Formulation and characterization of oil-in-water nanoemulsions stabilized by crude saponins isolated from onion skin waste.

In the current work, a detailed study of crude saponins isolated from onion skin waste (OSW) was carried out to minimize the interfacial tension ( γ ) at oil-water interfaces, and to evaluate important factors on the formulation and characterization of nanoemulsions (NEs) stabilized by OSW saponins as a new natural emulsifier. It was found that OSW saponins are moderately strong surface-active agents. The droplet size of OSW saponin-stabilized NEs formulated by a high-pressure homogenizer was significantly dependent on processing parameters, where volume mean droplet diameter ( d 4,3 ) decreased as homogenization pressure and emulsifier concentration increased. The d 4,3 of NEs formulated from soybean oil, sunflower oil, and medium-chain triglyceride (MCT) oil was <140 nm, which was much smaller than those prepared using orange oil ( d 4,3 > 800 nm). The OSW saponin-coated droplets in soybean-based NEs showed stability against thermal treatment (30-90 °C, 30 min), pH (7-9), and ionic strength (NaCl < 50 mM). The NEs exhibited instability with droplet coalescence at relatively low pH (3-6), and high ionic strength (NaCl > 50 mM). The NEs stabilized by 0.5-1% (w/w) OSW were highly stable without an increase in d 4,3 during the storage (60 days) at 25 °C. Overall, these results suggest that OSW saponins are an efficient natural emulsifier, with significant potential to replace synthetic emulsifiers in certain practical applications.