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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.

International audience; Cocoa powders are highly subjected to caking, a phenomenon of solid particles agglomeration that impairs powder functionalities such as rehydration and flowing properties. This study aimed at identifying the main caking mechanisms occurring during storage of cocoa powders, according to their fat content and water activity, as well as storage temperature. The formation of caked powder was monitored at macroscopic scale by sieve analysis, showing that caking was significant only for fatty powders at 40 degrees C, in agreement with a caking mechanism controlled by fat melting. The similarity of results obtained at 0.2 and 0.7 water activity indicated that the humidity caking mechanism was not significant for these powders. Observations performed by TEM' evidenced the formation of fat bridges, confirming the occurrence of the fat melting mechanism. Then, solvent extraction techniques designed to quantify fat fractions permitted to highlight that storage caused the conversion of encapsulated fat into free fat. At temperatures sufficient to melt cocoa fat (40 degrees C), this newly formed free fat migrates toward cocoa particle surface, enhancing its fat coverage, hence making cocoa particle sticky and prone to cake. Finally, XPS analysis of the extreme surface of cocoa powders confirmed that two conditions should be met to trigger significant cocoa powder caking: high fat content and elevated storage temperature.

The reconstitution of a beverage from a dehydrated powder involves several physical mechanisms that determine the practical difficulty to obtain a homogeneous drink in a convenient way and within an acceptable time for the preparation of a beverage. When pouring powder onto static water, the first hurdle to overcome is the air–water interface. We propose a model to predict the percentage of powder crossing the interface in 45 s, namely the duration relevant for this application. We highlight theoretically the determinant role of the contact angle and of the particle size distribution. We validate experimentally the model for single spheres and use it to predict the wettability performance of commercial food powders for different contact angles and particles sizes. A good agreement is obtained when comparing the predictions and the wettability of the tested powders.

Carbohydrates such as sucrose and maltodextrins are commonly used in dehydrated food beverages. However, these ingredients may have, in some cases, negative impacts on the reconstitution performance (e.g., lump formation), compromising key consumer's expectations. In this study, we propose to discuss the performance of carbohydrates with regard to major physical steps of reconstitution (wetting, capillarity, dispersion, and dissolution). We show how particle size and water temperature drive the kinetics of dissolution of crystalline sucrose and propose descriptive equations. For amorphous maltodextrin, we quantify variations in wetting, capillarity, and dissolution performance as a function of important solid properties (moisture content, molecular weight, and particle size) as well as the liquid temperature. By doing so, we highlight the important role of the glass-transition temperature in relation to the moisture content of the powder. The comprehensive understanding provided by this work may be used to optimize product formulation in term of reconstitution performance.

The spreading dynamics of a water droplet on a soluble polymer substrate is controlled by the rate of water transfers into the substrate. We provide a comprehensive description of the parameters controlling the wetting dynamics based on the analysis of the transfers between the spreading liquid and the solid substrate. Our model is supported by experimental results obtained on supported films of polymer with thickness e varied over two decades, on which droplets of water spread with a velocity U spanning two decades. Three different transfers are governing the hydration dynamics: (i) Water evaporates from the droplet, condenses on the substrate, and further diffuses through the polymer. (ii) Liquid water at the contact line diffuses in the polymer ahead of the contact line. (iii) Water in the droplet is convected at the droplet velocity U . The evaporative process is the most efficient at hydrating the substrate and controls the hydration of the polymer at distances ranging between a macroscopic cutoff length L and a microscopic length κ. L is set by the diffusion of vapor in air. κ results from a balance between the diffusion of water from the droplet at the contact line and the diffusion of water in the atmosphere. By analyzing the hydration profile at all distances to the contact line between L and κ, we define three different spreading regimes: a thick film regime where the film behaves as a semi-infinite medium, a thin film regime where hydration is homogeneous along the film thickness, and an intermediate regime where both situations coexist along the film away from the contact line.

