Abstract
The solubility and rheological properties of high-molecular-weight xanthan gum (XG) are crucial to its functional performance and determine its applications. Ultrasound modifies these properties mainly by altering acoustic propagation in viscous systems, which depends strongly on concentration and frequency mode. In this work, the propagation behavior of three frequency modes (33 kHz mono-frequency, 20-40 kHz dual-frequency, and 20-50-68 kHz triple-frequency) of arc-shaped flat-plate ultrasound was systematically investigated in XG solutions, as well as their effects on solubility and rheological properties. Results showed that multi-frequency ultrasound generated stronger and more uniform acoustic fields, maintaining higher space peak temporal peak acoustic intensity (I(SPTP)) over a wide concentration range, which was superior to the significant attenuation observed in mono-frequency mode above 10.0 g·L(-1). Ultrasonic treatment effectively increased solubility from 62.0 to 63.5% (untreated) to a maximum of 85.6% in the 20-40 kHz group. In terms of rheology, ultrasound reduced viscosity and altered viscoelastic behavior by disrupting the molecular network, with multi-frequency modes showing greater effects at higher concentrations. Surface tension decreased to a minimum of 58.4 mN·m(-1) under mono-frequency treatment. Frequency sweep and creep recovery tests indicated enhanced chain mobility and improved structural recovery after ultrasound. Microstructure analysis confirmed fiber fragmentation and the formation of a microporous structure, especially under multi-frequency modes. Overall, the key mechanism lies in the ability of multi-frequency ultrasound to maintain effective acoustic propagation in viscous media, thereby enhancing solubility and modulating rheological behavior.