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Fibrous plant-based materials are characterized by inhomogeneous structure and composition, which further evolve during wet grinding processes and affect the surface functionality of micronized particles. Therefore, the performance of aqueous microgrinding operations in stirred media mills can be optimized by investigating the interaction between process conditions and material properties of heterogeneous fibrous plant materials. In this experimental study it is shown how particle size reduction, tendency of re-agglomeration and stability of the suspension of micronized particles are driven by the specific energy input, residence time, temperature and presence of surfactants during the milling process. A structured experimental approach is described to optimize the achievable particle size reduction, expressed by the top cut diameter d90,3. It was found that the applied wet milling process determines the stability of particle suspensions throughout further downstream processing, making the grinding process the core unit operation with respect to the performance and formulation of food products containing micronized particles.

Compaction is commonly applied in pharmaceutical and food technology for enhancing the flowability, dosing accuracy as well as to reduce the dustiness of powdered products. It can also be applied to adjust the bulk density and to control reconstitution kinetics of beverage powders, leading to modified appearance or sensory profiles of the reconstituted beverages. While roller compaction of crystalline granules is well established, processing amorphous ingredients represents specific challenges linked to their moisture sensitivity. This contribution puts a spotlight on process-structure-property relationships for compaction of amorphous food particles. It relates the physical properties of the compacted granules to initial powder properties and process line settings. Linking food material science and process mastership, this work introduces the bonding regime map for roller compaction of amorphous materials. It frames the operational conditions and predicts the resulting product structure, which determines final product performance characteristics such as flowability, stability and reconstitution kinetics. Mastering the product structure allows prediction of its functionality.

Wetting as the first step during food powder reconstitution strongly depends on the interactions of wetting liquid and particle surface, expressed by the contact angle. Due to the heterogeneous composition of food materials including hydrophilic and hydrophobic surfaces, also the wetting process is of heterogeneous nature. Furthermore, the solubility of food ingredients, such as sugars, increases the complexity of understanding and describing the wettability. In this study, we compared experimental water penetration into inert model powders containing hydrophilic and hydrophobic surfaces with theoretical penetration rates which were calculated by modifying a Washburn-based approach for the application to heterogeneous systems. A fair agreement was found when adjusting the porosity of the powder bed. In a second step, the inert powder was replaced by sucrose to introduce the effect of solubility. Since in presence of sucrose the model for inert systems can no longer describe experimentally observed penetration rates, we developed a model for water penetration into food powders considering solute concentration dependent liquid properties. The model is based on the solution of a coupled system of two differential equations representing capillary rise into a pore network and the mass transfer equation. Viscosity was found to have a major influence on the wetting kinetics. Our model predicts penetration rates during the first few seconds which are close to the experimental data. This allows us to conclude that it is suitable for predicting the first seconds of capillary penetration into a particulate system consisting of soluble sucrose.

The dynamic wetting behavior as first step in reconstitution of heterogeneous model food powders was investigated in the present work. Therefore, we studied the capillary rise of water into single pores and pore systems containing hydrophilic and hydrophobic walls. A two-wall setup was developed to realize the capillary rise in a single gap containing walls with two different contact angles but a constant pore size. The equilibrium penetration height was measured and compared to calculated heights using advancing contact angles inserted in a model equation. Wetting experiments in a Washburn setup were performed with respect to the investigation of water penetration into a heterogeneous pore network which leads to a variable pore size during the rise. In order to determine the effect of contact angle, three different glass materials at varied levels of hydrophobicity were prepared. Furthermore, a focus was put on understanding the impact of the pore network on the wetting kinetics by mixing two size fractions of glass beads. The equilibrium heights determined in heterogeneous single gaps fit well with the calculated heights by inserting advancing contact angles into the model equation. The results of the powder systems show a significant impact on the wetting with an increasing hydrophobic fraction, while an increased hydrophobic contact angle further increased the wetting time only at higher quantities of the hydrophobic component in the sample. Furthermore, our findings emphasize the complexity of wetting into a system with heterogeneity in pore size and contact angle.

Properties of powders, such as flowability, reconstitution behaviour or particle adhesion, depend on the surface properties, which are related to the surface composition and environmental conditions. Many researchers reported redistribution of the feed components along a droplet radius during spray-drying of multicomponent food systems. This redistribution can be driven by the difference in the diffusivity of the components, component solubility, density, surface activity and hydrophobicity of components. The final composition on the surface of a dried particle is a resultant of all these material properties. This contribution presents an approach to estimate the surface composition of a particle formed during drying a multicomponent droplet. A continuum model of drying of a single droplet in a hot air stream was applied to predict the radial distribution of the components in the particle. The initial droplet consists of three components: water and two solutes, with different diffusion coefficients. In the model, the mass transfer is described by diffusion. The influences of the initial solid content and component ratio on the composition of the most outer layer of the formed particle were studied. An enrichment on the surface in the slowly diffusing component by 10–150% were predicted for different initial total solid contents, solid-based mass fractions of slowly diffusing component in the bulk and drying temperatures. Our model predicts that the initial ratio of the solid components has an impact on the surface composition only at low initial total solids content and that the drying air temperature has a weaker effect on the surface enrichment than the initial total solids content.

The wetting kinetics of powder-liquid systems represent an important factor with regards to many practical applications, such as coating, granulation, agglomeration or powder reconstitution. Wetting as the first step during food powder reconstitution strongly depends on the interactions of the wetting liquid and the particle surface, expressed by the contact angle. Due to the heterogeneous composition of food materials including hydrophilic and hydrophobic surfaces, also the wetting process heterogeneous in nature. Furthermore, the solubility of food ingredients, such as sugars, increases the complexity of understanding and describing the wettability. We investigated the liquid penetration into heterogeneous, soluble model food powders using a Washburn setup. Soluble sucrose was used as hydrophilic food component and silanized glass beads were prepared as hydrophobic, inert material. Both materials were characterized in terms of contact angle and powder properties. Different mixtures of hydrophilic, soluble sucrose and hydrophobic glass beads were produced and their wetting performance was determined. For predicting the liquid penetration into food powders, the model developed in our previous study (Kammerhofer et al., 2018a [1]) about the penetration of aqueous solutions into food powders was extended. It now considers solute concentration-dependent liquid properties and the heterogeneity of solid surfaces. The model is based on the solution of a coupled system of two differential equations representing capillary rise into a pore network and the mass transfer equation. Two main factors were identified as having a major influence on the wetting kinetics: the viscosity and the contact angle. Increasing either of these factors reduces the penetration performance, but each of them is dominant in a different concentration range. Our adapted model predicts penetration rates which are close to the experimental data. Thus, we can conclude that it is suitable for predicting the capillary penetration into heterogeneous, soluble food powders.