Comparative evaluation of protein- and non-protein-stabilized lycopene nanoemulsions: physicochemical properties, antioxidant activity, protein denaturation inhibition, and gastrointestinal stability

Abstract Lycopene is a highly hydrophobic carotenoid with remarkable antioxidant and anti-inflammatory properties; however, its application in aqueous food systems remains limited because of its poor water dispersibility and chemical instability. In this study, lycopene nanoemulsions were prepared using a combined solvent displacement–ultrasonication technique and stabilized with sodium caseinate (SC), Tween 20 (TW), and saponin (SP). The influence of emulsifier type on the physicochemical characteristics and functional properties of the nanoemulsions was systematically investigated. The prepared systems were characterized in terms of particle size, polydispersity index, ζ-potential, lycopene loading, antioxidant activity, protein denaturation inhibitory activity, gastrointestinal stability, and degradation kinetics. The type of emulsifier significantly affected the properties of the nanoemulsions. SP-stabilized nanoemulsions exhibited the smallest droplet size (~ 63 nm) with a narrow size distribution, whereas SC-stabilized systems showed the highest absolute ζ-potential ( ~ − 35 mV) and lycopene loading (~ 74 µg g⁻¹). Among the prepared formulations, SP nanoemulsions showed the highest 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity (~ 96%), while SC nanoemulsions exhibited the greatest ferric reducing antioxidant power (FRAP). Under simulated gastrointestinal conditions, SP-stabilized nanoemulsions retained the highest amount of lycopene after exposure to simulated gastric fluid (SGF) (~ 83%), whereas differences among the emulsifiers were also observed after exposure to simulated intestinal fluid (SIF). Compared with the macro-structured lycopene extract, nanoemulsification markedly improved the physicochemical characteristics and functional properties of lycopene. Overall, the results demonstrate that emulsifier type is an important factor governing the structural and functional behavior of lycopene nanoemulsions and may provide a useful basis for the development of food-grade nano-delivery systems for lipophilic bioactive compounds.