Effect of oscillating magnetic fields on freezing of a colloidal dispersion of superparamagnetic nanoparticles

Several studies suggest that single-domain biogenic magnetic nanoparticles could play a major role on the effectiveness of oscillating magnetic fields (OMFs) in improving food freezing. To evaluate the potential role of superparamagnetic biogenic particles, we performed freezing experiments in a colloidal dispersion of <10-nm maghemite nanoparticles (MNPs) subjected or not to a 31.3-mT OMF at 50 Hz. Our results show that, at our experimental conditions, Néel mechanism governs the alignment of magnetic dipole moments and, therefore, no significant MNPs rotation, able to disturb the ordering of water molecules, should be expected during OMF experiments. Accordingly, no effects of the applied OMF were observed either on supercooling, ice nucleation, or on freezing kinetics even though heat removal was slightly increased during the precooling step. These results suggest that weak OMFs usually applied in commercial freezers should not produce any significant effect on superparamagnetic nanoparticles present in food that could improve the freezing process.