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We apply here the previously developed mass transfer model (Part 1) for drying of aerogel monoliths to the supercritical drying of particles in a packed bed. We analyzed the influence of the operating conditions, flow rate, particle and autoclave size on the drying time and consumption of carbon dioxide. Although the model requires certain improvement to quantitatively predict the experimental drying kinetics, it captures all features of the supercritical drying: (i) transition of the limiting mass transfer step between diffusion in the gel and convection in the bulk fluid; (ii) role of CO2 density as natural variable for analysis of the CO2 consumption; (iii) influence of the autoclave dimensions on the drying process. We suggest a unified approach towards the analysis of drying process based on a dimensionless number. Practical recommendations on the rational selection of process parameters to achieve appropriate combinations of drying time and CO2 consumption are formulated.

Different numerical solutions of a previously developed mass transport model for supercritical drying of aerogel particles in a packed bed [Part 1: Selmer et al. 2018, Part 2: Selmer et al. 2019] are compared. Two finite difference discretizations and a finite volume method were used. The finite volume method showed a higher overall accuracy, in the form of lower overall Euclidean norm (l2 ) and maximum norm (l&infin, ) errors, as well as lower mole balance errors compared to the finite difference methods. Additionally, the finite volume method was more efficient when the condition numbers of the linear systems to be solved were considered. In case of fine grids, the computation time of the finite difference methods was slightly faster but for 16 or fewer nodes the finite volume method was superior. Overall, the finite volume method is preferable for the numerical solution of the described drying model for aerogel particles in a packed bed.

This study aims at investigating the effect of ethanol (EtOH) on the textural properties of whey protein and egg white protein hydrogels. The hydrogels were produced by thermally induced gel formation of aqueous protein solutions. The water contained in the gel network was subsequently exchanged by EtOH to assess structural changes upon exposure of hydrogels to ethanolic aqueous phases. The textural properties of the hydrogel and alcogel samples were analyzed by uniaxial compression tests. For both protein sources, the hardness increased exponentially when pH and EtOH concentration were increased. This increase correlated with a shrinkage of the gel samples. The gel texture was found to be elastic at low EtOH concentrations and became stiff and hard at higher EtOH concentrations. It was found that the solvent exchange influences the ion concentration within the gels and, therefore, the interactions between molecules in the gel structure. Non-covalent bonds were identified as substantially responsible for the gel structure., Molecules, 25 (19), ISSN:1420-3049

A novel technique for the measurement of the coating layer thickness of fine particles was developed in this work based on cross-sectioning of micrometre-sized single coated particles using focused ion beam (FIB) milling. This technique was tested on two batches of aerogel particles coated with thin coatings in a spouted bed. The FIB milling procedure consisted of two steps. First, the desired part of the coated particle was removed using a high ion beam current. The resulting cross-sectioned area was then polished using a lower ion beam current to make the cross-section clearly visible. The FIB milling process was controlled with simultaneous scanning electron microscopy (SEM). Afterwards, the coating layer thickness was evaluated using the SEM images. The coating layer was successfully applied on the porous aerogel microparticles in the spouted bed. The coating uniformity of the highly porous particles increased with increasing sprayed coating solution amount, with up to 91% of the particle pores being covered. The FIB-cross-sectioning technique using an ion beam of 2.50 nA for the first milling and 0.43 nA for polishing of the surface resulted in successful generation of cross-sections of representative particles with a visible particle core and coating layer. A coating layer thickness of approximately 700 nm was achieved on particles with sizes of below 45 μm.

A mass transport model for the supercritical drying of gels to aerogels in different sizes (monoliths or particles) and shapes (cylinder, sphere) was developed and evaluated. Physico-chemical data for the system CO2/ethanol from literature at relevant process conditions were analyzed for a precise description of relevant physical properties. In situ measurements of the supercritical drying kinetics of gel monoliths using Raman spectroscopy were used to fit the tortuosity factor of the gel network. Apart from the fitted tortuosity factor the presented model is predictive. The model was analyzed in detail for the case of spherical gel particles. Theoretical minimal drying times were found to range in seconds for microparticles. The mass transfer step limiting the overall drying kinetics was analyzed using the dimensionless Biot number. The transition Biot number can be used for rational selection of the drying conditions (pressure, temperature, mass flow) to achieve a fast drying with low CO2 consumption.

