Oscillating Magnetic Field-Based Supercooling Treatment for Solid Lipid Nanoparticles

Highlights OMF-based supercooling (-9°C) stabilized SC-SLNs, preventing ice and phase separation. SC-SLNs showed stable size (252.3 nm), PDI (0.255), and zeta potential (-52.7 mV). SC-SLNs and RF-SLNs had higher solid fat content compared to RT-SLNs, which behaved as a nanoemulsion. SC-SLNs exhibited a-form crystals with lower enthalpy and melting point than RF-SLNs. ABSTRACT. Solid lipid nanoparticles (SLNs) have emerged as effective carriers due to their ability to protect and control the release of encapsulated bioactive compounds. However, SLNs are susceptible to physical instability, such as the polymorphic transition from less ordered a-form to highly ordered ß'- or ß-form. Cooling to low temperatures is generally known to hinder lipid molecules from packing into more ordered crystalline structures. However, storing emulsion-based systems like SLNs at subzero temperatures can lead to instability due to ice formation. This study investigated the effects of an oscillating magnetic field (OMF)-based supercooling treatment on the physicochemical properties of SLNs made from cocoa butter and soy lecithin at subzero temperatures. The supercooled SLNs (SC-SLNs) were compared with frozen SLNs (FR-SLNs) treated at the same temperature without OMF treatment. The physicochemical properties were also compared with SLNs treated under traditional cooling and storage conditions at room temperature (20°C, RT-SLNs) and refrigeration (4°C, RF-SLNs). OMF-based supercooling treatment (15 mT, 5 Hz) maintained the supercooled state of the aqueous phase in SLNs at -9°C, preventing ice crystal formation and phase separation observed in FR-SLNs. SC-SLNs exhibited nanosized particles (252.3±0.6 nm), a low polydispersity index (0.255±0.002), and a high zeta potential (-52.7±0.7 mV), comparable to RT-SLNs and RF-SLNs. In addition, SC-SLNs and RF-SLNs exhibited higher solid fat content (SFC) compared to RT-SLNs, indicating that RT-SLNs behave as a nanoemulsion. Thermal analysis indicated that both SC-SLNs and RF-SLNs exhibited the a-form crystalline structure. However, SC-SLNs showed a lower melting enthalpy and faster melting rate compared to RF-SLNs, suggesting that SC-SLNs may possess a less ordered crystalline structure. These findings highlight the potential of OMF-based supercooling treatment in stabilizing SLNs at subzero temperatures while preserving their physicochemical properties. Keywords: Food emulsion, Freeze-thaw stability, Oscillating magnetic field, Polymorphic transition, Supercooling.