From organic waste compost to stable foams: pH-shifting and calcium ion synergy modulating protein interfacial behavior

Protein-based foaming agents exhibit potential for exceptional foaming capacity with broad industrial applicability, yet their high production costs drive the need for cost-effective and sustainable alternatives. This study unveils a novel synergistic strategy combining pH-shifting and calcium ion addition to transform organic waste compost protein (CP) into a high-performance foaming agent. Our approach utilizes pH-driven conformational adjustments to expose latent binding sites for calcium, enabling targeted interfacial reinforcement. The foaming capacity (FC) of the resulting CP was exceptional (>480 %). While untreated CP exhibited limited foam stability (FS, 58.86 %), 0.1 mol/L Ca 2+ alone boosted FS to 81.31 %, and pH-shifting alone achieved 70.21 %. The dual treatment synergized these effects, reaching 85.02 % FS while fully retaining FC. Advanced characterization revealed two mechanisms: (1) calcium-specific interfacial pattern, evidenced by SEM-EDS showing calcium enrichment at bubble surfaces (21.67 % vs. 5.39 % in controls), and (2) protein restructuring, where Ca 2+ coordinated with carboxyl/amino groups to compact CP’s secondary structure with an increase of α-helix content from 5.46 % to 6.96 % and (α-helix/(β-sheet+random coil) ratio increased by 32 %. Crucially, pH-shifting further amplified protein adsorption at air-water interfaces by 113 % (from 12.2 % to 26.06 %), creating viscoelastic films resistant to coalescence. The synergy between molecular-level structural changes and interfacial calcium-protein complexes accounts for the markedly enhanced FS. This study provides a theoretical foundation and a technical pathway for producing cost-effective, eco-friendly foaming agents derived from organic waste compost protein, thereby facilitating its high-value valorization.