Modulation and stabilization of silk fibroin-coated oil-in-water emulsions.

Summary The purpose of this study is to prepare and characterize stable oil-in-water emulsions containing droplets coated with silk fibroin. Silk fibroin, a native edible fibrous protein originating from silkworm cocoons, was used to prepare 10 % (by mass) corn oil-in-water emulsions at ambient temperature (pH=7.0, 10 mM phosphate buffer). Emulsions with relatively small mean particle diameter (d32=0.47 mm) and extremely good creaming stability (>7 days) could be produced at silk fibroin concentration of 1 % (by mass). The influence of pH (2–8), thermal processing (60–90 °C, 20 min), and concentration of salt (c(NaCl)=0–250 mM) on the properties and stability of the emulsions was analyzed using z-potential, particle size, and creaming stability measurements. The isoelectric point of droplets stabilized with silk fibroin was pH~4. The emulsions were stable to droplet flocculation and creaming at any pH except intermediate value (pH=4.0) when stored at room temperature, which was attributed to their relatively low z-potential. Their z-potential went from around 25 to –35 mV as the pH was increased from 2 to 8. The emulsions were also stable to thermal treatment (60 and 90 °C for 20 min, pH=3 and 7), with a slight decrease in the magnitude of z-potential at temperatures exceeding 60 °C. The emulsions were unstable to aggregation and creaming even at relatively low salt concentrations (c(NaCl)=0–250 mM, pH=3 and 7) as a result of electrostatic screening effects. These results suggest that bulk oil stabilized with silk fibroin has improved physical stability and may provide a new way of creating functional oil products and delivery systems.