Emulsifying properties of a dairy fat – whey protein – carboxymethylcellulose complex

Polyelectrolyte complexes formed by proteins and polysaccharides have gained increasing attention as food ingredients due to their techno-functional emulsifying properties. A coacervation method was developed to recover fat and proteins from cheese whey (a byproduct of small dairy facilities) by adding 1.8 g/L carboxymethylcellulose (CMC) and 15 g/L citric acid to raw whey, followed by recovery of the lower phase. The concentrate contained 25.6% protein and 39.2% fat (dry basis). This study evaluated the pH-dependent stability of oil-in-water (O/W) emulsions stabilized by this complex. The ζ-potential of the complex in aqueous media was measured across pH 2–12 (Nano Particle Analyzer). The emulsion stability was evaluated using conductometry by measuring the conductivity of the lower phase of the emulsion; t₁_/₂ was defined as the time at which the conductivity of the lower phase reached half of the conductivity of the aqueous phase. Coarse emulsions (oil fraction: 0.20) were prepared using a high-speed homogenizer (25,000 rpm, 1 min). To accelerate destabilization, diluted aqueous solutions were used for pH-specific emulsions, while emulsions prepared with the raw concentrate (without dilution) were analyzed for actual stability. The ζ-potential trended from ~0 mV at pH 3 to -43 mV at pH 10, revealing strong pH dependence. At pH 2–3, rapid phase separation occurred (t₁_/₂ = 18.2 ± 0.2 s), attributed to proximity to the isoelectric point of whey proteins, which impaired interfacial stabilization. Despite a ζ-potential of -29 mV at pH 4, immediate phase separation suggested insufficient electrostatic repulsion. Moderate stability (t₁_/₂ = 47 ± 3 s) was observed at pH 5–7, while alkaline conditions (pH ≥ 8) enhanced stability (t₁_/₂ > 60 s) due to stronger electrostatic repulsion. Notably, emulsions prepared with the raw concentrate at neutral pH remained stable for 96 h, demonstrating practical relevance. These results indicate that the dairy fat–whey protein–CMC complex acts as an effective emulsifier at neutral to alkaline pH but fails under highly acidic conditions. The pH-dependent behavior, linked to electrostatic interactions, underscores its potential for food applications where pH can be modulated, such as dairy-based beverages. This work also provides a scalable method to valorize cheese whey through functional coacervates.