Thermal effects induced by high-intensity ultrasound on the formation of whey protein isolate fibrils

High-intensity ultrasound (HIU) has been widely used to promote protein fibril formation. Although traditional HIU with temperature control can facilitate fibril formation, the thermal effects of HIU on protein fibril formation remain unclear. Herein, whey protein isolate (WPI) is selected to form whey protein isolate fibrils (WPF). The thermal effects of HIU on the structural and functional properties of WPF were investigated. WPF was obtained by treating with HIU (20 kHz, 400 W, 10-30 min) without temperature control at pH 7.0, followed by an acidic heating treatment (pH 2.0, 80 °C for 24 h). Thioflavin T (ThT) fluorescence showed that of WPF subjected to HIU for 30 min without temperature control and then heating for 24 h (UN₃₀-24H) exhibited higher ThT intensity than that WPF treated with HIU with temperature control (U₃₀-24H), indicating the production of more characteristic fibril structures due to the HIU thermal effects. Importantly, prior to the heating treatment, WPF produced without temperature control (UN₃₀-0H) showed decreased particle size (173.6 ± 2.7 nm), increased β-sheet content (35.11 ± 0.12%) and improved surface hydrophobicity (315.94 ± 21.54) compared with temperature control (U₃₀0H 200.1 ± 2.5 nm 33.49 ± 0.54% 238.31 ± 20.53), demonstrating that the HIU thermal effects accelerate the formation of protein fibril nuclei, leading to more ordered and dispersed WPF formation. Moreover, WPF treated by HIU without temperature control (UN) exhibited higher emulsifying and foaming properties relative to the sample treated with temperature control. Overall, our work provides a new approach for WPF formation by exploiting the HIU thermal effects.