The enhancement of mass transfer in foods by alternating electric fields.
50Hz alternating electric fields have been used to enhance the mass transfer of ionic solutes
within several food and model food materials under a range of conditions. Using image
analysis it has been possible to visualise diffusion and determine concentration profiles within
solid structures. It has been shown that in a uniform electric field diffusion can be enhanced
by ca. 40% above that of normal diffusion, when an electric field strength of approximately
1500 Ym-l is applied.
Modelling of the concentration profiles has demonstrated that the mechanism of enhanced
mass transfer is dependent on the material in which the ion is diffusing as well as the process
temperature and applied electric field strength. For alginate, a highly charged polymer
structure, at low temperature (30°C) using a "Fickian" diffusion model fitted the experimental
data well. At elevated temperature (100°C) the concentration profiles gave a poor fit to a
"Fickian" diffusion model, however fitted well to a "Type II" diffusion model where the
surface region of the material was flooded with dye. Within agar, a less highly charged
material, diffusion can be fitted well using a Fickian diffusion model in all cases. Although
diffusion is not truly Fickian as concentration dependence was seen.
A number of effects were seen, the rate of diffusion into the solid material increased with
decreasing molecular weight and increasing ionic charge of the dye molecules in solution
(methylene blue, rhodamine 60 and fluorescein), however not to the extent predicted by a
true electrophoretic model. Furthermore, enhanced diffusion increased by only 25% when the
motion of the ions was along the electric field lines, as opposed to at 90°.
The enhanced mass transfer effect has been demonstrated in a distorted electric field, local
electric field strengths within the system have been calculated, and the diffusion coefficients
obtained are in good agreement with diffusion coefficients in a constant electric field. Finally,
enhanced mass transfer has been demonstrated in two species of fish, here changes in the
diffusion coefficient demonstrate structural changes to the material.