Ultrasound-assisted preparation of zein particles: Insight into the effects and mechanisms of thermal factors

The research on the application of ultrasound in food processing has been increasing in recent years. In particular, using ice-bath temperature-controlled ultrasound (IBU) to minimize the effects of thermal aspects has been the main focus. However, this approach does not maximize the utilization of energy. Therefore, this study aims to compare the effect of non-temperature-controlled ultrasound (NTU) and temperature-controlled ultrasound (TU), with IBU on the structure and properties of zein. To further explore the role and mechanism of thermal effects in ultrasound processes. Firstly, particle size, zeta potential, hydrophobicity, and thermal stability of zein nanoparticles prepared using various ultrasound-assisted methods were characterized. Compared to the particle size in the IBU group, the NTU group consistently had smaller particles under the same power conditions (e.g., 197 nm versus 176 nm at U1). The particle size in the TU group changes depending on the temperature (e.g., from native 225 nm down to 185 nm). As for the denaturation temperatures, the NTU group (88.9 °C) was significantly higher than that of the IBU (75.76 °C) and TU groups (e.g., 76.9 °C at treatment temperatures of 30 °C). In all, IBU reduces the thermal stability, while NTU treatment enhances it. Furthermore, UV, fluorescence, and CD spectra results indicate that both NTU and TU groups significantly reduce the α-helix structure by unfolding and exposing more internal hydrophobic groups of zein than the IBU group. These results indicate that the effects of ultrasound on zein are closely related to the heating source during the ultrasonic process. IBU could be utilized to maximize the reduction of denaturation levels, but NTU may exhibit superior performance such as protein modification, reducing particle size, or enhancing stability in industrial production.