Pulsed hydrostatic pressure and ultrasound assisted extraction of soluble matter from mate leaves (Ilex paraguariensis): Experiments and modeling

• The kinetics of solute extraction from mate leaves was studied.
• Ultrasonic extraction alone was applied at moderate pressures.
• Ultrasonic extraction combined with pulses of hydrostatic pressure was applied.
• The equilibrium time was lower than that by infusion at atmospheric pressure.
• The equilibrium yield was higher than that by infusion at atmospheric pressure.

A set of 47 kHz ultrasonic extraction experiments assisted and non-assisted by pulsed hydrostatic pressure was performed in different pressures (91.4, 135.3, 202.9, 270.5, 338.2 kPa) at 16.5 ± 2.0 °C. A cylindrical extraction vessel fed with a mixture of distilled water and comminute dried leaves was internally equipped with a piston to have either a constant pressure or uniform 1:300 atmospheric and pressurized cycles of extraction per second. Kinetic results of solute mass fraction in the liquid phase determined by gravimetric method were obtained for 25,200 s at all the investigated conditions. A pure two-dimensional transient diffusive model was suggested to reproduce the experimental results of ultrasound-assisted extraction at constant pressure. A non-linear increase of the diffusion coefficient with pressure explained the reduction of extraction time by increasing the pressure. The mass transfer from the solid to the bulk liquid phase when involving ultrasonic extraction assisted by pulsed hydrostatic pressure was described by the Fick’s second law with a term to account for internal convection. A positive effect of pressure on velocity in the solid microchannels well-described by a rational function was able to explain the positive influence of hydrostatic pressure pulses on the kinetic results. In both the examined circumstances (only ultrasound or combined ultrasound-hydrostatic pressure pulse) the extraction yield at equilibrium was up to 200% higher than that obtained by conventional infusion at the same temperature.