Protein based aerogel particles derived from heat set whey and egg white protein hydrogels were prepared. The influence of the hydrogel preparation on aerogel properties was studied. Variations in pH, ionic strength and protein type and concentration were assessed as a means to vary hydrogel and corresponding aerogel structures in a targeted manner. An adjustment of pH of the precursor protein solution led to fragile hydrogels with obvious syneresis in the pH region around the isoelectric point. Corresponding aerogels were brittle and lacked the typically large specific inner surface area of aerogels. In an acidic environment, hydrogels were firm but also fragile. Aerogels were brittle but had a large specific inner BET-surface area. In an alkaline environment, the hydrogels were soft and elastic. Thus, resulting aerogels were mechanically stable and possessed a large BET-surface area. The addition of sodium chloride or calcium chloride to the protein solution had the same effect as lowering the pH towards the isoelectric point. The process of supercritical drying was also applied to enzymatically cross-linked sodium caseinate hydrogels with varying protein concentration. Corresponding aerogels had a smaller BET-surface area compared to heat set gels. The protein concentration did not affect the BET-surface area of the aerogel. Finally, an emulsion method was used to produce protein aerogel particles, suitable for microencapsulation purposes. All three types of protein formed single spherical aerogel particles with comparable aerogel properties to the related aerogel monolith. The properties of the protein aerogel particles may be tailored towards special needs for microencapsulation purposes in food, cosmetic or pharmaceutical applications.

The first evidence of solvent spillage under subcritical conditions during aerogel production is presented. The main objective was to understand the underlying phenomena controlling the solvent extraction kinetics during autoclave pressurization. Alginate, silica, and polyurethane as gels and ethanol, methyl ethyl ketone, and ethanol/water as solvents were investigated. When CO2 diffuses in the gel solvent, there is a volume expansion of the liquid solvent. This expanded liquid mixture spills out of the gel and accumulates as a separate liquid phase at the bottom of the autoclave. A lag time was observed between the start of the autoclave pressurization and the moment in which liquid starts to accumulate in the autoclave. The time needed for the solvent to start accumulating at the bottom of the autoclave is controlled by capillary forces and the saturation of the CO2-gas phase, on which temperature has an important effect. Low operating temperature and thereby low solubility of the solvent in the gas pha...

Supercritically dried protein aerogels made of whey protein, egg white protein or sodium caseinate were assessed as potential transportation and protection matrices for valuable components in food, nutraceutical or pharmaceutical applications. Swelling experiments of all three types of protein aerogels showed that such systems are very stable; they do not disintegrate in aqueous media, which is a major difference to silica or polysaccharide aerogels. The aerogels were loaded with fish oil where egg white protein aerogels showed the highest loading capacity, absorbing more fish oil than their own weight. Whey protein and sodium caseinate aerogels showed good controlled release properties in the conducted in-vitro digestion experiments. More than half of the encapsulated fish oil was released during intestinal digestion, whereas only a small amount of fish oil was released during oral or gastric digestion. The most fish oil from egg white protein aerogels was released during gastric digestion and only when the gel was formed in acidic environment. Despite the high loading capacity of egg white protein aerogels, those aerogels released the smallest amount of fish oil per mass of aerogel. The best ratio of released fish oil per mass of aerogel was achieved by whey protein aerogels.

Egg white protein hydrogels formed by heat coagulation were used to obtain aerogel structures by supercritical drying. Hydrogels were prepared from pasteurized and spray-dried egg white at different pH, ionic concentrations and protein content to modify the characteristics of the dried protein network. The largest BET-surface areas were found at low and high pH, the most mechanically stable aerogels – at alkaline pH. It was shown that the protein network is preserved during supercritical drying. Egg white as precursor for aerogels opens a new field of application for those proteins as microencapsulation material for sensitive or unpleasant tasting food additives.