Plant-protein Stabilized Emulsions as β-carotene Delivery Systems: Colloidal Stability and Behaviour during in vitro Digestion Conditions

Abstract

Plant proteins offer a promising clean-label alternative to synthetic surfactants, but their emulsifying properties are strongly dictated by pH. This study investigates how pH (3.0 and 7.0) governs the physicochemical stability, in vitro lipid digestion, and β-carotene bioaccessibility in emulsions stabilized by wheat, pea, and soy proteins, benchmarked against a Tween 80 nanoemulsion. Interfacial tension analysis confirmed Tween 80’s superior performance (reaching 2.3 mN/m at pH 3.0 and 1.7 mN/m at pH 7.0). Among the proteins, wheat was most effective, and all proteins showed higher activity at pH 7.0 (2.4-2.8 mN/m) than at pH 3.0 (3.3-4.2 mN/m). In terms of colloidal stability, at both pH levels the Tween 80 nanoemulsion remained stable with a consistent size of ~0.2 µm. At pH 3.0, wheat protein emulsions were highly stable, forming fine droplets (0.42 µm) and increased electrostatic repulsion (+15 mV). Conversely, soy and pea emulsions were unstable at pH 3.0 (droplets >6 µm) but stabilized at neutral pH, where wheat emulsions destabilized (droplets >7 µm). The small size of the Tween 80 nanoemulsion enabled the fastest (24.5 %/min0.5) and most complete (79.5%) lipid digestion, while protein emulsions digested slower. Notably, wheat protein emulsions at pH 3.0 achieved a superior β-carotene bioaccessibility (14.5%), comparable to the Tween 80 nanoemulsion (12.5%). This performance was attributed to efficient proteolysis and micelle formation. Soy and pea proteins had lower bioaccessibility (8-10%). These findings show that wheat protein at acidic pH is a highly effective clean-label strategy for enhancing both emulsion stability and nutrient bioaccessibility.