International audience; The surface of 11 wt.% fat cocoa particles was thoroughly investigated during a 2-month storage at 20 and 40 degrees C and at a(w) = 0.2. Surface fat migration was evidenced only for powders stored at 40 degrees C by X-ray Photoelectron Spectroscopy. This was confirmed by ToF-SIMS, which allowed visualizing bigger fat patches at the surface of 40 degrees C-stored powders. These observations emphasize that fat need to be under a melted form to move toward the surface during storage. However, an increase in the free fat content was observed at both 20 degrees C and 40 degrees C. These results suggest rearrangements of particle structure during storage; free fat being more accessible to solvents at both temperatures. Confirmation was done by Scanning and Transmission Electron Microscopy images. Free and encapsulated fat were also extracted and analyzed during storage and no significant difference in fatty acids distribution (C16, C18, C18:1, C18:2) was observed by gas chromatography.

Building sorption isotherms usually requires several weeks for the samples to equilibrate at different relative humidities. Such experimental durations are very long in view of present industrial timeframes. In this work, we propose a rapid and robust method using wise thermal treatment that significantly accelerates equilibration of samples without inducing chemical or physical degradation. To demonstrate the relevance of our approach, we used maltodextrin powders considered as model systems and coupled experimental work with theoretical prediction of water diffusion. By testing different temperatures, we prove that a thermal treatment at T g for approximately 2 h before measuring a w allows in most cases getting rid of moisture gradients within the powders; hence, we propose to introduce the notion of apparent vs. equilibrated water activity. By using equilibrated water activity data we find sorption isotherms which are less curved in the low water activity range in comparison with classical methods and propose to discuss the differences in view of the long time it takes for powders to fully equilibrate without thermal treatment. These results impact not only the methods and the accuracy of the data but also their practical use for instance in predicting shelf-life stability or modelling drying processes.

International audience; We study the wetting of polymer layers by polar solvents. As previously observed, when a dropletof solvent spreads, both its contact angle and velocity decrease with time as a result of solventtransfers from the droplet to the substrate. We show that, when the polymer is initially glassy, theangle decreases steeply for a given value of the velocity, Ug. We demonstrate that those variationsresult from a plasticization, i.e. a glass transition, undergone by the polymer layer during spreading,owing to the increase of its solvent content. By analyzing previous predictions on the wetting ofrigid and soft viscoelastic substrates, we relate Ug to the viscosity of the polymer gel close to glasstransition. Finally, we derive an analytical prediction for Ug, based on existing predictions for thewater transfer from the droplet to the substrate. Using polar solvents of different natures, we showthat the experimental data compares well to the predicted expression for Ug.

International audience; The wetting dynamics of a solvent on a soluble substrate interestingly results from the rates of the solvent transfers into the substrate. When a supported film of a hydrosoluble polymer with thickness e is wet by a spreading droplet of water with instantaneous velocity U, the contact angle is measured to be inversely proportionate to the product of thickness and velocity, eU, over two decades. As for many hydrosoluble polymers, the polymer we used (a polysaccharide) has a strongly nonlinear sorption isotherm phi(a(w)), where phi is the volume fraction of water in the polymer and a(w) is the activity of water. For the first time, this nonlinearity is accounted for in the dynamics of water uptake by the substrate. Indeed, by measuring the water content in the polymer around the droplet phi at distances as small as 5 mu m, we find that the hydration profile exhibits (i) a strongly distorted shape that results directly from the nonlinearities of the sorption isotherm and (ii) a cutoff length xi below which the water content in the substrate varies very slowly. The nonlinearities in the sorption isotherm and the hydration at small distances from the line were not accounted for by Tay et al., Soft Matter 2011, 7, 6953. Here, we develop a comprehensive description of the hydration of the substrate ahead of the contact line that encompasses the two water transfers at stake: (i) the evaporation condensation process by which water transfers into the substrate through the atmosphere by the condensation of the vapor phase, which is fed by the evaporation from the droplet itself, and (ii) the diffusion of liquid water along the polymer film. We find that the eU rescaling of the contact angle arises from the evaporation condensation process at small distances. We demonstrate why it is not modified by the second process

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.