Ratio-Dependent Engineering of Sunflower Protein–Phospholipid Interfaces in Flaxseed Oil-in-Water Emulsions: Toward Enhanced α-Linolenic Acid Bioaccessibility

• SPI–PL interactions reshape protein structure, enhancing solubility and dispersion. • SPI–PL ratios of 1:1 and 1:3 form more viscoelastic and stable interfacial films. • Higher PL content lowers droplet size, improving emulsion stability. • Higher-PL emulsions remain gastric-stable and boost lipid hydrolysis to 81.8%. • SPI–PL (1:3, 1:7) maximizes ALA bioaccessibility, reaching ∼1.35 mg/g in micelles. The rational design of plant protein–phospholipid composite interfacial structures offers a promising strategy for stabilizing and delivering hydrophobic nutrients in emulsion-based food systems. In this study, flaxseed oil–in–water emulsions co-stabilized by sunflower protein isolate (SPI) and phospholipids (PL) at varying ratios were systematically engineered to elucidate their structure–function–digestion relationships. Multispectral characterization revealed that SPI–PL interactions induced distinct protein conformational rearrangements, manifested by a reduction in α-helix content (from 27% to 11%) and a marked increase in surface hydrophobicity. At the oil–water interface, specific SPI/PL ratios (notably 1:1 and 1:3) promoted the formation of viscoelastic, deformation-resistant composite interfacial films with enhanced mechanical strength. These optimized interfacial architectures resulted in the formation of considerably finer emulsion droplets (0.24–0.44 μm, compared with ∼8.0 μm for SPI-only emulsions) and significantly improved physical stability. During in vitro gastrointestinal digestion, PL-enriched emulsions (SPI/PL ratios of 1:3–1:7) exhibited superior gastric stability and accelerated intestinal lipolysis, achieving a peak free fatty acid (FFA) release of 81.8%, compared with 59.1% for the SPI-only emulsion. Importantly, at these ratios, enhanced mixed-micelle formation and greater lipid-loading capacity led to the highest absolute bioaccessibility of α-linolenic acid (approximately 1.35 mg/g). Overall, this study demonstrates that ratio-dependent SPI–PL synergism allows for effective regulation of interfacial architecture, thereby providing an effective strategy for improving the stability, digestibility, and bioaccessibility of bioactive lipids in sustainable, plant-based emulsion delivery systems.