Modeling Mass and Heat Transfer in Multiphase Coffee Aroma Extraction.

Instant coffee manufacture involves the aqueous extraction of soluble coffee components followed by drying to form a soluble powder. Loss of volatile aroma compounds during concentration through evaporation can lower product quality. One method of retaining aroma is to steam-strip volatiles from the coffee and add them back to a concentrated coffee solution before the final drying stage. A better understanding of the impact of process conditions on the aroma content of the stripped solution will improve product design stages. In this context, we present a multiscale model for aroma extraction describing (i) the release from the matrix, (ii) intraparticle diffusion, (iii) transfer into water and steam, and (iv) advection through the column mechanisms. Results revealed (i) the existence of three different types of compound behavior, (ii) how aroma physiochemistry determines the limiting kinetics of extraction, and (iii) that extraction for some aromas can be inhibited by the interaction with other coffee components.

Toward a New Brewing Control Chart for the 21st Century.

This paper describes new results from a base model of brewing from a bed of packed coffee grains. The model solves for the diffusion of soluble species out of a distribution of particles into the flow through the bed pore space. It requires a limited set of input parameters. It gives a simple picture of the basic physics of coffee brewing and sets out a set of reduced variables for this process. The importance of bed extraction efficiency is elucidated. A coffee brewing control chart has been widely used to describe the region of ideal coffee brewing for some 50 years. A new chart is needed, however, one that connects actual brewing conditions (weight, flow rate, brew time, grind, etc.) to the yield and strength of brews. The paper shows a new approach to brewing control charts, including brew time and bed extraction efficiency as control parameters. Using the base model, an example chart will be given for a particular grind ratio of coarse to fine particles, and an "espresso regime" will be picked out. From such a chart yield, volume and strength of a brew can be read off at will.

Compact model for multi-phase liquid-liquid flows in micro-fluidic devices.

We present a compact model describing the laminar flow of viscous multiphase fluids in micro-channel networks. We apply this model to the flow of 2 immiscible fluids representing typically oil and water, in a network of micro-channels comprising one inlet for each fluid splitting into 2 branches meeting at a T-junction, where the 2 phases are combined before exiting the network through two outlets. This network is akin to an electrical "Wheatstone bridge" and represents a simplified interdigital micro-reactor, where the fluids to be mixed are separated into smaller branches and later re-combined together. We show from an analytical solution and a computational modelling that fluid flow inside this network is very sensitive to small differences in fluid resistance between the various branches of the network, which may lead to catastrophic error in fluid distribution between the various branches that can have a profound effect on mixing. These errors depend on the viscosity difference between the fluids, on the processing conditions, and also on the geometric resistance parameters of the various channels. Increasing the resistance of the distribution channels upstream of the fluid junctions allows minimisation of the distribution errors. Interaction between the fluids can also lead to transients that are orders of magnitude longer than the flooding time of the channels. This may be exploited to provide impedance-like terms in flui-logic operations.

Colloid flow during thickening--a particle level understanding for core-shell particles.

The flow curve of sheared concentrated colloids can show a region of shear thickening. The underlying mechanisms involved in this effect has long been an issue. Recently the author and co-workers have used Stokesian dynamics approximated for high concentrations to simulate models of polymer coated particles in the thickening regime. This work continues the search for a particle level of understanding these systems in the thickening regime. Previous work found that shear thickening is associated with the formation of a network of contacts between polymer coats. Previous work by the author has examined the fabric (geometry) and texture (density distribution) of the contact network. However despite this many-body effect, it was also found that a very simple mean-field type argument at the level of a pair of particles in contact relates the particle interaction laws to the thickening part of the flow curve up to an unknown factor. In the argument this factor is determined by aspects of the destruction of particle contacts. In this paper several new results are reported. The first concerns the definition of the network and its coordination. The paper contrasts systems with different thickness of polymer coats and gives tentative evidence that the network formed sits close to what is known as the iso-static coordination. The second set of issues concerns the analysis of the lifetimes of particle contacts--in effect an examination of the unknown factor and assumptions of the pair argument. A surprising result is that some particle pairs have relatively long lifetimes. Finally, data is reported for the stress fluctuations in the thickening regime.

Brownian dynamics simulations of aging colloidal gels.

The aging of colloidal gels is investigated using very long duration Brownian dynamics simulations. The Asakura-Oosawa description of the depletion interaction is used to model a simple colloid polymer mixture. Several regimes are identified during gel formation. The intermediate scattering function displays a double decay characteristic of systems where some kinetic processes are frozen. The beta relaxation at short times is explained in terms of the Krall-Weitz model for the decorrelation due to the elastic modes present. The alpha relaxation at long times is well described by a stretched exponential, showing a wide spectrum of relaxation times for which the q dependence is tau(alpha)=q(-2.2), lower than for diffusion. For the shortest waiting times, a combination of two stretched exponentials is used, suggesting a bimodal distribution. The extracted relaxation times vary with waiting time as tau(alpha)=tau(0.66)(w), more slowly than in the simple aging case. The real space displacements are found to be strongly non-Gaussian, correlated in space and time. We were unable to find clear evidence that the gel aging was driven by internal stresses. Rather, we hypothesize that in this case of weakly interacting gels, the aging behavior arises due to the thermal diffusion of strands, constrained by the percolating network, which ruptures discontinuously. Although the mechanisms differ, the similarity of some of the results to aging of glasses is striking.