Solutes In Heterogeneous Media Research Articles

Determination of the diffusion coefficients of small solutes in cheese: A review

In cheese technology, the mass transfer of small solutes, such as salt, moisture and metabolites during brining and ripening, is very important for the final quality of the cheese. This paper has the following objectives: (i) to review the data concerning the diffusion coefficients of solutes in different cheese types; (ii) to review the experimental methods available to model the mass transfer properties of small solutes in complex matrices such as cheese; and (iii) to consider some potential alternative approaches. Numerous studies have reported the transfer of salt in cheese during brining and ripening. Regardless of the type of cheese and its composition, the effective diffusion coefficients of salt have been reported to be between 1 and 5.3 × 10−10 m2·s−1 at 10–15 °C. However, few papers have dealt with the mass transfer properties of other small solutes in cheese. Most of the reported effective diffusion coefficient values have been obtained by macroscopic and destructive concentration profile methods. More recently, some other promising techniques, such as nuclear magnetic resonance, magnetic resonance imaging or fluorescence recovery after photobleaching, are currently being developed to measure the mass transfer properties of solutes in heterogeneous media at microscopic scales. However, these methods are still difficult to apply to complex matrices such as cheese. Further research needs to focus on: (i) the development of nondestructive techniques to determine the mass transfer properties of small solutes at a microscopic level in complex matrices such as cheese; and (ii) the determination of the mass transfer properties of metabolites that are involved in enzymatic reactions during cheese ripening.

Itô equation model for dispersion of solutes in heterogeneous media

Transport processes in heterogeneous media such as ionized plasmas, natural porous media, and turbulent atmosphere are often modeled as diffusion processes in random velocity fields. Using the Itô formalism, we decompose the second spatial moments of the concentration and the equivalent effective dispersion coefficients in terms corresponding to various physical factors which influence the transport. We explicitly define "ergodic'' dispersion coefficients, independent of the initial conditions and completely determined by local dispersion coefficients and velocity correlations. Ergodic coefficients govern an upscaled process which describes the transport at large tine-space scales. The non-ergodic behavior at finite times shown by numerical experiments for large initial plumes is explained by "memory terms'' accounting for correlations between initial positions and velocity fluctuations on the trajectories of the solute molecules.

A method of control of transfer processes

The effective medium theory for the coupled phenomena of heat conduction and mass diffusion of solute in heterogeneous media has been derived. The theory has been applied to a barrier with adaptive conductive and diffusive characteristics to be used for control of heat flow and mass flow of species. The barrier consists of a mixture of small solid particles of ellipsoidal shape which are randomly distributed in a carrier fluid. Volume fraction of the particles is small and variation in conductive–diffusive properties of the barrier is achieved by changing orientation of the particles. This analysis gives an estimate to what a degree the solute flow can be controlled by both bulk concentration and temperature gradient applied to the walls of the barrier. Influence of different parameters like ratios of conductive and diffusive properties of particles and the carrier fluid, solubility ratio of the diffusing species in the fluid and in the particle, class of particle shapes, particle aspect ratio and volume fraction as well as thickness of the barrier on the species mass flow have been studied in detail.