Co-surfactant roles of amino acids at oil-water interface: Application in low-pH emulsions to regulate physical and oxidative stabilities

Sucrose monopalmitate (SMP) is an effective surfactant for emulsification, but exhibits poor stability in low pH environments, due to neutralized surface charges. To improve SMP-based emulsions, we used food-grade amino acids as co-surfactants. Lysine, histidine, phenylalanine, and tryptophan lowered the water-oil interfacial tension. Tryptophan was the most effective, providing sufficient electrostatic repulsion to stabilize emulsion droplets by adsorbing onto the oil surface and becoming protonated at pH 3. However, tryptophan was counterproductive to stabilize SMP-based emulsions between pH 4 and 5. A minimum concentration (0.4 w/v%) of tryptophan prevented droplet coalescence and creaming. Incorporating tryptophan with SMP before emulsification resulted in larger droplets compared to post-emulsification addition. Tryptophan-costabilized emulsions induced flocculation with kappa-carrageenan via electrostatic adsorption but showed higher compatibility with polysaccharides with weaker charges. Tryptophan enhanced oxidative stability of unsaturated lipids creating a cationic shield to repel transition metals in the aqueous phase and stabilizing cleavage of lipid hydroperoxides.