al, 2017). Resolution, and thereby image quality of light microscopes can be improved using suitable contrast enhancement methods.…”

“…Moreover, the effect of processes such as thermal annealing (Sittipod & Shi, 2016), gelatinisation (Díaz-Calderón et al, 2018) and pulsed electric field (PEF) (Li et al, 2019) on the crystallinity of starch particles can be monitored as well, by comparing the birefringence intensity of starch samples prior to and after treatment. A further example of food processing characterization can be found in (Ray et al, 2017), where the lyophilisation of a lactose solution was investigated by coupling an optical microscope with a temperature and vacuum stage. In this work, the effect of different processing conditions (e.g.…”