Rapid pickling of tuber mustard by using flowing saline solution under magnetic field

Abstract: Pickling is considered to be the most effective method of preserving food materials and of preventing spoilage. Commercial pickled tuber mustard with special favor is very popular in China. Most agricultural raw materials and food itself is porous to a certain extent, favoring ion diffusion. Salting is time-consuming process; therefore, some techniques in salting process has been applied to reduce salting time, such as vacuum impregnation, ultrasound technique and pulsed pressure treatment. Electrolyte solution contains a large number of charged ions such as Na+, Cl-, H+, Ca2+, Mg2+ and OH-. The movement of charged ions can be affected by the Lorentz force in the perpendicular magnetic field which induces large-scale mass transfer and migration in a fixed direction. In this study, the low-cost static magnetic field system has been designed which utilized modern NdFeB permanent magnets as the source for the required magnetic flux. The two blocks of NdFeB magnets(100 mm× 100 mm×15 mm, Br=0.5 T) were arranged in parallel, and placed with iron yokes in a way that a magnetic flux density B was generated by adjusting its distance. The magnetic flux densities between the parallel magnets measured by Teslameter, were 200 Gs, 1000 Gs and 2000 Gs, respectively, which were known to be strong enough to have the magnetic effect on the movement of ions. Simultaneously, these magnet systems provided sufficient free space to place the samples. The sample was placed parallel to the direction of magnetic field lines. Then tuber mustard salting by flowing saline was conducted in magnetic field at different treatment conditions. The flow rates of saline solution were respectively 0.03 m/s (Re=563), 0.06 m/s (Re=1127) and 0.22 m/s (Re=4132) corresponding to laminar flow and turbulence. In the experiment, the sampling period was 10 minutes, and then mass fraction of NaCl in the sample was determined after a 3-hour brining period. The mass transfer kinetic model has been investigated for evaluation of salt content in the samples during pickling. All measurements were performed in the laboratory at average room temperature of 20 oC. Three replicates of each sample were used for statistical analysis. The results showed that mass fraction of NaCl in the sample reached maximum at the magnetic flux density of 2000 Gs and flowing velocity of 0.06 m/s within the salting period. At this point the salinity diffusion coefficient was highest (k=2.35·10-2 min-1). The turbulence in saline solution was not conducive to the diffusion of salt into the sample with the lowest diffusion coefficient (k=0.34·10-2 min-1 and 0.31·10-2 min-1) at 0.22 m/s. There was no significant increase in mass fraction of NaCl in tuber mustard immersed by the static saline solution under the perpendicular magnetic field. However, saline solution in laminar flow conditions was conducive to the diffusion of free ions into the porous plant materials with the action of the Lorentz force. Analysis of mass transfer in sample demonstrated that using the Logistic kinetics model, the highest correlation coefficient of the equations was 0.989. The technology achieves the rapid salting for agric-products.