Achieving high-performance Spirulina platensis protein isolate-based foam via ultrasound coupled with pH shift-induced interfacial remodeling: Dynamic adsorption and interfacial rheology behavior

To enhance the foaming properties of Spirulina platensis protein isolate (SPP) for broader food applications, an ultrasound coupled with pH-shifting method was applied to modify the structural characteristics of SPP. The effects on air-water interfacial behavior and foaming properties were systematically investigated to elucidate the underlying mechanism. Circular dichroism, surface hydrophobicity, and intrinsic fluorescence results indicated that the secondary and tertiary structure of SPP was altered under cotreatment, resulting in increased exposure of hydrophobic groups and the formation of more flexible structures of SPP compared to individual treatments. Furthermore, combining ultrasound with pH-shifting treatment induced the unfolding, fragmentation, and subsequent refolding of SPP, which substantially reduced its particle size. Notably, U500-SPP-12 (SPP treated by pH-shifting coupled with ultrasound at 500 W) exhibited the smallest particle size of 58.17 nm. These improved structural properties endowed U500-SPP-12 with a faster diffusion rate (0.50 mN/m/s^1/2) at air-water interface, shorter lag time (151.04 s), and higher interfacial adsorbed protein content (23.61 %), effectively lowering the interfacial energy barrier. Upon adsorption at the interface, the cotreated SPP formed a solid-like interfacial film with superior viscoelastic properties, thereby preventing bubble instability. Consequently, the SPP subjected to the cooperated treatment possessed superior foaming performance, with U500-SPP-12 achieving the highest foaming capacity (221.01 %) and foaming stability (63.72 %). This study demonstrates that the application of ultrasound coupled with pH-shifting technology represents a simple yet effective strategy for enhancing the foaming properties of SPP, offering significant potential for the development of innovative foam-based foods.