Atmospheric Cold Plasma-Assisted Egg White Protein-Casein Dual Complexation: Insights into Effects on Structural and Functional Properties

Atmospheric cold plasma (ACP) offers a promising approach for tailoring protein interactions through controlled oxidative modification, but its effect on mixed egg white protein (EWP) and dairy casein (CN) systems remains unclear. Hence, this study investigated the physicochemical, structural, and functional properties of EWP-CN dual systems (2:1) treated with ACP at 70 kV for 0–5 min, followed by heating at 90 ± 2 °C for 30 min. The results indicated that moderate ACP exposure (0.5–2 min) reduced particle size and turbidity, maintained solubility, and induced conformational rearrangement. Surface hydrophobicity increased from 535.16 ± 2.95 in the 0 min control to 718.60 ± 4.15 after 2 min of ACP treatment, representing a 34.3% increase and indicating partial unfolding and exposure of nonpolar regions. Over the same treatment interval, the carbonyl content increased significantly from 1.85 ± 0.10 to 3.09 ± 0.19 μmol carbonyl/g protein. When treatment was extended to 3–5 min, the carbonyl content increased nonlinearly, with the sharpest rise occurring between 2 and 3 min, reaching 6.02 ± 0.34 μmol/g at 5 min, approximately 3.25-fold higher than the control. These changes were accompanied by increased turbidity and particle size, as well as decreased solubility after heating, resulting in the formation of a visible gel after 5 min of treatment. Additionally, ACP treatment for 1 min improved emulsion activity and foaming performance, but emulsion stability remained below that of the control at all treatment times. Under the tested conditions, ACP treatment for 1–2 min represented a favorable modification window for balancing structural flexibility, colloidal stability, and functionality in EWP-CN dual systems. Treatment for 1 min generally enhanced colloidal and initial interfacial properties, while 2 min achieved the highest surface hydrophobicity. However, further formulation optimization is required to achieve long-term emulsion stability.